Taking and Importing Marine Mammals; Taking Marine Mammals Incidental to Geophysical Surveys in the Gulf of America

DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

50 CFR Part 217

[Docket No. 260220-0051]

RIN 0648-BO19

Taking and Importing Marine Mammals; Taking Marine Mammals Incidental to Geophysical Surveys in the Gulf of America

AGENCY:

National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA), Commerce.

ACTION:

Proposed rule; request for comments.

SUMMARY:

NMFS has received a request for the reimplementation of incidental take regulations (ITR) governing the incidental taking of marine mammals during geophysical survey activity conducted in the Gulf of America (GOA). Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments on its proposed rule and will consider public comments relevant to this proposed rule prior to issuing any final rule.

DATES:

Comments and information must be received no later than March 26, 2026.

ADDRESSES:

Submit all electronic public comments via the Federal e-Rulemaking Portal. Visit

https://www.regulations.gov

and enter NOAA-NMFS-2025-0638 in the Search box. Click on the “Comment” icon, complete the required fields, and enter or attach your comments. A plain language summary of the rule is also available on the Federal e-Rulemaking Portal.

Instructions:

Comments sent by any other method, to any other address or individual, or received after the end of the comment period, may not be considered by NMFS. All comments received are a part of the public record and will generally be posted for public viewing on

https://www.regulations.gov

without change. All personal identifying information (

e.g.,

name, address), confidential business information, or otherwise sensitive information submitted voluntarily by the sender will be publicly accessible. NMFS will accept anonymous comments (enter “N/A” in the required fields if you wish to remain anonymous).

FOR FURTHER INFORMATION CONTACT:

Ben Laws, Office of Protected Resources, NMFS, (301) 427-8401.

SUPPLEMENTARY INFORMATION:

Purpose and Need for Regulatory Action

On January 19, 2021 (86 FR 5322), in response to a petition request from BOEM, NMFS issued a final rule implementing ITRs under the MMPA, 16 U.S.C. 1361

et seq.,

governing the take of marine mammals incidental to the conduct of geophysical survey activities in the GOA.

1

The ITRs provide a framework for authorization of incidental take through Letters of Authorization (LOAs) upon request from individual applicants planning specific geophysical survey activities The ITRs became effective on April 19, 2021, and are effective through April 19, 2026 (86 FR 5322, January 19, 2021).

1

Pursuant to Executive Order 14172, “Restoring Names That Honor American Greatness,” and Department of the Interior Secretarial Order 3423, “The Gulf of America,” the body of water formerly known as the Gulf of Mexico is now called the Gulf of America. Accordingly, NMFS amended the incidental take regulations to reflect the change. See 90 FR 38001 (August 7, 2025).

NMFS subsequently discovered that the 2021 rule was based on erroneous take estimates. We conducted another rulemaking to reassess the statutorily required findings for issuance of the 2021 ITRs using correct take estimates and other newly available and pertinent information relevant to the analyses supporting some of the findings in the 2021 final rule and the taking allowable under the regulations. We issued a final rule affirming those findings in April 2024, effective through April 19, 2026 (89 FR 31488, April 24, 2024). The 2024 rule did not result in any changes to the existing ITRs.

On March 25, 2025, NMFS received an application from the EnerGeo Alliance (EnerGeo) requesting development of ITRs governing the taking of marine mammals incidental to geophysical survey activity conducted in the GOA over the course of 5 years following the expiration of the existing ITRs. Following receipt of NMFS' comments on the draft application on April 15, 2025, EnerGeo submitted revised versions of the application on July 14, August 8, and August 12, 2025, the last of which was determined to be adequate and complete. NMFS determined at that time, based on the date of submission of the adequate and complete application, that it was unlikely a new rulemaking process could be completed prior to expiration of the existing ITRs on April 19, 2026.

On August 28, 2025, NMFS Office of Protected Resources (OPR) received a request from NMFS Office of Policy (Policy) for reimplementation of the current ITR to avoid a lapse in ITRs offering incidental take coverage for GOA geophysical survey activities. The request notes that the pending April 2026 expiration of the current ITRs would affect regulatory certainty through loss of an efficient permitting framework, and that reimplementation of the existing ITRs on the basis of the same specified activity defined in the initial 2021 final rule and associated estimates of incidental take evaluated in the 2024 corrective rulemaking is consistent with the MMPA and appropriate pursuant to Executive Orders 14156, “Declaring a National Energy Emergency,” and 14154, “Unleashing American Energy.” On October 20, 2025, BOEM (the original petitioner for the current ITRs) submitted a request to be included in the process as a co-petitioner.

NMFS has received multiple requests from industry survey operators relating to specific survey activities that would extend beyond the expiration date of the current ITRs, establishing the ongoing need for the ITRs. The requested reimplementation of regulations would continue the current established framework for authorization of incidental take through LOAs until superseded by a new ITR promulgated on the basis of the separate EnerGeo request.

Legal Authority for the Action

Section 101(a)(5)(A) of the MMPA (16 U.S.C. 1371(a)(5)(A)) directs the Secretary of Commerce to allow, upon request, the incidental, but not intentional, taking of small numbers of marine mammals by U.S. citizens who engage in a specified activity (other than commercial fishing) within a specified geographical region for up to 5 years if, after notice and public comment, the agency makes certain findings and issues regulations that set forth permissible methods of taking pursuant to that activity and other means of effecting the “least practicable adverse impact” (LPAI) on the affected species or stocks and their habitat (see the discussion below in the Proposed Mitigation section), as well as monitoring and reporting requirements. Under NMFS' implementing regulations for section 101(a)(5)(A), NMFS issues LOAs to individuals (including entities) seeking authorization for take under the activity-specific incidental take regulations (50 CFR 216.106).

Severability

In the event a court declares NMFS' interpretation of small numbers to be invalid, NMFS intends that the remaining aspects of the rule and ITR be severable. This is because the negligible impact analysis for this rule is the

biologically relevant inquiry, and that analysis is based on the total annual estimated taking for all activities the regulations will govern. The issuance of LOAs to authorize the incidental take of marine mammals, subject to the mitigation, monitoring, and reporting requirements in those LOAs, is based on a finding that the total taking over the five-year period will have a negligible impact on the affected species or stocks; and that the mitigation and related monitoring will effect the least practicable adverse impact on those species or stocks. The small numbers standard is a statutory requirement that could be satisfied on an LOA by LOA basis in accordance with the ruling of a court that invalidates the interpretation set forth in this proposed rule. NMFS is including a provision in the proposed regulatory text to that effect.

Summary of Major Provisions Within the Regulations

Following is a summary of the major provisions of this proposed rule regarding geophysical survey activities. The regulations contain requirements for mitigation, monitoring, and reporting, including:

• Standard detection-based mitigation measures, including use of visual and acoustic observation to detect marine mammals and shutdown of acoustic sources in certain circumstances;

• A time-area restriction designed to avoid effects to bottlenose dolphins in times and places of particular importance;

• Vessel strike avoidance measures; and

• Monitoring and reporting requirements.

These measures are unchanged from those included in the current ITRs. See 50 CFR 217.180

et seq.

Background

Sections 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361

et seq.

) direct the Secretary of Commerce (as delegated to NMFS) to allow, upon request, the incidental, but not intentional, taking of small numbers of marine mammals by U.S. citizens who engage in a specified activity (other than commercial fishing) within a specified geographical region if certain findings are made and either regulations are issued or, if the taking is limited to harassment, a notice of a proposed authorization is provided to the public for review.

An incidental take authorization shall be granted if NMFS finds that the taking will have a negligible impact on the species or stock(s), will not have an unmitigable adverse impact on the availability of the species or stock(s) for subsistence uses (where relevant), and if the permissible methods of taking and requirements pertaining to the mitigation, monitoring, and reporting of such takings are set forth.

NMFS has defined “negligible impact” in 50 CFR 216.103 as an impact resulting from the specified activity that cannot be reasonably expected to, and is not reasonably likely to, adversely affect the species or stock through effects on annual rates of recruitment or survival. The MMPA states that the term “take” means to harass, hunt, capture, kill or attempt to harass, hunt, capture, or kill any marine mammal.

Except with respect to certain activities not pertinent here, the MMPA defines “harassment” as: any act of pursuit, torment, or annoyance, which (i) has the potential to injure a marine mammal or marine mammal stock in the wild (Level A harassment); or (ii) has the potential to disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioral patterns, including, but not limited to, migration, breathing, nursing, breeding, feeding, or sheltering (Level B harassment).

On January 19, 2021, we issued a final rule with ITRs to govern the unintentional taking of marine mammals incidental to geophysical survey activities conducted in U.S. waters of the GOA over the course of the statutory maximum of 5 years (86 FR 5322, January 19, 2021). NMFS subsequently discovered that the 2021 rule was based on erroneous take estimates. We conducted another rulemaking to reassess the statutorily required findings for issuance of the 2021 ITRs using correct take estimates and other newly available and pertinent information relevant to the analyses supporting some of the findings in the 2021 final rule and the taking allowable under the regulations. We issued a final rule affirming those findings in April 2024 (89 FR 31488, April 24, 2024). The 2024 rule did not result in any changes to the existing ITRs, which provide a framework for authorization of incidental take through LOAs upon request from individual applicants planning specific geophysical survey activities. The existing ITRs are in effect through April 19, 2026.

On March 25, 2025, NMFS received an application from EnerGeo requesting development of ITRs governing the taking of marine mammals incidental to geophysical survey activity conducted in the GOA over the course of 5 years following the date of issuance. Following receipt of NMFS' comments on the draft application on April 15, 2025, EnerGeo submitted revised versions of the application on July 14, August 8, and August 12, 2025. On September 24, 2025 (90 FR 45936), we published a notice of receipt of the request in the

Federal Register

, requesting comments and information related to the request.

On August 28, 2025, NMFS OPR received a request from NMFS Policy for reimplementation of the current ITR. The request notes that the pending April 2026 expiration of the current ITR would affect regulatory certainty with loss of an efficient permitting framework, and that reimplementation of the existing ITR on the basis of the same specified activity defined in the initial 2021 final rule and associated estimates of incidental take evaluated in the 2024 corrective rulemaking is consistent with the MMPA and appropriate pursuant to Executive Orders 14156, “Declaring a National Energy Emergency,” and 14154, “Unleashing American Energy.” On September 3, 2025 (90 FR 42569), we published a notice of receipt of the request in the

Federal Register

, requesting comments and information related to the request. All comments received are available online at

https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america.

Among the comments was a letter from EnerGeo and other industry trade associations expressing support for NMFS' proposed issuance of reimplemented ITRs until superseded by a new ITR promulgated on the basis of the separate EnerGeo request. Please see the letters for full comments.

On October 20, 2025, BOEM (the original petitioner for the current ITRs) submitted a request to be included in the process as a co-petitioner, expressing support for the requested reimplementation of the existing ITRs. Both the NMFS Policy and BOEM requests are available online at:

https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america.

This proposed rule provides analysis of the same activities and activity levels considered for the 2021 final rule, which were unchanged in the 2024 final rule, and utilizes the same modeling methodology described in the 2024 final rule. We incorporate the best available information, including information that was newly evaluated in the 2024 final rule and any information that is newly available since issuance of the 2024 final rule. The 2024 final rule incorporated expanded modeling results relative to the 2021 final rule that

estimate take utilizing the existing methodology but also consider the effects of using smaller airgun arrays (relative to the proxy source originally defined by BOEM) that are currently prevalent as evidenced by LOA applications received by NMFS to date (see

https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america

).

There are no changes to the nature or level of the specified activities within or across years or to the geographic scope of the activity. There is no new information pertaining to the estimates of marine mammal take presented in the 2024 final rule and, therefore, no changes to those take numbers. Based on our assessment of the specified activity in light of the revised take estimates and other new information, we have preliminarily determined that the 2024 ITRs at 50 CFR 217.180

et seq.,

which include the required mitigation and associated monitoring measures, satisfy the MMPA requirement to prescribe the means of effecting the LPAI on the affected species or stocks and their habitat, and therefore, do not change those regulations, nor do we change the requirements pertaining to monitoring and reporting.

National Environmental Policy Act (NEPA)

In 2017, BOEM produced a final Programmatic Environmental Impact Statement (PEIS) to evaluate the direct, indirect, and cumulative impacts of geological and geophysical survey activities in the GOA, pursuant to requirements of NEPA. The PEIS is available online at:

https://www.boem.gov/Gulf-of-Mexico-Geological-and-Geophysical-Activities-Programmatic-EIS/.

NOAA, through NMFS, participated in preparation of the PEIS as a cooperating agency due to its legal jurisdiction and special expertise in conservation and management of marine mammals, including its authority to authorize incidental take of marine mammals under the MMPA.

In 2020, NMFS prepared a Record of Decision (ROD): (1) to adopt BOEM's Final PEIS to support NMFS' analysis associated with issuance of incidental take authorizations pursuant to section 101(a)(5)(A) or (D) of the MMPA and the regulations governing the taking and importing of marine mammals (50 CFR part 216); and (2) to announce and explain the basis for NMFS' decision to review and potentially issue incidental take authorizations under the MMPA on a case-by-case basis, if appropriate.

The 2017 NOAA NEPA Companion Manual required supplements to Environmental Impact Statements if (1) the agency made substantial changes in the proposed action that are relevant to environmental concerns or (2) there were significant new circumstances or information relevant to environmental issues and bearing on the proposed action or its impacts. For the 2024 final rule, NMFS considered these criteria and the criteria relied upon for the 2020 ROD to determine whether any new circumstances or information were “significant,” thereby requiring supplementation of the 2017 PEIS. NMFS reevaluated its findings related to the MMPA negligible impact standard and the LPAI standard governing its regulations in light of the corrected take estimates and other relevant new information. Based on that evaluation, NMFS reaffirmed its negligible impact determinations and determined that the existing regulations prescribed the means of effecting the LPAI on the affected species or stocks and their habitat, and therefore made no changes to the regulations. NMFS considered updated take estimates that corrected the take estimate errors and incorporated other new information,

e.g.,

modeling of a more representative airgun array and updated marine mammal density information. NMFS also consulted scientific publications from 2021 through 2024, data that were collected by the agency and other entities after the PEIS was completed, field reports, reports produced under the BOEM-funded Gulf of Mexico Marine Assessment Program for Protected Species (GoMMAPPS) project), and other sources (

e.g.,

updated NMFS Stock Assessment Reports (SARs)). In addition, NMFS considered new circumstances and information related to updated information on Rice's whales in the action area (population abundance, mortality and sources of mortality, distribution and occurrence) and any new data, analysis, or information on the effects of geophysical survey activity on marine mammals and relating to the effectiveness and practicability of measures to reduce the risk associated with impacts of such survey activity. Based on the review applying the 2017 supplementation standard and the 2020 ROD criteria, NMFS determined for its 2024 final rule that supplementation of the 2017 PEIS was not warranted.

In 2025, NOAA revised its NEPA procedures. As required by the 2025 procedures, environmental documents must be supplemented when (1) the agency makes substantial changes to the proposed activity or decision that are relevant to environmental concerns; or (2) the agency decides, in its discretion, that there are substantial new circumstances or information about the significance of the adverse effects that bear on the proposed activity or decision or its effects. Under this standard, NMFS has again considered whether there are any substantial new circumstances or information that bear on this proposed action or its impacts. For NMFS' consideration of new circumstances and information, NMFS has consulted any new scientific information available since issuance of the 2024 final rule. Again, NMFS has not made any changes to the proposed action relevant to environmental concerns, and has made no changes to the regulations. Based on the current review, NMFS has again determined preliminarily that supplementation of the 2017 PEIS is not warranted.

Summary of the Proposed Action

This proposed rule provides analysis of the same activities and activity levels considered for the 2024 final rule, and utilizes the same modeling methodology described in the 2024 final rule. There are no changes to the nature or level of the specified activities within or across years or to the geographic scope of the activity. Based on our preliminary assessment of the specified activity in light of the take estimates, which remain unchanged, we have determined that the specified activity will have a negligible impact on the affected species or stocks of marine mammals.

2

Additionally, the regulations at 50 CFR 217.180 satisfy the MMPA requirement to prescribe the means of effecting the least practicable adverse impact on the affected species or stocks and their habitat and contain monitoring and reporting requirements pertaining to the taking. Therefore, as requested, we propose to reimplement those regulations.

2

There are no relevant subsistence uses implicated by this action. Therefore the taking from the specified activity will not have an unmitigable adverse impact on the availability of the species for taking for relevant subsistence uses. See 16 U.S.C. 1371(a)(5)(A).

Description of the Specified Activity

Overview

The specified activity for this proposed action as requested by the NMFS' Policy petition is unchanged from the specified activity considered for the 2021 and 2024 rules, consisting of geophysical surveys conducted for a variety of reasons. Actual total amounts of effort (including by survey type and

location) are not known in advance of receiving LOA requests, but take in excess of what is analyzed in this rule would not be authorized. Applicants seeking authorization for take of marine mammals incidental to survey activities outside the geographic scope of the rule (

i.e.,

within the former Gulf of Mexico Energy Security Act (GOMESA) (Sec. 104, Pub. L. 109-432)

3

moratorium area) would need to pursue a separate MMPA incidental take authorization (see figure 1).

3

The Congressional moratorium in GOMESA was in place until June 30, 2022. On September 8, 2020, the President withdrew, under section 12 of the Outer Continental Shelf Lands Act, the same area covered by the prior GOMESA moratorium from disposition by leasing for 10 years, beginning on July 1, 2022, and ending on June 30, 2032.

EnerGeo's 2025 ITR petition suggests that the existing level of effort estimates, by survey type and location, are a reasonable representation of the activities expected to occur under our proposed ITR reimplementation rule (which EnerGeo supports). That petition, available online at:

https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america,

carries forward the same survey types and similar estimated annual levels of effort by survey type and location as specified over a 10-year period in BOEM's 2016 petition (as adjusted in 2020 by BOEM to account for removal from consideration of the area then under a Congressional leasing moratorium under GOMESA). The most notable difference is EnerGeo's estimate that approximately 40 percent of forecast survey effort may be accomplished using less environmentally impactful alternative sources to airgun arrays (

e.g.,

tuned pulse or dual barbell sources; additional descriptions of these source types may be found in

Federal Register

notices of LOA issuance under the existing ITR,

e.g.,

86 FR 37309, July 15, 2021; 87 FR 55790, September 12, 2022; 88 FR 72739, October 23, 2023). NMFS will address these changes to survey effort in a future rulemaking on EnerGeo's petition. For the current rulemaking, we have determined the specified activity that is the subject of this proposed rule is a reasonable projection on which to proceed.

Geophysical surveys are conducted to obtain information on marine seabed and subsurface geology for a variety of reasons, including to obtain data for hydrocarbon and mineral exploration and production; aid in siting of oil and gas structures, facilities, and pipelines; identify possible seafloor or shallow depth geologic hazards; and locate potential archaeological resources and benthic habitats that should be avoided.

Deep penetration seismic surveys using airgun arrays as an acoustic source (sound sources are described in the

Detailed Description of Activities

section) are a primary method of obtaining geophysical data used to characterize subsurface structure. These surveys are designed to illuminate deeper subsurface structures and formations. A deep penetration survey uses an acoustic source suited to provide data on geological formations that may be thousands of meters (m) beneath the seafloor, as compared with a shallow penetration or high resolution geophysical (HRG) survey that may be intended to evaluate shallow subsurface formations or the seafloor itself (

e.g.,

for hazards).

Deep penetration surveys may be two-dimensional (2D) or three-dimensional (3D), and there are a variety of survey methodologies designed to provide the specific data of interest. 2D surveys are designed to acquire data over large areas (thousands of square miles) in order to screen for potential hydrocarbon prospectivity, and provide a cross-sectional image of the structure. In contrast, 3D surveys may use similar acoustic sources but are designed to cover smaller areas with greater resolution (

e.g.,

with closer survey line spacing), providing a volumetric image of underlying geological structures. Repeated 3D surveys are referred to as four-dimensional (4D), or time-lapse, surveys that assess the depletion of a reservoir.

Shallow penetration and high-resolution surveys are designed to highlight seabed and near-surface potential obstructions, archaeology, and geohazards that may have safety implications during rig installation or well and development facility siting. Shallow penetration surveys may use a small airgun array, single airgun, or similar sources, while high-resolution surveys (which are limited to imaging the seafloor itself) may use a variety of sources, such as sub-bottom profilers, single or multibeam echosounders, or side-scan sonars.

Dates and Duration

The specified activities may occur at any time during the 5-year period of validity of the proposed regulations. Actual dates and duration of individual surveys are not known. Although the proposed period of validity is for 5 years, we reiterate the requested reimplementation of regulations would continue only until superseded by a new ITR promulgated on the basis of the separate EnerGeo request.

Specified Geographical Region

Generally speaking, projected survey activity may occur within U.S. waters within the GOA, outside of the former GOMESA moratorium area. The specified geographical region (with modeling zones and depicting the area withdrawn from leasing consideration) is depicted in figure 1.

EP24FE26.010

Figure 1—Specified Geographical Region

Detailed Description of Activities

An airgun is a device used to emit acoustic energy pulses into the seafloor, and generally consists of a steel cylinder that is charged with high-pressure air. There are different types of airguns; differences between types of airguns are generally in the mechanical parts that release the pressurized air, and the bubble and acoustic energy released are effectively the same. Airguns are typically operated at a firing pressure of 2,000 pounds per square inch (psi). Release of the compressed air into the water column generates a signal that reflects (or refracts) off the seafloor and/or subsurface layers having acoustic impedance contrast. Individual airguns are available in different volumetric sizes and, for deep penetration seismic surveys, are towed in arrays (

i.e.,

a certain number of airguns of varying sizes in a certain arrangement) designed according to a given company's method of data acquisition, seismic target, and data processing capabilities.

Airgun arrays are typically configured in subarrays of 6-12 airguns each. The airgun array is typically towed at a speed of approximately 4.5 to 5 knots (kn). The output of an airgun array is directly proportional to airgun firing pressure or to the number of airguns, and is expressed as the cube root of the total volume of the array.

Airguns are considered to be low-frequency acoustic sources, producing sound with energy in a frequency range from less than 10 hertz (Hz) to 2 kHz (though there may be energy at higher frequencies), with most energy radiated at frequencies below 500 Hz. Frequencies of interest to industry are below approximately 100 Hz. The amplitude of the acoustic wave emitted from the source is equal in all directions (

i.e.,

omnidirectional) for a single airgun, but airgun arrays do possess some directionality due to phase delays between guns in different directions. Airgun arrays are typically tuned to maximize functionality for data acquisition purposes, meaning that sound transmitted in horizontal directions and at higher frequencies is minimized to the extent possible.

When fired, a brief (~0.1 second) pulse of sound is emitted by all airguns in an array nearly simultaneously, in order to increase the amplitude of the overall source pressure signal. The combined signal amplitude and directivity is dependent on the number and sizes of individual airguns and their geometric positions within the array. The airguns are silent during the intervening periods, with the array typically fired on a fixed distance (or shot point) interval. The intervals are optimized for water depth and the distance of important geological features below seafloor, but a typical interval in relatively deep water might be approximately every 10-20 seconds (or 25-50 m, depending on vessel speed). The return signal is recorded by a listening device, and later analyzed with computer interpretation and mapping systems used to depict the subsurface. There must be enough time between shots for the sound signals to propagate down to and reflect from the feature of interest, and then to propagate upward to be received on hydrophones or geophones. Reverberation of sound from previous shots must also be given time to dissipate. The receiving hydrophones can be towed behind or in front of the airgun array (may be towed from the source vessel or from a separate receiver vessel), or ocean bottom nodes (OBN) containing geophone receivers can be deployed on the seabed. Receivers may be displaced several kilometers (km) horizontally away from the source, so horizontal propagation time is also considered in setting the interval between shots.

Sound levels for airgun arrays are typically modeled or measured at some distance from the source and a nominal source level then back-calculated. Because these arrays constitute a distributed acoustic source rather than a single point source (

i.e.,

the “source” is actually comprised of multiple sources with some predetermined spatial arrangement), the highest sound levels measurable at any location in the water will be less than the nominal source level. At sufficient distance—in the far field—the array may be perceived as a single point source but individual sources, each with less intensity than that of the whole, may be discerned at closer distances (Caldwell and Dragoset (2000) define the far field as greater than 250 m; though this distance is dependent on the array dimensions). Therefore, back-calculated source levels are not typically considered to be accurate indicators of the true maximum amplitude of the output in the far field, which is what is typically of concern in assessing potential impacts to marine mammals. In addition, the effective source level for sound propagating in near-horizontal directions (

i.e.,

directions likely to impact most marine mammals in the vicinity of an array) is likely to be substantially lower (

e.g.,

15-24 decibels (dB); Caldwell and Dragoset, 2000) than the nominal source level applicable to downward propagation because of the directional nature of the sound from the airgun array. The horizontal propagation of sound is reduced by noise cancellation effects created when sound from neighboring airguns on the same horizontal plane partially cancel each other out.

Alternative sources to conventional airgun arrays are increasingly used in deep penetration surveys. These sources, such as the tuned pulse source (TPS) or dual barbell sources, are expected to present lower potential for impacts to marine mammals but they operate on the same basic principles as traditional airgun sources in that they use compressed air to create a bubble in the water column which then goes through a series of collapses and expansions creating primarily low-frequency sounds. Because of the increasing potential for use of these sources, we describe them briefly here to show that they (and their potential impacts) fall within the scope of this proposed rule. However, the acoustic exposure modeling supporting this rule, and the estimated marine mammal take numbers evaluated herein, assume that airgun sources are used during all projected survey effort.

The difference between the TPS and airgun sources is that the TPS releases a larger volume of air, but at lower pressure. This creates a larger bubble resulting in more of the energy being concentrated in low-frequencies. The release of the air is also “tuned” so that the primary signal has an extended rise time and lower peak pressure level than that of a traditional airgun array source. Field data confirm that the TPS produces more sound at lower frequencies (approximately 2-4 Hz) compared to an airgun source, while producing much less sound (lower decibel levels) at frequencies above 4 Hz, meaning that the source produces significantly reduced energy at frequencies used by marine mammals for hearing and communication. This means that even for species in the low-frequency hearing group (mysticete whales) most affected by seismic survey sounds, the TPS is expected to have less impact than a traditional airgun array in terms of overlap with frequencies the species use. Potential impacts on high- and very high-frequency hearing groups will be reduced even more.

Dual barbell sources consist of one physical element with two large chambers, similarly creating a larger bubble resulting in more of the energy being concentrated in low frequencies. In addition to concentrating energy at lower frequencies, these sources are expected to produce lower overall sound levels than conventional airgun sources. The number of airguns in an array is highly influential on overall sound energy output, because the output increases approximately linearly with the number of airgun elements. In this case, because the same air volume is used to operate two very large guns, rather than tens of smaller guns, the array produces lower sound levels than a conventional array of equivalent total volume.

Survey protocols generally involve a predetermined set of survey, or track, lines. The seismic acquisition vessel(s) (source vessel) will travel down a linear track for some distance until a line of data is acquired, then turn and acquire data on a different track. In some cases, data is acquired as the source vessel(s) turns continuously rather than moving on a linear track (

i.e.,

coil surveys). The spacing between track lines and the length of track lines can vary greatly, depending on the objectives of a survey. Spacing and length of tracks varies by survey.

The general activities described here could occur pre- or post-leasing and/or on- or off-lease. Pre-lease surveys are more likely to involve larger-scale activity designed to explore or evaluate geologic formations. Post-lease activities may also include deep penetration surveys, but would be expected to be smaller in spatial and temporal scale as they are associated with specific leased blocks. Shallow penetration and HRG surveys are more likely to be associated with specific leased blocks and/or facilities, with HRG surveys used along pipeline routes and to search for archaeological resources and/or benthic communities.

2D and 3D Surveys (Deep Penetration Surveys)

—Deep penetration surveys may use an airgun array(s) as the acoustic source and may be 2D or 3D (with repeated 3D surveys termed 4D). Surveys may be designed as either multi-source (

i.e.,

multiple arrays towed by one or more source vessel(s)) or single source.

We described previously the basic differences between 2D and 3D surveys. A typical 2D survey deploys a single array, whereas a 3D vessel may deploy multiple source arrays. Among 3D surveys in particular, there are a variety of survey designs employed to acquire the specific data of interest. Conventional, single-vessel 3D surveys are referred to as narrow azimuth (NAZ) surveys. Survey techniques using multiple source vessels, often referred to as wide-azimuth (WAZ) surveys, help to provide better data quality than that achievable using traditional NAZ surveys, including better illumination, higher signal-to-noise ratios, and higher resolution. This is useful in imaging subsurface areas containing complex geologic structures, particularly those beneath salt bodies with irregular geometries.

In summary, 3D survey design involves a vessel with one or more acoustic sources covering an area of interest with relatively tight spatial configuration. In order to provide richer, more useful data, particularly in areas with more difficult geology, survey designs become more complicated with additional source and/or receiver vessels operating in potentially increasingly complicated choreographies. The time required to complete one pass of a trackline for a single NAZ vessel and the time required for one pass by a multi-vessel entourage conducting a WAZ survey will be essentially the same. Turn times will be somewhat longer during multi-vessel surveys to ensure that all vessels are properly aligned prior to beginning the next trackline. Coil surveys, described previously, reduce the total survey time due to elimination of the trackline-turn methodology. Note that, while coil surveys occur infrequently in the GOA, the coil survey simulation is applicable to a variety of survey types that are

conducted within smaller areas than 2D and 3D survey types.

Borehole Seismic Surveys

—The placement of seismic sensors in a drilled well or borehole is another way data can be acquired. These surveys, typically referred to as vertical seismic profiles (VSP), provide information about geologic structure, lithology, and fluids that is intermediate between that obtained from sea surface surveys and well-log scale information (well logging is the process of recording various physical, chemical, electrical, or other properties of the rock/fluid mixtures penetrated by drilling a borehole). VSP surveying is conducted by placing receivers at many (50-200) depths in a wellbore and recording both direct-arriving and reflection energy from an acoustic source. The acoustic source usually is a single airgun or small airgun array hung from a platform or deployed from a source vessel. The airguns used for VSPs may be the same or similar to those used for 2D and 3D surveys; however, the number of airguns and the total volume of an array used are typically less. Some VSP surveys take less than a day, and most are completed in a few days. Borehole seismic surveys include 2D VSPs, 3D VSPs, and other types of surveys.

Shallow Penetration/HRG Surveys

—These surveys are conducted to provide data informing initial site evaluation, drilling rig emplacement, and platform or pipeline design and emplacement. Identification of geohazards (

e.g.,

gas hydrates, buried channels) is necessary to avoid drilling and facilities emplacement problems, and operators are required to identify and avoid archaeological resources and certain benthic communities. In most cases, conventional 2D and 3D deep penetration surveys do not have the correct resolution to provide the required information. Shallow penetration surveys typically use small airgun arrays, paired or single airguns, or non-airgun impulsive sources such as sparkers or boomers. HRG surveys generally use electromechanical sources, including sources that are not likely to cause incidental take of marine mammals, such as sub bottom profilers, echosounders, and side-scan sonars (Ruppel

et al.,

2022).

Representative Sound Sources

Because the specifics of acoustic sources to be used cannot be known in advance of receiving LOA requests from industry operators, it is necessary to define representative acoustic source parameters, as well as representative survey patterns. The supporting modeling for the 2021 ITR considered two specific airgun array sizes/configurations (4,130 and 8,000 in

3

arrays) as well as a single, 90-in

3

airgun. For the 2024 rule, modeling of a third representative airgun array size (5,110-in

3

) was also specifically considered. In its petition for the 2021 ITR, BOEM determined realistic representative proxy sound sources and survey patterns. We note that EnerGeo's 2025 petition for a new ITR carries forward these assumed proxies regarding survey patterns, as well as the 5,110-in

3

array modeled for the 2024 rule, as representative of ongoing industry survey activities in the GOA.

Acoustic exposure modeling for the 8,000-in

3

airgun array and 90-in

3

single airgun, which provided support for the 2021 rule, was described in detail in “Acoustic Propagation and Marine Mammal Exposure Modeling of Geological and Geophysical Sources in the Gulf of Mexico” and “Addendum to Acoustic Propagation and Marine Mammal Exposure Modeling of Geological and Geophysical Sources in the Gulf of Mexico” (Zeddies

et al.,

2015, 2017a). Additional information, including evaluation of the 4,130-in

3

airgun array, was provided in “Gulf of Mexico Acoustic Exposure Model Variable Analysis” (Zeddies

et al.,

2017b).

Modeling of the more representative 5,110-in

3

airgun array for NMFS' 2024 rule (in view of LOA applications received to date under the current ITR) was described in a 2022 memorandum (Weirathmueller

et al.,

2022). These reports provide full detail regarding the modeled acoustic sources and survey types and are available online at:

www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america.

Representative sources for the modeling include the three different airgun arrays, the single airgun, and an acoustic source package including a sub-bottom profiler in combination with multibeam echosounder and side-scan sonar. Two major survey types were considered: large-area seismic (including 2D, 3D NAZ, 3D WAZ, and coil surveys) and small-area, high-resolution geotechnical (including single airgun surveys and HRG surveys using the aforementioned package of sources). The nominal airgun sources used for analysis of this proposed rule include a small single airgun (90-in

3

airgun) and a large airgun array (8,000-in

3

). While the 5,110-in

3

airgun array is considered most representative of the airgun sources that are likely to be used during deep penetration surveys during the period of effectiveness of this proposed ITR, the 8,000-in

3

airgun array results in larger take numbers for most species for which acoustic exposures were modeled, and therefore provide the basis for the analysis herein, thus allowing the flexibility for applicants to use larger sources when survey objectives dictate. The modeling for the alternative 4,130- and 5,110-in

3

arrays provides more realistic estimates of take for use in survey-specific LOAs, depending on the actual acoustic sources planned for use (see Letters of Authorization section). We note that while high-resolution geophysical sources were included for consideration in the 2021 final rule to allow for take authorization if necessary, these types of sources would not typically be expected to cause the incidental take of marine mammals (Ruppel

et al.,

2022).

New technologies and/or uses of existing technologies may come into practice during the period of validity of these proposed regulations. As under the 2021 and 2024 final rules, NMFS will evaluate any such developments on a case-specific basis to determine whether expected impacts on marine mammals are consistent with those described or referenced in this document and, therefore, whether any anticipated take incidental to use of those new technologies or practices may appropriately be authorized under the existing regulatory framework. See Letters of Authorization for additional information.

Estimated Levels of Effort

Actual total amounts of effort by survey type and location cannot be known in advance of receiving LOA requests from survey operators. Therefore, BOEM's 2017 PEIS provided projections of survey level of effort for the different survey types for a 10-year period (and BOEM refined those projections following removal of the GOMESA area from the scope of activity in 2020). As noted above, these estimated levels of effort remain representative of expected survey activity on an ongoing basis and, therefore, are carried forward unchanged. Table 1 provides those effort projections for the next 5-year period.

In order to provide some spatial resolution to the projections of survey effort and to provide reasonably similar areas within which acoustic modeling might be conducted, the geographic region was divided into seven zones, largely on the basis of water depth, seabed slope, and defined BOEM planning area boundaries. Shelf regions typically extend from shore to approximately 100-200 m water depths

where bathymetric relief is gradual. The slope starts where the seabed relief is steeper and extends into deeper water. In the GOA water deepens from 100-200 m to 1,500-2,500 m over as little as a 50 km horizontal distance. As the slope ends, water depths become more consistent, though depths can vary from 2,000 to 3,300 m. Three primary bathymetric areas were defined as shelf (0-200 m water depth), slope (200-2,000 m), and deep (>2,000 m).

Available information regarding cetacean density in the GOA shows that, in addition to water depth, animal distribution tends to vary from east to west in the GOA and appears correlated with the width of shelf and slope areas from east to west. The western region is characterized by a relatively narrow shelf and moderate-width slope. The central region has a moderate-width shelf and moderate-width slope, and the eastern region has a wide shelf and a very narrow slope. Therefore, BOEM's western, central, and eastern planning area divisions provide appropriate longitudinal separations for the shelf and slope areas. Due to relative consistency in both physical properties and predicted animal distribution, the deep area was not subdivided. As shown in figure 1, zones 1-3 represent the shelf area (from east to west), zones 4-6 represent the slope area (from east to west), and zone 7 is the deep area. Removal of the GOMESA moratorium area from the scope of activity entirely eliminated zone 1 from consideration, and reduced zone 4 by approximately 98 percent and zone 7 by 33 percent. Smaller portions of zones 2 and 5 were also removed from consideration (figure 1).

EP24FE26.011

Description of Marine Mammals in the Area of the Specified Activities

Table 2 lists all species with expected potential for occurrence in the GOA and summarizes information related to the population or stock, including potential biological removal (PBR). PBR, defined by the MMPA as the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population, is considered in concert with known sources of ongoing anthropogenic mortality (as described in NMFS' stock assessment reports (SAR)). For status of species, we provide information regarding U.S. regulatory status under the MMPA and Endangered Species Act (ESA).

In some cases, species are treated as guilds. In general ecological terms, a guild is a group of species that have similar requirements and play a similar role within a community. However, for purposes of stock assessment or density modeling, certain species may be treated together as a guild because they are difficult to distinguish visually and many observations are ambiguous. For example, NMFS' GOA SARs assess stocks of

Mesoplodon

spp. and

Kogia

spp. as guilds. Following this approach, we consider beaked whales and

Kogia

spp. as guilds. In this rule, reference to “beaked whales” includes the goose-beaked whale

4

and Blainville's and Gervais' beaked whales, and reference to “

Kogia

spp.” includes both the dwarf and pygmy sperm whale.

4

Note that this species is referred to in NMFS' SARs as the “Cuvier's beaked whale.”

The use of guilds herein follows the best available density information (

i.e.,

Garrison

et al.,

2023). The density models treat beaked whales and

Kogia

spp. as guilds and consolidate four species into an undifferentiated blackfish guild. These species include the melon-headed whale, false killer whale, pygmy killer whale, and killer whale. The model authors determined that, for this group of species, there were insufficient sightings of any individual species to generate a species-specific model (Garrison

et al.,

2023). Therefore, reference to blackfish hereafter includes the melon-headed whale, false killer whale, pygmy killer whale, and killer whale.

5

Twenty-one species (with 24 managed stocks) have the potential to co-occur with the prospective survey activities. All managed stocks in this region are assessed in NMFS' U.S. Atlantic SARs. All values presented in table 2 are the most recent available. For more information, please see information presented in the SARs (available online at:

https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports

).

5

This rule provides a single take estimate for the melon-headed whale, false killer whale, pygmy killer whale, and killer whale grouped together as the “blackfish.” This approach reflects the best available scientific information (Garrison

et al.,

2023). These species are encountered only occasionally during any given vessel survey, and these relatively infrequent encounters make it difficult to fit species-specific detection and habitat models. For each of these models, the authors detail analyses and decisions relevant to model development, as well as notes of caution regarding use of the models given the associated uncertainty resulting from development of a model based on few sightings. The Garrison

et al.

(2023) models are based on survey data from 2003 to 2019. Notably, surveys conducted after 2009 were conducted in “passing” mode, where the ship did not deviate from the trackline to approach and verify species identifications for detected marine mammal groups, resulting in an increase in observed marine mammal groups that could not be identified to species. As a result of these factors, the model authors determined it appropriate to develop a single spatial model based on sightings of unidentified blackfish, in addition to the relatively few sightings where species identification could be confirmed.

EP24FE26.012

EP24FE26.013

EP24FE26.014

In table 2 above, we report two sets of abundance estimates: those from NMFS' SARs and those predicted by habitat-based cetacean density models. Please see footnote 3 of table 2 for more detail. NMFS' SAR estimates are typically generated from the most recent shipboard and/or aerial surveys conducted. GOA oceanography is dynamic, and the spatial scale of the GOA is small relative to the ability of most cetacean species to travel. U.S. waters only comprise about 40 percent of the entire GOA, and 65 percent of GOA oceanic waters are south of the U.S. EEZ. Studies based on abundance and distribution surveys restricted to U.S. waters are unable to detect temporal shifts in distribution beyond U.S. waters that might account for any changes in abundance within U.S. waters. NMFS' SAR estimates also in some cases do not incorporate correction for detection bias. Therefore, for cryptic or long-diving species (

e.g.,

beaked whales,

Kogia

spp., sperm whales), they should generally be considered underestimates (see footnotes 5 and 7 of table 2).

The model-based abundance estimates represent the output of predictive models derived from multi-year observations and associated environmental parameters and which incorporate corrections for detection bias (the same models and data from which the density estimates are derived). Incorporating more data over multiple years of observation can yield different results in either direction, as the result is not as readily influenced by fine-scale shifts in species habitat preferences or by the absence of a species in the study area during a given year. NMFS' SAR abundance estimates show substantial year-to-year variability in some cases. Incorporation of correction for detection bias should systematically result in greater abundance predictions. For these reasons, the model-based estimates are generally more realistic and, for the purposes of assessing estimated exposures relative to abundance—used in this case to understand the scale of the predicted takes compared to the population—NMFS generally believes that the model-based abundance predictions are the best available information and most appropriate because they were used to generate the exposure estimates and therefore provide the most relevant comparison.

As part of our evaluation of the environmental baseline, which is considered as part of the negligible impact analysis, we consider any known areas of importance as marine mammal habitat. We also consider other relevant information, such as unusual mortality events (UME) and the 2010

Deepwater Horizon

oil spill.

Habitat

—Important habitat areas may include areas of known importance for reproduction, feeding, or migration, or areas where small and resident populations are known to occur. They may have independent regulatory status such as designated critical habitat for ESA-listed species (as defined by section 3 of the ESA) or be identified through other means (

e.g.,

recognized Biologically Important Areas (BIA)).

No critical habitat has yet been designated for the Rice's whale, though a proposed rule to do so was published (88 FR 47453, July 24, 2023). The proposal references the same supporting information discussed herein in suggesting that GOA continental slope waters between 100 and 400 m water depth be designated as critical habitat. In addition, a BIA has been recognized since 2015 (LaBrecque

et al.,

2015).

Our knowledge of Rice's whale distribution is based on a combination of historic and contemporary sightings, passive acoustic detections, and spatial modeling. The Rice's whale was historically typically observed only within a narrowly circumscribed area within the eastern GOA, leading to the area being described as a year-round BIA by LaBrecque

et al.

(2015). In sightings data available to support that description, whales were observed only between approximately the 100- and 300-m isobaths in the eastern GOA from the head of the De Soto Canyon (south of Pensacola, Florida) to northwest of Tampa Bay, Florida (Maze-Foley and Mullin, 2006; Waring

et al.,

2016; Rosel and Wilcox, 2014; Rosel

et al.,

2016). NOAA's ESA status review of the species (formerly the GOM Bryde's whale) (Rosel

et al.,

2016) expanded the 2015 BIA description by stating that, due to the depth of some sightings, the area is appropriately defined to the 400-m isobath and westward to Mobile Bay, Alabama, in order to provide some buffer around the deeper sightings and to include all sightings in the northeastern GOA. This area is now considered to mark a core habitat area for the species, versus its entire range within the GOA (as described in the 2023 proposed critical habitat designation). The core habitat area contains the highest known densities of Rice's whale and has defined the movements of previously tagged Rice's whales.

More recent scientific data, including visual and acoustic detections, now indicate that Rice's whales occupy waters along the continental shelf and slope and adjacent waters throughout the U.S. GOA, and in particular, waters between 100 and 400 m deep. The widest swath of habitat occurs in the species' aforementioned core habitat area in the northeastern GOA, south and west of Alabama and Florida. However, a contiguous strip of habitat also extends south of the core habitat area toward the Florida Keys, and westward along the continental shelf and slope offshore of Mississippi, Louisiana, and Texas (Garrison

et al.,

2023). Passive acoustic monitoring (PAM) recordings have been especially valuable for confirming the species' year-round presence in the central and western GOA (Soldevilla

et al.,

2022, 2024), helping to offset the limited visual

survey effort in those locations. The shallowest and deepest waters where Rice's whales have been confirmed visually to date are 117 m and 408 m, respectively, but Rice's whales may use waters that are deeper or shallower than those values at times, and unconfirmed sightings from protected species observers (PSOs) have occurred at a wider range of locations and depths (Barkaszi and Kelley, 2018, 2024).

Current understanding regarding Rice's whale occurrence in the central and western GOA is largely based on passive acoustic detections (Soldevilla

et al.,

2022; 2024). As background, a NOAA survey reported observation of a Rice's whale in the western GOA in 2017 (Garrison

et al.,

2020). Genetic analysis of a skin biopsy that was collected from the whale confirmed it to be a Rice's whale. There had not previously been a genetically verified sighting of a Rice's whale in the western GOA, and given the importance of this observation, additional survey effort was conducted in an attempt to increase effort in the area. However, no additional sightings were recorded (note that there were two sightings of unidentified large baleen whales in 1992 in the western GOA, recorded as

Balaenoptera

sp. or Bryde's/sei whale (Rosel

et al.,

2021)). Subsequently, during 2023 survey effort in the western GOA, a sighting of what has been described as a group of two probable Rice's whales was recorded (

https://www.fisheries.noaa.gov/science-blog/successful-final-leg-gulf-america-marine-mammal-and-seabird-vessel-survey

). In addition, there are occasional sightings by PSOs of baleen whales in the GOA that may be Rice's whales. Rosel

et al.

(2021) reviewed 13 whale sightings reported by PSOs in the GOA from 2010-2014 that were recorded as baleen whales. No sightings were close enough for the PSOs to see the diagnostic three lateral ridges on the whales' rostrums required to confirm them as Rice's whales. Rosel

et al.

ruled out five of the sightings as more likely being sperm whales based on water depth and descriptions of the whales' behavior. The remaining eight sightings may have been Rice's whales based on one or more lines of evidence (

i.e.,

photographs, behavioral description, and/or water depth consistent with Rice's whales). Of these sightings, three occurred in the northeastern GOA core habitat area, while the remaining five occurred along the GOA shelf break south of Louisiana.

The acoustic detections provide evidence of year-round Rice's whale presence outside of the northeastern GOA core habitat area. Soldevilla

et al.

(2022) deployed autonomous passive acoustic recorders at 5 sites along the GOA shelf break in predicted Rice's whale habitat (Roberts

et al.,

2016) for 1 year (2016-2017) to (1) determine if Rice's whales occur in waters beyond the northeastern GOA and, if so, (2) evaluate their seasonal occurrence and site fidelity at the five sites. Over the course of the 1-year study, sporadic, year-round recordings of calls assessed as belonging to Rice's whales were made south of Louisiana within approximately the same depth range (200-400 m), indicating that some Rice's whales occurred regularly in waters beyond the northeastern GOA core habitat area during the study period. Based on the detection range of the sonobuoys and acoustic monitors used in the study, actual occurrence could be in water depths up to 500 m (M. Soldevilla, pers. comm.), though the deepest confirmed Rice's whale sighting is at 408 m water depth. Data were successfully collected at four of the five sites; of those four sites, Rice's whale calls were detected at three. Detection of calls ranged from 1 to 16 percent of total days at the three sites. Calls were present in all seasons at two sites, with no obvious seasonality. It remains unknown whether animals are moving between the northwestern and the northeastern GOA or whether these represent different groups of animals (Soldevilla

et al.,

2022).

A subsequent follow-up study (Soldevilla

et al.,

2024) similarly involved deployment of autonomous passive acoustic recorders for approximately 1 year (2019-2020) at two shelf break sites, including one central GOA site included in the previous study and one new site further west, offshore Corpus Christi, Texas (recorders were also deployed at a site in Mexican waters for almost 2 years (2020-2022)). The study objectives were to (1) determine if Rice's whales occur in Mexican waters and to (2) evaluate how frequently they occur at all three sites. Rice's whale calls were detected on 33 and 25 percent of days at the central and western GOA sites, respectively, with calls recorded throughout the year, though no distinct seasonality was detected. These findings reflect an increase in the frequency and number of detections at the central GOA site compared with the 2016-2017 study. The authors note that these findings highlight persistence of Rice's whale detections at this site over multiple years, as well as variability among years (Soldevilla

et al.,

2024). Rice's whale calls were also detected at the site in Mexican waters (see Soldevilla

et al.

(2024) for additional discussion). The authors also describe differences in Rice's whale call types recorded in the eastern GOA compared with those recorded in the western GOA, suggesting that whales may indeed have a broader distribution than the northeastern GOA (Soldevilla

et al.,

2024).

The rate of call detections throughout the year is considerably higher in the eastern GOA than at the central/western GOA site where calls were most commonly detected, with at least 8.3 calls/hour among four eastern GOA sites within the core habitat area over 110 deployment days (Rice

et al.,

2014) compared to 0.27 calls/hour over the 299-day deployment at the central/western GOA site where calls were detected most frequently in the 2016-2017 study. Approximately 2,000 total calls were detected at the central/western GOA site over 10 months in 2016-2017, compared to more than 66,000 total detections at the eastern GOA deployment site over 11 months (

i.e.,

approximately 30 times more calls were detected at the eastern GOA site; Soldevilla

et al.,

2022). Although ambient noise conditions were higher at the central/western GOA site, thus influencing maximum detection range, accounting for this difference in conditions would be expected to result in only 4-8 times as many call detections if all other factors (including presence and number of whales) were consistent (versus 30 times as many detections). Overall, Soldevilla

et al.

(2022) assessed that there seem to be fewer whales or more sparsely spaced whales in the central/western GOA compared to the eastern GOA, with calls present on fewer days, lower call detection rates, and far fewer call detections in the central/western GOA.

The passive acoustic data discussed above provide evidence that waters 100-400 m deep in the central and western GOA are Rice's whale habitat and are being used by Rice's whales in all seasons. This could imply that the population size is larger than previously estimated, or it could indicate that some individual Rice's whales have a broader distribution in the GOA than previously understood (Soldevilla

et al.,

2024). Either way, the acoustic findings, combined with the low numbers of visual sightings in the central and western GOA, suggest that density and abundance of Rice's whales in the central and western GOA are less than in the core habitat in the northeastern GOA. Therefore, while we expect that some individual Rice's whales occur outside the core habitat area and/or that whales from the northeastern GOA core

habitat area occasionally travel outside the area, the currently available data are not sufficient to make inferences about Rice's whale density and abundance in the central and western GOA. More research is needed to answer key questions about Rice's whale density, abundance, habitat use, demography, and stock structure in the central and western GOA.

While these acoustic data and few confirmed sightings support the presence of Rice's whales in western and central GOA waters (within the 100-400 m water depth), the information is consistent with the predictions of Rice's whale density modeling, on which basis NMFS has anticipated and evaluated the potential for and effects of takes of Rice's whale in western and central GOA waters. Little is known about the number of whales that may be present, the nature of these individuals' use of the habitat, or the timing, duration, or frequency of occurrence for individual whales. Conversely, the importance of northeastern GOA waters to Rice's whale recovery is clear (Rosel

et al.,

2016). A comparison of acoustic and sightings data from the central/western and eastern GOA, even acknowledging the limitations of those data, suggests that occurrence of whales in the northeastern GOA core habitat is significantly greater and that the area provides the habitat of greatest importance to the species.

Finally, we acknowledge the “core distribution area” described in the 2024 final rule. Delineation of the core distribution area was an effort by NMFS SEFSC (Rosel and Garrison, 2022) to more systematically delimit the previously described core habitat area, including through the addition of buffers around confirmed sightings and location data from tagged whales to account for potential uncertainty in whale locations and possible movements from those locations. However, the result of this precautionary approach was that areas outside of Rice's whale habitat (NMFS, 2023) were included in the core distribution area. We discussed the relevance of this area in relation to our understanding of Rice's whale habitat in detail in the 2024 final rule. In summary, while the actual Rice's whale core habitat area (

i.e.,

the aforementioned area containing the majority of Rice's whale sightings, containing the movements of previously tagged whales, and where the volume and rate of acoustic detections is highest) is entirely outside the geographic scope of the rule; 5 percent of the core distribution area overlaps the scope of this rule. Within that small portion of the core distribution area, 76 percent covers waters shallower than 100 m (36 percent) or deeper than 400 m (40 percent),

i.e.,

three-quarters of the area covers waters considered outside of most suitable Rice's whale habitat. Therefore, we have determined that the “core distribution area” described by Rosel and Garrison (2022) has no relevance within the geographic scope of this rule beyond consideration of Rice's whale habitat (assumed to be within waters 100-400 m in depth) throughout the geographic scope. We do not further discuss the core distribution area.

Deepwater Horizon Oil Spill

—In 2010, the

Macondo

well blowout and explosion aboard the

Deepwater Horizon

drilling rig (also known as the

Deepwater Horizon

explosion, oil spill, and response; hereafter referred to as the DWH oil spill) caused oil, natural gas, and other substances to flow into the GOA for 87 days before the well was sealed. Total oil discharge was estimated at 3.19 million barrels (134 million gallons), resulting in the largest marine oil spill in history (DWH NRDA Trustees, 2016). In addition, the response effort involved extensive application of dispersants at the seafloor and at the surface, and controlled burning of oil at the surface was also used extensively as a response technique. The oil, dispersant, and burn residue compounds continue to present ecological challenges in the region. NMFS discussed the impacts of the DWH oil spill on marine mammals in detail in its 2018 notice of proposed rulemaking (83 FR 29212; June 22, 2018), and we refer the reader to that document for additional detail. The 2018 proposed rule provided detailed discussion of the DWH oil spill. There is no new information regarding the DWH oil spill. Estimates of annual mortality for many stocks over the period 2014-2018 include mortality attributed to the effects of the DWH oil spill (see table 2) (Hayes

et al.,

2023), and these mortality estimates are considered as part of the environmental baseline.

An Unusual Mortality Event (UME) affecting multiple cetacean species in the northern GOA occurred from 2010 to 2014. Additional information on the UME is available online at:

https://www.fisheries.noaa.gov/national/marine-life-distress/2010-2014-cetacean-unusual-mortality-event-northern-gulf-mexico.

In summary, the event included all cetaceans stranded during this time in Alabama, Mississippi, and Louisiana and all cetaceans other than bottlenose dolphins stranded in the Florida Panhandle (Franklin County through Escambia County), with a total of 1,141 cetaceans stranded or reported dead offshore. For reference, the same area experienced a normal average of 75 strandings per year from 2002 to 2009 (Litz

et al.,

2014). The majority of stranded animals were bottlenose dolphins, though at least 10 additional species were reported as well. Since not all cetaceans that die wash ashore where they may be found, the number reported stranded is likely a fraction of the total number of cetaceans that died during the UME. The UME investigation and the

Deepwater Horizon

Natural Resource Damage Assessment determined that the DWH oil spill was the most likely explanation of the persistent, elevated stranding numbers in the northern GOA after the 2010 spill.

In summary, coastal and oceanic marine mammals were injured by exposure to oil from the DWH spill. Nearly all of the stocks that overlap with the oil spill footprint have demonstrable, quantifiable injuries, and the remaining stocks (for which there is no quantifiable injury) were also likely injured, though there is not currently enough information to make a determination. Injuries included elevated mortality rates, reduced reproduction, and disease. Due to these effects, affected populations may require decades to recover absent successful efforts at restoration (

e.g.,

DWH NRDA Trustees, 2017). The ability of the stocks to recover and the length of time required for that recovery are tied to the carrying capacity of the habitat, and to the degree of other population pressures. NMFS treats the effects of the DWH oil spill as part of the baseline in considering the likely resilience of these populations to the effects of the activities considered in this proposed rule.

Marine Mammal Hearing

Hearing is the most important sensory modality for marine mammals underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Not all marine mammal species have equal hearing capabilities (

e.g.,

Richardson

et al.,

1995; Wartzok and Ketten, 1999; Au and Hastings, 2008). To reflect this, Southall

et al.

(2007, 2019) recommended that marine mammals be divided into hearing groups based on directly measured (behavioral or auditory evoked potential techniques) or estimated hearing ranges

(behavioral response data, anatomical modeling,

etc.

). Generalized hearing ranges were chosen based on the ~65 decibel (dB) threshold from composite audiograms, previous analyses in NMFS (2018), and/or data from Southall

et al.

(2007) and Southall

et al.

(2019).

EP24FE26.015

For more detail concerning these groups and associated frequency ranges, please see NMFS (2024) for a review of available information.

Potential Effects of the Specified Activities on Marine Mammals and Their Habitat

This section provides a discussion of the ways in which components of the specified activity may impact marine mammals and their habitat. The Estimated Take section later in this document includes a quantitative analysis of the number of individuals that are expected to be taken by this activity. The Negligible Impact Analysis and Determination section considers the content of this section, the Estimated Take section, and the Proposed Mitigation section, to draw conclusions regarding the likely impacts of these activities on the reproductive success or survivorship of individuals and whether those impacts are reasonably expected to, or reasonably likely to, adversely affect the species or stock through effects on annual rates of recruitment or survival.

Description of Active Acoustic Sound Sources

This section contains a brief technical background on sound, the characteristics of certain sound types, and on metrics used in this proposal inasmuch as the information is relevant to the specified activity and to a discussion of the potential effects of the specified activity on marine mammals found later in this document.

Sound travels in waves, the basic components of which are frequency, wavelength, velocity, and amplitude. Frequency is the number of pressure waves that pass by a reference point per unit of time and is measured in Hz or cycles per second. Wavelength is the distance between two peaks or corresponding points of a sound wave (length of one cycle). Higher frequency sounds have shorter wavelengths than lower frequency sounds, and typically attenuate (decrease) more rapidly, except in certain cases in shallower water. Amplitude is the height of the sound pressure wave or the “loudness” of a sound and is typically described using the relative unit of the dB. A sound pressure level (SPL) in dB is described as the ratio between a measured pressure and a reference pressure (for underwater sound, this is 1 micropascal (μPa)) and is a logarithmic unit that accounts for large variations in amplitude; therefore, a relatively small change in dB corresponds to large changes in sound pressure. The source level (SL) represents the SPL referenced at a distance of 1 m from the source (referenced to 1 μPa) while the received level is the SPL at the listener's position (referenced to 1 μPa).

Root mean square (RMS) is the quadratic mean sound pressure over the duration of an impulse. RMS is calculated by squaring all of the sound amplitudes, averaging the squares, and then taking the square root of the average (Urick, 1983). RMS accounts for both positive and negative values; squaring the pressures makes all values positive so that they may be accounted for in the summation of pressure levels (Hastings and Popper, 2005). This measurement is often used in the context of discussing behavioral effects, in part because behavioral effects, which often result from auditory cues, may be better expressed through averaged units than by peak pressures.

Sound exposure level (SEL; represented as dB re 1 μPa

2

-s) represents the total energy contained within a pulse and considers both intensity and

duration of exposure. Peak sound pressure (also referred to as zero-to-peak sound pressure or 0-p) is the maximum instantaneous sound pressure measurable in the water at a specified distance from the source and is represented in the same units as the RMS sound pressure. Another common metric is peak-to-peak sound pressure (pk-pk), which is the algebraic difference between the peak positive and peak negative sound pressures. Peak-to-peak pressure is typically approximately 6 dB higher than peak pressure (Southall

et al.,

2007).

When underwater objects vibrate or activity occurs, sound-pressure waves are created. These waves alternately compress and decompress the water as the sound wave travels. Underwater sound waves radiate in a manner similar to ripples on the surface of a pond and may be either directed in a beam or beams or may radiate in all directions (omnidirectional sources), as is the case for pulses produced by the airgun array considered here. The compressions and decompressions associated with sound waves are detected as changes in pressure by aquatic life and man-made sound receptors such as hydrophones.

Even in the absence of sound from the specified activity, the underwater environment is typically loud due to ambient sound. Ambient sound is defined as environmental background sound levels lacking a single source or point (Richardson

et al.,

1995), and the sound level of a region is defined by the total acoustical energy being generated by known and unknown sources. These sources may include physical (

e.g.,

wind and waves, earthquakes, ice, atmospheric sound), biological (

e.g.,

sounds produced by marine mammals, fish, and invertebrates), and anthropogenic (

e.g.,

vessels, dredging, construction) sound. A number of sources contribute to ambient sound, including the following (Richardson

et al.,

1995):

Wind and waves

—The complex interactions between wind and water surface, including processes such as breaking waves and wave-induced bubble oscillations and cavitation, are a main source of naturally occurring ambient sound for frequencies between 200 Hz and 50 kHz (Mitson, 1995). In general, ambient sound levels tend to increase with increasing wind speed and wave height. Surf sound becomes important near shore, with measurements collected at a distance of 8.5 km from shore showing an increase of 10 dB in the 100 to 700 Hz band during heavy surf conditions;

Precipitation

—Sound from rain and hail impacting the water surface can become an important component of total sound at frequencies above 500 Hz, and possibly down to 100 Hz during quiet times;

Biological

—Marine mammals can contribute significantly to ambient sound levels, as can some fish and snapping shrimp. The frequency band for biological contributions is from approximately 12 Hz to over 100 kHz; and

Anthropogenic

—Sources of anthropogenic sound related to human activity include transportation (surface vessels), dredging and construction, oil and gas drilling and production, seismic surveys, sonar, explosions, and ocean acoustic studies. Vessel noise typically dominates the total ambient sound for frequencies between 20 and 300 Hz. In general, the frequencies of anthropogenic sounds are below 1 kHz and, if higher frequency sound levels are created, they attenuate rapidly. Sound from identifiable anthropogenic sources other than the activity of interest (

e.g.,

a passing vessel) is sometimes termed background sound, as opposed to ambient sound.

The sum of the various natural and anthropogenic sound sources at any given location and time—which comprise “ambient” or “background” sound—depends not only on the source levels (as determined by current weather conditions and levels of biological and human activity) but also on the ability of sound to propagate through the environment. In turn, sound propagation is dependent on the spatially and temporally varying properties of the water column and sea floor, and is frequency-dependent. As a result of this dependence on a large number of varying factors, ambient sound levels can be expected to vary widely over both coarse and fine spatial and temporal scales. Sound levels at a given frequency and location can vary by 10-20 dB from day to day (Richardson

et al.,

1995). The result is that, depending on the source type and its intensity, sound from a given activity may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals. Details of source types are described in the following text.

Sounds are often considered to fall into one of two general types: Pulsed and non-pulsed. The distinction between these two sound types is important because they have differing potential to cause physical effects, particularly with regard to hearing (

e.g.,

NMFS, 2018; Ward, 1997 in Southall

et al.,

2007). Please see Southall

et al.

(2007) for an in-depth discussion of these concepts.

Pulsed sound sources (

e.g.,

airguns, explosions, gunshots, sonic booms, impact pile driving) produce signals that are brief (typically considered to be less than 1 second), broadband, atonal transients (American National Standards Institute (ANSI), 1986, 2005; Harris, 1998; National Institute for Occupational Health and Safety (NIOSH), 1998; International Organization for Standardization, 2003) and occur either as isolated events or repeated in some succession. Pulsed sounds are all characterized by a relatively rapid rise from ambient pressure to a maximal pressure value followed by a rapid decay period that may include a period of diminishing, oscillating maximal and minimal pressures, and generally have an increased capacity to induce physical injury as compared with sounds that lack these features.

Non-pulsed sounds can be tonal, narrowband, or broadband, brief or prolonged, and may be either continuous or non-continuous (ANSI, 1995; NIOSH, 1998). Some of these non-pulsed sounds can be transient signals of short duration but without the essential properties of pulses (

e.g.,

rapid rise time). Examples of non-pulsed sounds include those produced by vessels, aircraft, machinery operations such as drilling or dredging, vibratory pile driving, and active sonar systems (such as those used by the U.S. Navy). The duration of such sounds, as received at a distance, can be greatly extended in a highly reverberant environment.

Airgun arrays produce pulsed signals with energy in a frequency range from about 10-2,000 Hz, with most energy radiated at frequencies below 200 Hz. The amplitude of the acoustic wave emitted from the source is equal in all directions (

i.e.,

omnidirectional), but airgun arrays do possess some directionality due to different phase delays between guns in different directions. Airgun arrays are typically tuned to maximize functionality for data acquisition purposes, meaning that sound transmitted in horizontal directions and at higher frequencies is minimized to the extent possible.

Acoustic Effects

Here, we discuss the effects of active acoustic sources on marine mammals.

Potential Effects of Underwater Sound

6

—Anthropogenic sounds cover a broad range of frequencies and sound

levels and can have a range of highly variable impacts on marine life, from none or minor to potentially severe responses, depending on received levels, duration of exposure, behavioral context, and various other factors. The potential effects of underwater sound from active acoustic sources can potentially result in one or more of the following: Temporary or permanent hearing impairment; non-auditory physical or physiological effects; behavioral disturbance; stress; and masking (Richardson

et al.,

1995; Gordon

et al.,

2004; Nowacek

et al.,

2007; Southall

et al.,

2007; Götz

et al.,

2009). The degree of effect is intrinsically related to the signal characteristics, received level, distance from the source, and duration of the sound exposure. In general, sudden, high level sounds can cause hearing loss, as can longer exposures to lower level sounds. Temporary or permanent loss of hearing, if it occurs at all, will occur almost exclusively in cases where a noise is within an animal's hearing frequency range. We first describe specific manifestations of acoustic effects before providing discussion specific to the use of airgun arrays.

6

Please refer to the information given previously (

Description of Active Acoustic Sound Sources

) regarding sound, characteristics of sound types, and metrics used in this document.

Richardson

et al.

(1995) described zones of increasing intensity of effect that might be expected to occur, in relation to distance from a source and assuming that the signal is within an animal's hearing range. First is the area within which the acoustic signal would be audible (potentially perceived) to the animal, but not strong enough to elicit any overt behavioral or physiological response. The next zone corresponds with the area where the signal is audible to the animal and of sufficient intensity to elicit behavioral or physiological response. Third is a zone within which, for signals of high intensity, the received level is sufficient to potentially cause discomfort or tissue damage to auditory or other systems. Overlaying these zones to a certain extent is the area within which masking (

i.e.,

when a sound interferes with or masks the ability of an animal to detect a signal of interest that is above the absolute hearing threshold) may occur; the masking zone may be highly variable in size.

We describe the more severe effects of certain non-auditory physical or physiological effects only briefly as we do not expect that use of airgun arrays are reasonably likely to result in such effects (see below for further discussion). Potential effects from impulsive sound sources can range in severity from effects such as behavioral disturbance or tactile perception to physical discomfort, slight injury of the internal organs and the auditory system, or mortality (Yelverton

et al.,

1973). Non-auditory physiological effects or injuries that theoretically might occur in marine mammals exposed to high level underwater sound or as a secondary effect of extreme behavioral reactions (

e.g.,

change in dive profile as a result of an avoidance reaction) caused by exposure to sound include neurological effects, bubble formation, resonance effects, and other types of organ or tissue damage (Cox

et al.,

2006; Southall

et al.,

2007; Zimmer and Tyack, 2007; Tal

et al.,

2015). The survey activities considered here do not involve the use of devices such as explosives or mid-frequency tactical sonar that are associated with these types of effects.

Marine mammals, like all mammals, develop increased hearing thresholds over time due to age-related degeneration of auditory pathways and sensory cells of the inner ear. This natural, age-related hearing loss is contrasted by noise-induced hearing loss (Møller, 2012). Marine mammals exposed to high-intensity sound or to lower-intensity sound for prolonged periods can experience a noise-induced hearing threshold shift (TS), which NMFS defines as a change, usually an increase, in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level as a result of noise exposure (NMFS, 2018, 2024). The amount of TS is customarily expressed in dB. Noise-induced hearing TS can be temporary (TTS) or permanent (PTS), and higher-level sound exposures are more likely to cause PTS or other auditory injury. As described in NMFS (2018, 2024) there are numerous factors to consider when examining the consequence of TS, including, but not limited to, the signal temporal pattern (

e.g.,

impulsive or non-impulsive), likelihood an individual would be exposed for a long enough duration or to a high enough level to induce a TS, the magnitude of the TS, time to recovery (seconds to minutes or hours to days), the frequency range of the exposure (

i.e.,

spectral content), the hearing frequency range of the exposed species relative to the signal's frequency spectrum (

i.e.,

how an animal uses sound within the frequency band of the signal;

e.g.,

Kastelein

et al.,

2014), and the overlap between the animal and the source (

e.g.,

spatial, temporal, and spectral).

Auditory Injury (AUD INJ)

NMFS (2024) defines AUD INJ as damage to the inner ear that can result in destruction of tissue, such as the loss of cochlear neuron synapses or auditory neuropathy (Houser 2021; Finneran 2024). AUD INJ may or may not result in a PTS. PTS is subsequently defined as a permanent, irreversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2024). PTS does not generally affect more than a limited frequency range, and an animal that has incurred PTS has some level of hearing loss at the relevant frequencies; typically, animals with PTS or other AUD INJ are not functionally deaf (Au and Hastings, 2008; Finneran, 2016). For marine mammals, AUD INJ is considered to be possible when sound exposures are sufficient to produce 40 dB of TTS measured after exposure (Southall

et al.

2007, 1019). AUD INJ levels for marine mammals are estimates; with the exception of a single study unintentionally inducing PTS in a harbor seal (

Phoca vitulina

) (Kastak

et al.,

2008; Reichmuth

et al.

2019), there are no empirical data measuring AUD INJ in marine mammals largely due to the fact that, for various ethical reasons, experiments involving anthropogenic noise exposure at levels inducing AUD INJ are not typically pursued or authorized (NMFS, 2024).

Temporary Threshold Shift (TTS)

TTS is the mildest form of hearing impairment that can occur during exposure to sound. TTS is a temporary, reversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2024) that represents primarily tissue fatigue (Henderson

et al.,

2008), and is not considered an AUD INJ. Based on data from marine mammal TTS measurements (see Southall

et al.,

2007, 2019), a TTS of 6 dB is considered the minimum threshold shift clearly larger than any day-to-day or session-to-session variation in a subject's normal hearing ability (Finneran

et al.,

2000, 2002; Schlundt

et al.,

2000). While experiencing TTS, the hearing threshold rises, and a sound must be at a higher level in order to be heard.

In terrestrial and marine mammals, TTS can last from minutes or hours to days (

i.e.,

there is recovery back to baseline/pre-exposure levels), can occur within a specific frequency range (

i.e.,

an animal might only have a temporary loss of hearing sensitivity within a limited frequency band of its auditory range), and can be of varying amounts (

e.g.,

an animal's hearing sensitivity might be reduced by only 6 dB or reduced by 30 dB). In many cases, hearing sensitivity recovers rapidly after exposure to the sound ends. While there

are data on sound levels and durations necessary to elicit mild TTS for marine mammals, recovery is complicated to predict and dependent on multiple factors.

Relationships between TTS and AUD INJ thresholds have not been studied in marine mammals, and there are no measured PTS data for cetaceans, but such relationships are assumed to be similar to those in humans and other terrestrial mammals. AUD INJ typically occurs at exposure levels at least several dB above that inducing mild TTS (

e.g.,

a 40-dB threshold shift approximates AUD INJ onset (Kryter

et al.,

1966; Miller, 1974), while a 6-dB threshold shift approximates TTS onset (Southall

et al.,

2007, 2019). Based on data from terrestrial mammals, a precautionary assumption is that the AUD INJ thresholds for impulsive sounds (such as airgun pulses as received close to the source) are at least 6 dB higher than the TTS threshold on a peak sound pressure level (PK SPL) basis and AUD INJ cumulative SEL (SEL

24h

) thresholds are 15 (impulsive sound criteria) to 20 dB (non-impulsive criteria) higher than TTS cumulative SEL thresholds (Southall

et al.,

2007, 2019). Given the higher level of sound or longer exposure duration necessary to cause AUD INJ as compared with TTS, it is considerably less likely that AUD INJ could occur.

Marine mammal hearing plays a critical role in communication with conspecifics and interpretation of environmental cues for purposes such as predator avoidance and prey capture. Depending on the degree (elevation of threshold in dB), duration (

i.e.,

recovery time), and frequency range of TTS, and the context in which it is experienced, TTS can have effects on marine mammals ranging from discountable to serious. For example, a marine mammal may be able to readily compensate for a brief, relatively small amount of TTS in a non-critical frequency range that occurs during a time where ambient noise is lower and there are not as many competing sounds present. Alternatively, a larger amount and longer duration of TTS sustained during a time when communication is critical for successful mother/calf interactions could have more serious impacts.

Finneran

et al.

(2015) measured hearing thresholds in 3 captive bottlenose dolphins before and after exposure to 10 pulses produced by a seismic airgun in order to study TTS induced after exposure to multiple pulses. Exposures began at relatively low levels and gradually increased over a period of several months, with the highest exposures at peak SPLs from 196 to 210 dB and cumulative (unweighted) SELs from 193 to 195 dB. No substantial TTS was observed. In addition, behavioral reactions were observed that indicated that animals can learn behaviors that effectively mitigate noise exposures (although exposure patterns must be learned, which is less likely in wild animals than for the captive animals considered in this study). The authors note that the failure to induce more significant auditory effects was likely due to the intermittent nature of exposure, the relatively low peak pressure produced by the acoustic source, and the low-frequency energy in airgun pulses as compared with the frequency range of best sensitivity for dolphins and other high-frequency cetaceans.

Currently, TTS data only exist for four species of cetaceans (bottlenose dolphin, beluga whale (

Delphinapterus leucas

), harbor porpoise (

Phocoena phocoena

), and Yangtze finless porpoise (

Neophocaena asiaeorientalis

)) exposed to a limited number of sound sources (

i.e.,

mostly tones and octave-band noise) in laboratory settings (Finneran, 2015). In general, harbor porpoises have a lower TTS onset than other measured cetacean species (Finneran, 2015). Additionally, the existing marine mammal TTS data come from a limited number of individuals within these species.

Critical questions remain regarding the rate of TTS growth and recovery after exposure to intermittent noise and the effects of single and multiple pulses. Data at present are also insufficient to construct generalized models for recovery and determine the time necessary to treat subsequent exposures as independent events. More information is needed on the relationship between auditory evoked potential and behavioral measures of TTS for various stimuli. For summaries of data on TTS in marine mammals or for further discussion of TTS onset thresholds, please see Southall

et al.

(2007, 2019), Finneran and Jenkins (2012), Finneran (2015), and NMFS (2018, 2024).

Behavioral Effects

—Behavioral disturbance may include a variety of effects, including subtle changes in behavior (

e.g.,

minor or brief avoidance of an area or changes in vocalizations), more conspicuous changes in similar behavioral activities, and more sustained and/or potentially severe reactions, such as displacement from or abandonment of high-quality habitat. Behavioral responses to sound are highly variable and context-specific, and any reactions depend on numerous intrinsic and extrinsic factors (

e.g.,

species, state of maturity, experience, current activity, reproductive state, auditory sensitivity, time of day), as well as the interplay between factors (

e.g.,

Richardson

et al.,

1995; Wartzok

et al.,

2003; Southall

et al.,

2007, 2019; Weilgart, 2007; Archer

et al.,

2010). Behavioral reactions can vary not only among individuals but also within an individual, depending on previous experience with a sound source, context, and numerous other factors (Ellison

et al.,

2012), and can vary depending on characteristics associated with the sound source (

e.g.,

whether it is moving or stationary, number of sources, distance from the source). Please see appendices B-C of Southall

et al.

(2007) for a review of studies involving marine mammal behavioral responses to sound.

Habituation can occur when an animal's response to a stimulus wanes with repeated exposure, usually in the absence of unpleasant associated events (Wartzok

et al.,

2003). Animals are most likely to habituate to sounds that are predictable and unvarying. It is important to note that habituation is appropriately considered as a “progressive reduction in response to stimuli that are perceived as neither aversive nor beneficial,” rather than as, more generally, moderation in response to human disturbance (Bejder

et al.,

2009). The opposite process is sensitization, when an unpleasant experience leads to subsequent responses, often in the form of avoidance, at a lower level of exposure. As noted, behavioral state may affect the type of response. For example, animals that are resting may show greater behavioral change in response to disturbing sound levels than animals that are highly motivated to remain in an area for feeding (Richardson

et al.,

1995; National Research Council (NRC), 2003; Wartzok

et al.,

2003). Controlled experiments with captive marine mammals have shown pronounced behavioral reactions, including avoidance of loud sound sources (Ridgway

et al.,

1997). Observed responses of wild marine mammals to loud pulsed sound sources (typically seismic airguns or acoustic harassment devices) have been varied but often consist of avoidance behavior or other behavioral changes suggesting discomfort (Morton and Symonds, 2002; see also Richardson

et al.,

1995; Nowacek

et al.,

2007). However, many delphinids approach acoustic source vessels with no apparent discomfort or obvious behavioral change (

e.g.,

Barkaszi

et al.,

2012, Barkaszi and Kelly, 2018).

Available studies show wide variation in response to underwater sound;

therefore, it is difficult to predict specifically how any given sound in a particular instance might affect marine mammals perceiving the signal. If a marine mammal does react briefly to an underwater sound by changing its behavior or moving a small distance, the impacts of the change are unlikely to be significant to the individual, let alone the stock or population. However, if a sound source displaces marine mammals from an important feeding or breeding area for a prolonged period, impacts on individuals and populations could be significant (

e.g.,

Lusseau and Bejder, 2007; Weilgart, 2007; NRC, 2005). There are broad categories of potential response, which we describe in greater detail here, that include alteration of dive behavior, alteration of foraging behavior, effects to breathing, interference with or alteration of vocalization, avoidance, and flight.

Changes in dive behavior can vary widely, and may consist of increased or decreased dive times and surface intervals as well as changes in the rates of ascent and descent during a dive (

e.g.,

Frankel and Clark, 2000; Ng and Leung, 2003; Nowacek

et al.,

2004; Goldbogen

et al.,

2013a, b). Variations in dive behavior may reflect disruptions in biologically significant activities (

e.g.,

foraging) or they may be of little biological significance. The impact of an alteration to dive behavior resulting from an acoustic exposure depends on what the animal is doing at the time of the exposure and the type and magnitude of the response.

Disruption of feeding behavior can be difficult to correlate with anthropogenic sound exposure, so it is usually inferred by observed displacement from known foraging areas, the appearance of secondary indicators (

e.g.,

bubble nets or sediment plumes), or changes in dive behavior. As for other types of behavioral response, the frequency, duration, and temporal pattern of signal presentation, as well as differences in species sensitivity, are likely contributing factors to differences in response in any given circumstance (

e.g.,

Croll

et al.,

2001; Nowacek

et al.,

2004; Madsen

et al.,

2006; Yazvenko

et al.,

2007a, b). A determination of whether foraging disruptions affect fitness consequences would require information on or estimates of the energetic requirements of the affected individuals and the relationship between prey availability, foraging effort and success, and the life history stage of the animal.

Visual tracking, PAM, and movement recording tags were used to quantify sperm whale behavior prior to, during, and following exposure to airgun arrays at received levels in the range 140-160 dB at distances of 7-13 km, following a phase-in of sound intensity and full array exposures at 1-13 km (Madsen

et al.,

2006; Miller

et al.,

2009). Sperm whales did not exhibit horizontal avoidance behavior at the surface. However, foraging behavior may have been affected. The sperm whales exhibited 19 percent less vocal, or buzz, rate during full exposure relative to post exposure, and the whale that was approached most closely had an extended resting period and did not resume foraging until the airguns had ceased firing. The remaining whales continued to execute foraging dives throughout exposure; however, swimming movements during foraging dives were 6 percent lower during exposure than control periods (Miller

et al.,

2009). These data raise concerns that seismic surveys may impact foraging behavior in sperm whales, although more data are required to understand whether the differences were due to exposure or natural variation in sperm whale behavior (Miller

et al.,

2009).

Changes in respiration naturally vary with different behaviors and alterations to breathing rate as a function of acoustic exposure and can be expected to co-occur with other behavioral reactions, such as a flight response or an alteration in diving. However, respiration rates in and of themselves may be representative of annoyance or an acute stress response. Various studies have shown that respiration rates may either be unaffected or could increase, depending on the species and signal characteristics, again highlighting the importance in understanding species differences in the tolerance of underwater noise when determining the potential for impacts resulting from anthropogenic sound exposure (

e.g.,

Kastelein

et al.,

2001, 2005, 2006; Gailey

et al.,

2007, 2016).

Marine mammals vocalize for different purposes and across multiple modes, such as whistling, echolocation click production, calling, and singing. Changes in vocalization behavior in response to anthropogenic noise can occur for any of these modes and may result from a need to compete with an increase in background noise or may reflect increased vigilance or a startle response. For example, in the presence of potentially masking signals, humpback whales and killer whales have been observed to increase the length of their songs or amplitude of calls (Miller

et al.,

2000; Fristrup

et al.,

2003; Foote

et al.,

2004; Holt

et al.,

2012), while right whales have been observed to shift the frequency content of their calls upward while reducing the rate of calling in areas of increased anthropogenic noise (Parks

et al.,

2007). In some cases, animals may cease sound production during production of aversive signals (Bowles

et al.,

1994).

Cerchio

et al.

(2014) used PAM to document the presence of singing humpback whales off the coast of northern Angola and to opportunistically test for the effect of seismic survey activity on the number of singing whales. Two recording units were deployed between March and December 2008 in the offshore environment; numbers of singers were counted every hour. Generalized additive mixed models were used to assess the effect of survey day (seasonality), hour (diel variation), moon phase, and received levels of noise (measured from a single pulse during each 10 minutes sampled period) on singer number. The number of singers significantly decreased with increasing received level of noise, suggesting that humpback whale communication was disrupted to some extent by the survey activity.

Castellote

et al.

(2012) reported acoustic and behavioral changes by fin whales in response to shipping and airgun noise. Acoustic features of fin whale song notes recorded in the Mediterranean Sea and northeast Atlantic Ocean were compared for areas with different shipping noise levels and traffic intensities and during a seismic airgun survey. During the first 72 hours of the survey, a steady decrease in song received levels and bearings to singers indicated that whales moved away from the acoustic source and out of the study area. This displacement persisted for a time period well beyond the 10-day duration of seismic airgun activity, providing evidence that fin whales may avoid an area for an extended period in the presence of increased noise. The authors hypothesize that fin whale acoustic communication is modified to compensate for increased background noise and that a sensitization process may play a role in the observed temporary displacement.

Seismic pulses at average received levels of 131 dB re 1 μPa

2

-s caused blue whales to increase call production (Di Iorio and Clark, 2009). In contrast, McDonald

et al.

(1995) tracked a blue whale with seafloor seismometers and reported that it stopped vocalizing and changed its travel direction at a range of 10 km from the acoustic source vessel (estimated received level 143 dB pk-pk). Blackwell

et al.

(2013) found that bowhead whale call rates dropped significantly at onset of airgun use at sites with a median distance of 41-45 km from the survey. Blackwell

et al.

(2015) expanded this analysis to show that whales actually increased calling rates as soon as airgun signals were detectable before ultimately decreasing calling rates at higher received levels (

i.e.,

10-minute cumulative SEL (SEL

cum

) of ~127 dB). Overall, these results suggest that bowhead whales may adjust their vocal output in an effort to compensate for noise before ceasing vocalization effort and ultimately deflecting from the acoustic source (Blackwell

et al.,

2013, 2015). These studies demonstrate that even low levels of noise received far from the source can induce changes in vocalization and/or behavior for mysticetes.

Avoidance is the displacement of an individual from an area or migration path as a result of the presence of sound or other stressors, and is one of the most obvious manifestations of disturbance in marine mammals (Richardson

et al.,

1995). For example, gray whales are known to change direction—deflecting from customary migratory paths—in order to avoid noise from seismic surveys (Malme

et al.,

1984). Humpback whales show avoidance behavior in the presence of an active seismic array during observational studies and controlled exposure experiments in western Australia (McCauley

et al.,

2000). Avoidance may be short-term, with animals returning to the area once the noise has ceased (

e.g.,

Bowles

et al.,

1994; Goold, 1996; Stone

et al.,

2000; Morton and Symonds, 2002; Gailey

et al.,

2007). Longer-term displacement is possible, however, which may lead to changes in abundance or distribution patterns of the affected species in the affected region if habituation to the presence of the sound does not occur (

e.g.,

Bejder

et al.,

2006; Teilmann

et al.,

2006).

Forney

et al.

(2017) detail the potential effects of noise on marine mammal populations with high site fidelity, including displacement and auditory masking, noting that a lack of observed response does not imply absence of fitness costs and that apparent tolerance of disturbance may have population-level impacts that are less obvious and difficult to document. Avoidance of overlap between disturbing noise and areas and/or times of particular importance for sensitive species may be critical to avoiding population-level impacts because (particularly for animals with high site fidelity) there may be a strong motivation to remain in the area despite negative impacts. Forney

et al.

(2017) state that, for these animals, remaining in a disturbed area may reflect a lack of alternatives rather than a lack of effects.

Forney

et al.

(2017) specifically discuss beaked whales, stating that until recently most knowledge of beaked whales was derived from strandings, as they have been involved in atypical mass stranding events associated with mid-frequency active (MFA) sonar training operations. Given these observations and recent research, beaked whales appear to be particularly sensitive and vulnerable to certain types of acoustic disturbance relative to most other marine mammal species. Individual beaked whales reacted strongly to experiments using simulated MFA sonar at low received levels, by moving away from the sound source and stopping foraging for extended periods. These responses, if on a frequent basis, could result in significant fitness costs to individuals (Forney

et al.,

2017). Additionally, difficulty in detection of beaked whales due to their cryptic surfacing behavior and silence when near the surface pose problems for mitigation measures employed to protect beaked whales. Forney

et al.

(2017) specifically state that failure to consider both displacement of beaked whales from their habitat and noise exposure could lead to more severe biological consequences.

A flight response is a dramatic change in normal movement to a directed and rapid movement away from the perceived location of a sound source. The flight response differs from other avoidance responses in the intensity of the response (

e.g.,

directed movement, rate of travel). Relatively little information on flight responses of marine mammals to anthropogenic signals exist, although observations of flight responses to the presence of predators have occurred (Connor and Heithaus, 1996). The result of a flight response could range from brief, temporary exertion and displacement from the area where the signal provokes flight to, in extreme cases, marine mammal strandings (Evans and England, 2001). However, it should be noted that response to a perceived predator does not necessarily invoke flight (Ford and Reeves, 2008), and whether individuals are solitary or in groups may influence the response.

Behavioral disturbance can also impact marine mammals in more subtle ways. Increased vigilance may result in costs related to diversion of focus and attention (

i.e.,

when a response consists of increased vigilance, it may come at the cost of decreased attention to other critical behaviors such as foraging or resting). These effects have generally not been demonstrated for marine mammals, but studies involving fish and terrestrial animals have shown that increased vigilance may substantially reduce feeding rates (

e.g.,

Beauchamp and Livoreil, 1997; Fritz

et al.,

2002; Purser and Radford, 2011). In addition, chronic disturbance can cause population declines through reduction of fitness (

e.g.,

decline in body condition) and subsequent reduction in reproductive success, survival, or both (

e.g.,

Harrington and Veitch, 1992; Daan

et al.,

1996; Bradshaw

et al.,

1998). However, Ridgway

et al.

(2006) reported that increased vigilance in bottlenose dolphins exposed to sound over a 5-day period did not cause any sleep deprivation or stress effects.

Many animals perform vital functions, such as feeding, resting, traveling, and socializing, on a diel cycle (24-hour cycle). Disruption of such functions resulting from reactions to stressors, such as sound exposure, are more likely to be significant if they last more than one diel cycle or recur on subsequent days (Southall

et al.,

2007). Consequently, a behavioral response lasting less than 1 day and not recurring on subsequent days is not considered particularly severe unless it could directly affect reproduction or survival (Southall

et al.,

2007). Note that there is a difference between multi-day substantive behavioral reactions and multi-day anthropogenic activities. For example, just because an activity lasts for multiple days does not necessarily mean that individual animals are either exposed to activity-related stressors for multiple days or, further, exposed in a manner resulting in sustained multi-day substantive behavioral responses.

Stone (2015) reported data from at-sea observations during 1,196 seismic surveys from 1994 to 2010. When large arrays of airguns (considered to be 500 in

3

or more in that study) were firing, lateral displacement, more localized avoidance, or other changes in behavior were evident for most odontocetes. However, significant responses to large arrays were found only for the minke whale and fin whale. Behavioral responses observed included changes in swimming or surfacing behavior, with indications that cetaceans remained near the water surface at these times. Cetaceans were recorded as feeding less often when large arrays were active. Behavioral observations of gray whales during a seismic survey monitored whale movements and respirations pre-, during, and post-seismic survey (Gailey

et al.,

2016). Behavioral state and water depth were the best “natural” predictors of whale movements and respiration and, after considering natural variation, none of the response variables were significantly associated with seismic survey or vessel sounds.

Stress Responses

—An animal's perception of a threat may be sufficient to trigger stress responses consisting of some combination of behavioral responses, autonomic nervous system responses, neuroendocrine responses, or immune responses (

e.g.,

Seyle, 1950; Moberg, 2000). In many cases, an animal's first and sometimes most economical (in terms of energetic costs) response is behavioral avoidance of the potential stressor. Autonomic nervous system responses to stress typically involve changes in heart rate, blood pressure, and gastrointestinal activity. These responses have a relatively short duration and may or may not have a significant long-term effect on an animal's fitness.

Neuroendocrine stress responses often involve the hypothalamus-pituitary-adrenal system. Virtually all neuroendocrine functions that are affected by stress—including immune competence, reproduction, metabolism, and behavior—are regulated by pituitary hormones. Stress-induced changes in the secretion of pituitary hormones have been implicated in failed reproduction, altered metabolism, reduced immune competence, and behavioral disturbance (

e.g.,

Moberg, 1987; Blecha, 2000). Increases in the circulation of glucocorticoids are also equated with stress (Romano

et al.,

2004).

The primary distinction between stress (which is adaptive and does not normally place an animal at risk) and distress is the cost of the response. During a stress response, an animal uses glycogen stores that can be quickly replenished once the stress is alleviated. In such circumstances, the cost of the stress response would not pose serious fitness consequences. However, when an animal does not have sufficient energy reserves to satisfy the energetic costs of a stress response, energy resources must be diverted from other functions. This state of distress will last until the animal replenishes its energetic reserves sufficiently to restore normal function.

Relationships between these physiological mechanisms, animal behavior, and the costs of stress responses are well-studied through controlled experiments and for both laboratory and free-ranging animals (

e.g.,

Holberton

et al.,

1996; Hood

et al.,

1998; Jessop

et al.,

2003; Krausman

et al.,

2004; Lankford

et al.,

2005). Stress responses due to exposure to anthropogenic sounds or other stressors and their effects on marine mammals have also been reviewed (Fair and Becker, 2000; Romano

et al.,

2002b) and, more rarely, studied in wild populations (

e.g.,

Romano

et al.,

2002a). For example, Rolland

et al.

(2012) found that noise reduction from reduced ship traffic in the Bay of Fundy was associated with decreased stress in North Atlantic right whales. These and other studies lead to a reasonable expectation that some marine mammals will experience physiological stress responses upon exposure to acoustic stressors and that it is possible that some of these would be classified as “distress.” In addition, any animal experiencing TTS would likely also experience stress responses (NRC, 2003).

Auditory Masking

—Sound can disrupt behavior through masking, or interfering with, an animal's ability to detect, recognize, or discriminate between acoustic signals of interest (

e.g.,

those used for intraspecific communication and social interactions, prey detection, predator avoidance, navigation) (Richardson

et al.,

1995; Erbe

et al.,

2016). Masking occurs when the receipt of a sound is interfered with by another coincident sound at similar frequencies and at similar or higher intensity, and may occur whether the sound is natural (

e.g.,

snapping shrimp, wind, waves, precipitation) or anthropogenic (

e.g.,

shipping, sonar, seismic exploration) in origin. The ability of a noise source to mask biologically important sounds depends on the characteristics of both the noise source and the signal of interest (

e.g.,

signal-to-noise ratio, temporal variability, direction), in relation to each other and to an animal's hearing abilities (

e.g.,

sensitivity, frequency range, critical ratios, frequency discrimination, directional discrimination, age or TTS hearing loss), and existing ambient noise and propagation conditions.

Under certain circumstances, significant masking could disrupt behavioral patterns, which in turn could affect fitness for survival and reproduction. It is important to distinguish TTS and PTS, which persist after the sound exposure, from masking, which occurs during the sound exposure. Because masking (without resulting in a TS) is not associated with abnormal physiological function, it is not considered a physiological effect, but rather a potential behavioral effect.

The frequency range of the potentially masking sound is important in predicting any potential behavioral impacts. For example, low-frequency signals may have less effect on high-frequency echolocation sounds produced by odontocetes but are more likely to affect detection of mysticete communication calls and other potentially important natural sounds such as those produced by surf and some prey species. The masking of communication signals by anthropogenic noise may be considered as a reduction in the communication space of animals (

e.g.,

Clark

et al.,

2009) and may result in energetic or other costs as animals change their vocalization behavior (

e.g.,

Miller

et al.,

2000; Foote

et al.,

2004; Parks

et al.,

2007; Di Iorio and Clark, 2009; Holt

et al.,

2009). Masking may be less in situations where the signal and noise come from different directions (Richardson

et al.,

1995), through amplitude modulation of the signal, or through other compensatory behaviors (Houser and Moore, 2014). Masking can be tested directly in captive species (

e.g.,

Erbe, 2008), but in wild populations it must be either modeled or inferred from evidence of masking compensation. There are few studies addressing real-world masking sounds likely to be experienced by marine mammals in the wild (

e.g.,

Branstetter

et al.,

2013).

Masking affects both senders and receivers of acoustic signals and can potentially have long-term chronic effects on marine mammals at the population level as well as at the individual level. Low-frequency ambient sound levels have increased by as much as 20 dB (more than three times in terms of SPL) in the world's ocean from pre-industrial periods, with most of the increase from distant commercial shipping (Hildebrand, 2009). All anthropogenic sound sources, but especially chronic and lower-frequency signals (

e.g.,

from vessel traffic), contribute to elevated ambient sound levels, thus intensifying masking.

Masking effects of pulsed sounds (even from large arrays of airguns) on marine mammal calls and other natural sounds are expected to be limited, although there are few specific data on this. Because of the intermittent nature and low duty cycle of seismic pulses, animals can emit and receive sounds in the relatively quiet intervals between pulses. However, in exceptional situations, reverberation occurs for much or all of the interval between pulses (

e.g.,

Simard

et al.,

2005; Clark and Gagnon 2006), which could mask calls. Situations with prolonged strong reverberation are infrequent. However, it is common for reverberation to cause some lesser degree of elevation of the background level between airgun pulses (

e.g.,

Gedamke 2011; Guerra

et al.,

2011, 2016; Klinck

et al.,

2012; Guan

et al.,

2015), and this weaker reverberation presumably reduces the detection range of calls and other natural sounds to some degree. Guerra

et al.

(2016)

reported that ambient noise levels between seismic pulses were elevated as a result of reverberation at ranges of 50 km from the seismic source. Based on measurements in deep water of the Southern Ocean, Gedamke (2011) estimated that the slight elevation of background noise levels during intervals between seismic pulses reduced blue and fin whale communication space by as much as 36-51 percent when a seismic survey was operating 450-2,800 km away. Based on preliminary modeling, Wittekind

et al.

(2016) reported that airgun sounds could reduce the communication range of blue and fin whales 2,000 km from the seismic source. Nieukirk

et al.

(2012) and Blackwell

et al.

(2013) noted the potential for masking effects from seismic surveys on large whales.

Some baleen and toothed whales are known to continue calling in the presence of seismic pulses, and their calls usually can be heard between the pulses (

e.g.,

Nieukirk

et al.,

2012; Thode

et al.,

2012; Bröker

et al.,

2013; Sciacca

et al.,

2016). Cerchio

et al.

(2014) suggested that the breeding display of humpback whales off Angola could be disrupted by seismic sounds, as singing activity declined with increasing received levels. In addition, some cetaceans are known to change their calling rates, shift their peak frequencies, or otherwise modify their vocal behavior in response to airgun sounds (

e.g.,

Di Iorio and Clark 2009; Castellote

et al.,

2012; Blackwell

et al.,

2013, 2015). The hearing systems of baleen whales are more sensitive to low-frequency sounds than are the ears of the small odontocetes that have been studied directly (

e.g.,

MacGillivray

et al.,

2014). The sounds important to small odontocetes are predominantly at much higher frequencies than are the dominant components of airgun sounds, thus limiting the potential for masking. In general, masking effects of seismic pulses are expected to be minor, given the normally intermittent nature of seismic pulses.

Vessel Noise

Vessel noise from survey vessels could affect marine animals in the proposed survey areas. Houghton

et al.

(2015) proposed that vessel speed is the most important predictor of received noise levels, and Putland

et al.

(2017) also reported reduced sound levels with decreased vessel speed. However, some energy is also produced at higher frequencies (Hermannsen

et al.,

2014); low levels of high-frequency sound from vessels has been shown to elicit responses in harbor porpoise (Dyndo

et al.,

2015).

Vessel noise, through masking, can reduce the effective communication distance of a marine mammal if the frequency of the sound source is close to that used by the animal, and if the sound is present for a significant fraction of time (

e.g.,

Richardson

et al.,

1995; Clark

et al.,

2009; Jensen

et al.,

2009; Gervaise

et al.,

2012; Hatch

et al.,

2012; Rice

et al.,

2014; Dunlop, 2015; Jones

et al.,

2017; Putland

et al.,

2017). In addition to the frequency and duration of the masking sound, the strength, temporal pattern, and location of the introduced sound also play a role in the extent of the masking (Branstetter

et al.,

2013, 2016; Finneran and Branstetter 2013; Sills

et al.,

2017). Branstetter

et al.

(2013) reported that time-domain metrics are also important in describing and predicting masking.

Baleen whales are thought to be more sensitive to sound at these low frequencies than are toothed whales (

e.g.,

MacGillivray

et al.,

2014), possibly causing localized avoidance of the survey area during seismic operations. Many odontocetes show considerable tolerance of vessel traffic, although they sometimes react at long distances if confined by ice or shallow water, if previously harassed by vessels, or have had little or no recent exposure to vessels (Richardson

et al.,

1995). Pirotta

et al.

(2015) noted that the physical presence of vessels, not just ship noise, disturbed the foraging activity of bottlenose dolphins. There is little data on the behavioral reactions of beaked whales to vessel noise, though they seem to avoid approaching vessels (

e.g.,

Würsig

et al.,

1998) or dive for an extended period when approached by a vessel (

e.g.,

Kasuya, 1986).

In summary, survey vessel sounds would not be at levels expected to cause anything more than possible localized and temporary behavioral changes in marine mammals, and would not be expected to result in significant negative effects on individuals or at the population level. In addition, in all oceans of the world, large vessel traffic is currently so prevalent that it is commonly considered a usual source of ambient sound (NSF-USGS, 2011).

Vessel Strike

Vessel collisions with marine mammals, or vessel strikes, can result in death or serious injury of the animal. Wounds resulting from vessel strike may include massive trauma, hemorrhaging, broken bones, or propeller lacerations (Knowlton and Kraus, 2001). An animal at the surface may be struck directly by a vessel, a surfacing animal may hit the bottom of a vessel, or an animal just below the surface may be cut by a vessel's propeller. Superficial strikes may not kill or result in the death of the animal. These interactions are typically associated with large whales (

e.g.,

fin whales), which are occasionally found draped across the bulbous bow of large commercial vessels upon arrival in port. Although smaller cetaceans are more maneuverable in relation to large vessels than are large whales, they may also be susceptible to strike. The severity of injuries typically depends on the size and speed of the vessel, with the probability of death or serious injury increasing as vessel speed increases (Knowlton and Kraus, 2001; Laist

et al.,

2001; Vanderlaan and Taggart, 2007; Conn and Silber, 2013). Impact forces increase with speed, as does the probability of a strike at a given distance (Silber

et al.,

2010; Gende

et al.,

2011).

Pace and Silber (2005) also found that the probability of death or serious injury increased rapidly with increasing vessel speed. Specifically, the predicted probability of serious injury or death increased from 45 to 75 percent as vessel speed increased from 10 to 14 knots (kn, 26 kilometer per hour (kph)), and exceeded 90 percent at 17 kn (31 kph). Higher speeds during collisions result in greater force of impact, but higher speeds also appear to increase the chance of severe injuries or death through increased likelihood of collision by pulling whales toward the vessel (Clyne, 1999; Knowlton

et al.,

1995). In a separate study, Vanderlaan and Taggart (2007) analyzed the probability of lethal mortality of large whales at a given speed, showing that the greatest rate of change in the probability of a lethal injury to a large whale as a function of vessel speed occurs between 8.6 and 15 kn (28 kph). The chances of a lethal injury decline from approximately 80 percent at 15 kn (28 kph) to approximately 20 percent at 8.6 kn (16 kph). At speeds below 11.8 kn (22 kph), the chances of lethal injury drop below 50 percent, while the probability asymptotically increases toward one hundred percent above 15 kn (28 kph).

Survey vessels will travel at a speed of 5 kn (9 kph) while towing seismic survey gear. At this speed, both the possibility of striking a marine mammal and the possibility of a strike resulting in serious injury or mortality are discountable. At average transit speed, the probability of serious injury or mortality resulting from a strike is less than 50 percent. However, the likelihood of a strike actually happening is again discountable. Vessel strikes, as analyzed in the studies cited above, generally involve commercial shipping,

which is much more common in both space and time than is geophysical survey activity. No such incidents have been reported for geophysical survey vessels.

Although the likelihood of the vessel striking a marine mammal is low, we propose a robust vessel strike avoidance protocol (see Proposed Mitigation), which we believe eliminates any foreseeable risk of vessel strike during transit. We anticipate that vessel collisions involving a seismic data acquisition vessel towing gear, while not impossible, represent unlikely, unpredictable events for which there are no preventive measures. Given the proposed mitigation measures, the relatively slow speed of the vessel towing gear, the presence of bridge crew watching for obstacles at all times (including marine mammals), and the presence of marine mammal observers, the possibility of vessel strike is discountable and, further, were a strike of a large whale to occur, it would be unlikely to result in serious injury or mortality. No incidental take resulting from vessel strike is anticipated, and this potential effect of the specified activity will not be discussed further in the following analysis.

Stranding

—When a living or dead marine mammal swims or floats onto shore and becomes “beached” or incapable of returning to sea, the event is a “stranding” (Geraci

et al.,

1999; Perrin and Geraci, 2002; Geraci and Lounsbury, 2005; NMFS, 2007). The legal definition for a stranding under the MMPA is that a marine mammal is dead and is on a beach or shore of the United States; or in waters under the jurisdiction of the United States (including any navigable waters); or a marine mammal is alive and is on a beach or shore of the United States and is unable to return to the water; on a beach or shore of the United States and, although able to return to the water, is in need of apparent medical attention; or in the waters under the jurisdiction of the United States (including any navigable waters), but is unable to return to its natural habitat under its own power or without assistance.

Marine mammals strand for a variety of reasons, such as infectious agents, biotoxicosis, starvation, fishery interaction, vessel strike, unusual oceanographic or weather events, sound exposure, or combinations of these stressors sustained concurrently or in series. However, the cause or causes of most strandings are unknown (Geraci

et al.,

1976; Eaton, 1979; Odell

et al.,

1980; Best, 1982). Numerous studies suggest that the physiology, behavior, habitat relationships, age, or condition of cetaceans may cause them to strand or might predispose them to strand when exposed to another phenomenon. These suggestions are consistent with the conclusions of numerous other studies that have demonstrated that combinations of dissimilar stressors commonly combine to kill an animal or dramatically reduce its fitness, even though one exposure without the other does not produce the same result (Chroussos, 2000; Creel, 2005; DeVries

et al.,

2003; Fair and Becker, 2000; Foley

et al.,

2001; Moberg, 2000; Relyea, 2005a; 2005b, Romero, 2004; Sih

et al.,

2004).

There is no conclusive evidence that exposure to airgun noise results in behaviorally-mediated forms of injury. Behaviorally-mediated injury (

i.e.,

mass stranding events) has been primarily associated with beaked whales exposed to MFA sonar. MFA sonar and the alerting stimulus used in Nowacek

et al.

(2004) are very different from the noise produced by airguns. One should therefore not expect the same reaction to airgun noise as to these other sources. As explained below, military MFA sonar is very different from airguns, and one should not assume that airguns will cause the same effects as MFA sonar (including strandings).

To understand why military MFA sonar affects beaked whales differently than airguns do, it is important to note the distinction between behavioral sensitivity and susceptibility to auditory injury. To understand the potential for auditory injury in a particular marine mammal species in relation to a given acoustic signal, the frequency range the species is able to hear is critical, as well as the species' auditory sensitivity to frequencies within that range. Current data indicate that not all marine mammal species have equal hearing capabilities across all frequencies and, therefore, species are grouped into hearing groups with generalized hearing ranges assigned on the basis of available data (Southall

et al.,

2007, 2019). Hearing ranges as well as auditory sensitivity/susceptibility to frequencies within those ranges vary across the different groups. For example, in terms of hearing range, the very high-frequency cetaceans (

e.g., Kogia

spp.) have a generalized hearing range of frequencies between 200 Hz and 165 kHz, while high-frequency cetaceans—such as dolphins and beaked whales—have a generalized hearing range between 150 Hz to 160 kHz. Regarding auditory susceptibility within the hearing range, while high-frequency cetaceans and very high-frequency cetaceans have roughly similar hearing ranges, the very high-frequency group is much more susceptible to noise-induced hearing loss during sound exposure,

i.e.,

these species have lower thresholds for these effects than other hearing groups (NMFS, 2018, 2024). Referring to a species as behaviorally sensitive to noise simply means that an animal of that species is more likely to respond to lower received levels of sound than an animal of another species that is considered less behaviorally sensitive. So, while dolphin species and beaked whale species—both in the high-frequency cetacean hearing group—are assumed to generally hear the same sounds equally well and be equally susceptible to noise-induced hearing loss (auditory injury), the best available information indicates that a beaked whale is more likely to behaviorally respond to that sound at a lower received level compared to an animal from other high-frequency cetacean species that are less behaviorally sensitive. This distinction is important because, while beaked whales are more likely to respond behaviorally to sounds than are many other species (even at lower levels), they cannot hear the predominant, lower frequency sounds from seismic airguns as well as sounds that have more energy at frequencies that beaked whales can hear better (such as military MFA sonar).

Military MFA sonar effects beaked whales differently than airguns do because it produces energy at different frequencies than airguns. High-frequency cetacean hearing is generically thought to be best between 8.8 to 110 kHz,

i.e.,

these cutoff values define the range above and below which a species in the group is assumed to have declining auditory sensitivity, until reaching frequencies that cannot be heard (NMFS, 2018, 2024). However, beaked whale hearing is likely best within a higher, narrower range (20-80 kHz, with best sensitivity around 40 kHz), based on a few measurements of hearing in stranded beaked whales (Cook

et al.,

2006; Finneran

et al.,

2009; Pacini

et al.,

2011) and several studies of acoustic signals produced by beaked whales (

e.g.,

Frantzis

et al.,

2002; Johnson

et al.,

2004, 2006; Zimmer

et al.,

2005). While precaution requires that the full range of audibility be considered when assessing risks associated with noise exposure (Southall

et al.,

2007, 2019), animals typically produce sound at frequencies where they hear best. More recently, Southall

et al.

(2019) suggested that certain species in the high-frequency hearing group (beaked whales, sperm whales, and killer whales) are likely more sensitive to lower frequencies

within the group's generalized hearing range than are other species within the group, and state that the data for beaked whales suggest sensitivity to approximately 5 kHz. However, this information is consistent with the general conclusion that beaked whales (and other high-frequency cetaceans) are relatively insensitive to the frequencies where most energy of an airgun signal is found. Military MFA sonar is typically considered to operate in the frequency range of approximately 3-14 kHz (D'Amico

et al.,

2009),

i.e.,

outside the range of likely best hearing for beaked whales but within or close to the lower bounds, whereas most energy in an airgun signal is radiated at much lower frequencies, below 500 Hz (Dragoset, 1990).

It is important to distinguish between energy (loudness, measured in dB) and frequency (pitch, measured in Hz). In considering the potential impacts of mid-frequency components of airgun noise (1-10 kHz, where beaked whales can be expected to hear) on marine mammal hearing, one needs to account for the energy associated with these higher frequencies and determine what energy is truly “significant.” Although there is mid-frequency energy associated with airgun noise (as expected from a broadband source), airgun sound is predominantly below 1 kHz (Breitzke

et al.,

2008; Tashmukhambetov

et al.,

2008; Tolstoy

et al.,

2009). As stated by Richardson

et al.

(1995), “[. . .] most emitted [seismic airgun] energy is at 10-120 Hz, but the pulses contain some energy up to 500-1,000 Hz.” Tolstoy

et al.

(2009) conducted empirical measurements, demonstrating that sound energy levels associated with airguns were at least 20 dB lower at 1 kHz (considered “mid-frequency”) compared to higher energy levels associated with lower frequencies (below 300 Hz) (“all but a small fraction of the total energy being concentrated in the 10-300 Hz range” (Tolstoy

et al.,

2009)), and at higher frequencies (

e.g.,

2.6-4 kHz), power might be less than 10 percent of the peak power at 10 Hz. Energy levels measured by Tolstoy

et al.

(2009) were even lower at frequencies above 1 kHz. In addition, as sound propagates away from the source, it tends to lose higher-frequency components faster than low-frequency components (

i.e.,

low-frequency sounds typically propagate longer distances than high-frequency sounds) (Diebold

et al.,

2010). Although higher-frequency components of airgun signals have been recorded, it is typically in surface-ducting conditions (

e.g.,

DeRuiter

et al.,

2006; Madsen

et al.,

2006) or in shallow water, where there are advantageous propagation conditions for the higher frequency (but low-energy) components of the airgun signal (Hermannsen

et al.,

2015). This should not be of concern because the likely behavioral reactions of beaked whales that can result in acute physical injury would result from noise exposure at depth (because of the potentially greater consequences of severe behavioral reactions). In summary, the frequency content of airgun signals is such that beaked whales will not be able to hear the signals well (compared to MFA sonar), especially at depth where we expect the consequences of noise exposure could be more severe.

Aside from frequency content, there are other significant differences between MFA sonar signals and the sounds produced by airguns that minimize the risk of severe behavioral reactions that could lead to strandings or deaths at sea,

e.g.,

significantly longer signal duration, horizontal sound direction, typical fast and unpredictable source movement. All of these characteristics of MFA sonar tend towards greater potential to cause severe behavioral or physiological reactions in exposed beaked whales that may contribute to stranding. Although both sources are powerful, MFA sonar contains significantly greater energy in the mid-frequency range, where beaked whales hear better. Short-duration, high energy pulses—such as those produced by airguns—have greater potential to cause damage to auditory structures (though this is unlikely for high-frequency cetaceans, as explained later in this document), but it is longer duration signals that have been implicated in the vast majority of beaked whale strandings. Faster, less predictable movements in combination with multiple source vessels are more likely to elicit a severe, potentially anti-predator response. Of additional interest in assessing the divergent characteristics of MFA sonar and airgun signals and their relative potential to cause stranding events or deaths at sea is the similarity between the MFA sonar signals and stereotyped calls of beaked whales' primary predator: the killer whale (Zimmer and Tyack, 2007). Although generic disturbance stimuli—as airgun noise may be considered in this case for beaked whales—may also trigger antipredator responses, stronger responses should generally be expected when perceived risk is greater, as when the stimulus is confused for a known predator (Frid and Dill, 2002). In addition, because the source of the perceived predator (

i.e.,

MFA sonar) will likely be closer to the whales (because attenuation limits the range of detection of mid-frequencies) and moving faster (because it will be on faster-moving vessels), any antipredator response would be more likely to be severe (with greater perceived predation risk, an animal is more likely to disregard the cost of the response; Frid and Dill, 2002). Indeed, when analyzing movements of a beaked whale exposed to playback of killer whale predation calls, Allen

et al.

(2014) found that the whale engaged in a prolonged, directed avoidance response, suggesting a behavioral reaction that could pose a risk factor for stranding. Overall, these significant differences between sound from MFA sonar and the mid-frequency sound component from airguns and the likelihood that MFA sonar signals will be interpreted in error as a predator are critical to understanding the likely risk of behaviorally-mediated injury due to seismic surveys.

The available scientific literature also provides a useful contrast between airgun noise and MFA sonar regarding the likely risk of behaviorally-mediated injury. There is strong evidence for the association of beaked whale stranding events with MFA sonar use, and particularly detailed accounting of several events is available (

e.g.,

a 2000 Bahamas stranding event for which investigators concluded that MFA sonar use was responsible; Evans and England, 2001). D'Amico

et al.

(2009) reviewed 126 beaked whale mass stranding events over the period from 1950 (

i.e.,

from the development of modern MFA sonar systems) through 2004. Of these, there were two events where detailed information was available on both the timing and location of the stranding and the concurrent nearby naval activity, including verification of active MFA sonar usage, with no evidence for an alternative cause of stranding. An additional 10 events were at minimum spatially and temporally coincident with naval activity likely to have included MFA sonar use and, despite incomplete knowledge of timing and location of the stranding or the naval activity in some cases, there was no evidence for an alternative cause of stranding. The U.S. Navy has publicly stated agreement that five such events since 1996 were associated in time and space with MFA sonar use, either by the U.S. Navy alone or in joint training exercises with the North Atlantic Treaty Organization. The U.S. Navy additionally noted that, as of 2017, a 2014 beaked whale stranding event in Crete coincident with naval exercises was under review and had not yet been

determined to be linked to sonar activities (U.S. Navy, 2017). Separately, the International Council for the Exploration of the Sea reported in 2005 that, worldwide, there have been about 50 known strandings, consisting mostly of beaked whales, with a potential causal link to MFA sonar (International Council for the Exploration for the Sea, 2005). In contrast, very few such associations have been made to seismic surveys, despite widespread use of airguns as a geophysical sound source in numerous locations around the world.

A review of possible stranding associations with seismic surveys (Castellote and Llorens, 2016) states that, “[s]peculation concerning possible links between seismic survey noise and cetacean strandings is available for a dozen events but without convincing causal evidence.” The authors' search of available information found 10 events worth further investigation via a ranking system representing a rough metric of the relative level of confidence offered by the data for inferences about the possible role of the seismic survey in a given stranding event. Only three of these events involved beaked whales. Whereas D'Amico

et al.

(2009) used a 1-5 ranking system, in which “1” represented the most robust evidence connecting the event to MFA sonar use, Castellote and Llorens (2016) used a 1-6 ranking system, in which “6” represented the most robust evidence connecting the event to the seismic survey. As described above, D'Amico

et al.

(2009) found that two events were ranked “1” and 10 events were ranked “2” (

i.e.,

12 beaked whale stranding events were found to be associated with MFA sonar use). In contrast, Castellote and Llorens (2016) found that none of the three beaked whale stranding events achieved their highest ranks of 5 or 6. Of the 10 total events, none achieved the highest rank of 6. Two events were ranked as 5: one stranding in Peru involving dolphins and porpoises and a 2008 stranding in Madagascar. This latter ranking can only be broadly associated with the survey itself, as opposed to use of seismic airguns. An investigation of the 2008 Madagascar stranding event, which did not involve beaked whales, concluded that use of a high-frequency mapping system (12-kHz multibeam echosounder) was the most plausible and likely initial behavioral trigger of the event, which was likely exacerbated by several site- and situation-specific secondary factors. The review panel found that seismic airguns were used after the initial strandings and animals entering a lagoon system, that airgun use clearly had no role as an initial trigger, and that there was no evidence that airgun use dissuaded animals from leaving (Southall

et al.,

2013).

However, one of these stranding events, involving two goose-beaked beaked whales, was contemporaneous with and reasonably associated spatially with a 2002 seismic survey in the Gulf of California, as was the case for the 2007 Gulf of Cadiz seismic survey discussed by Castellote and Llorens (also involving two goose-beaked beaked whales). Neither event was considered a “true atypical mass stranding” (according to Frantzis (1998)) as used in the analysis of Castellote and Llorens (2016). While we agree with Castellote and Llorens that this lack of evidence associating seismic surveys and stranding events should not be considered conclusive, it is clear that there is very little evidence that seismic surveys should be considered as posing a significant risk of acute harm to beaked whales or other high-frequency cetaceans. We have considered the potential for the proposed surveys to result in marine mammal stranding and, based on the best available information, do not expect a stranding to occur.

Other Potential Impacts

Here, we briefly address the potential risks due to entanglement and contaminant spills. We are not aware of any records of marine mammal entanglement in towed arrays such as those considered here, and we address measures designed to eliminate the potential for entanglement in gear used by OBN surveys in Proposed Mitigation. The discharge of trash and debris is prohibited (33 CFR 151.51 through 151.77) unless it is passed through a machine that breaks up solids such that they can pass through a 25-mm mesh screen. All other trash and debris must be returned to shore for proper disposal with municipal and solid waste. Some personal items may be accidentally lost overboard. However, U.S. Coast Guard and Environmental Protection Act regulations require operators to become proactive in avoiding accidental loss of solid waste items by developing waste management plans, posting informational placards, manifesting trash sent to shore, and using special precautions such as covering outside trash bins to prevent accidental loss of solid waste. Entanglement risks are essentially eliminated by the proposed requirements, and entanglement risks are not discussed further in this document.

Marine mammals could be affected by accidentally spilled diesel fuel from a vessel associated with proposed survey activities. Quantities of diesel fuel on the sea surface may affect marine mammals through various pathways: surface contact of the fuel with skin and other mucous membranes, inhalation of concentrated petroleum vapors, or ingestion of the fuel (direct ingestion or by the ingestion of contaminated prey) (

e.g.,

Geraci and St. Aubin, 1980, 1985, 1990). However, the likelihood of a fuel spill during any particular geophysical survey is considered to be remote, and the potential for impacts to marine mammals would depend greatly on the size and location of a spill and meteorological conditions at the time of the spill. Spilled fuel would rapidly spread to a layer of varying thickness and break up into narrow bands or windrows parallel to the wind direction. The rate at which the fuel spreads would be determined by the prevailing conditions such as temperature, water currents, tidal streams, and wind speeds. Lighter, volatile components of the fuel would evaporate to the atmosphere almost completely in a few days. Evaporation rate may increase as the fuel spreads because of the increased surface area of the slick. Rougher seas, high wind speeds, and high temperatures also tend to increase the rate of evaporation and the proportion of fuel lost by this process (Scholz

et al.,

1999). We do not anticipate potentially meaningful effects to marine mammals as a result of any contaminant spill resulting from the proposed survey activities, and contaminant spills resulting from the specified activity are not discussed further in this document.

Anticipated Effects on Marine Mammal Habitat

Physical Disturbance

—Sources of seafloor disturbance related to geophysical surveys that may impact marine mammal habitat include placement of anchors, nodes, cables, sensors, or other equipment on or in the seafloor for various activities. Equipment deployed on the seafloor has the potential to cause direct physical damage and could affect bottom-associated fish resources.

Placement of equipment, such as nodes, on the seafloor could damage areas of hard bottom where direct contact with the seafloor occurs and could crush epifauna (organisms that live on the seafloor or surface of other organisms). Damage to unknown or unseen hard bottom could occur, but because of the small area covered by most bottom-founded equipment, the patchy distribution of hard bottom habitat, and typical BOEM permit

conditions related to avoidance of such areas, contact with unknown hard bottom is expected to be rare and impacts minor. Seafloor disturbance in areas of soft bottom can cause loss of small patches of epifauna and infauna due to burial or crushing, and bottom-feeding fishes could be temporarily displaced from feeding areas. Overall, any effects of physical damage to habitat are expected to be minor and temporary.

Effects to Prey

—Marine mammal prey varies by species, season, and location and, for some, is not well documented. Fish react to sounds which are especially strong and/or intermittent low-frequency sounds, and behavioral responses such as flight or avoidance are the most likely effects. However, the reaction of fish to airguns depends on the physiological state of the fish, past exposures, motivation (

e.g.,

feeding, spawning, migration), and other environmental factors. Several studies have demonstrated that airgun sounds might affect the distribution and behavior of some fishes, potentially impacting foraging opportunities or increasing energetic costs (

e.g.,

Fewtrell and McCauley, 2012; Pearson

et al.,

1992; Skalski

et al.,

1992; Santulli

et al.,

1999; Paxton

et al.,

2017), though the bulk of studies indicate no or slight reaction to noise (

e.g.,

Miller and Cripps, 2013; Dalen and Knutsen, 1987; Peña

et al.,

2013; Chapman and Hawkins, 1969; Wardle

et al.,

2001; Jorgenson and Gyselman, 2009; Blaxter

et al.,

1981; Cott

et al.,

2012; Boeger

et al.,

2006), and that, most commonly, while there are likely to be impacts to fish as a result of noise from nearby airguns, such effects will be temporary. For example, investigators reported significant, short-term declines in commercial fishing catch rate of gadid fishes during and for up to 5 days after seismic survey operations, but the catch rate subsequently returned to normal (Engås

et al.,

1996; Engås and Lokkeborg, 2002). Other studies have reported similar findings (Hassel

et al.,

2004).

Skalski

et al.,

(1992) also found a reduction in catch rates—for rockfish (

Sebastes

spp.) in response to controlled airgun exposure—but suggested that the mechanism underlying the decline was not dispersal but rather decreased responsiveness to baited hooks associated with an alarm behavioral response. A companion study showed that alarm and startle responses were not sustained following the removal of the sound source (Pearson

et al.,

1992). Therefore, Skalski

et al.

(1992) suggested that the effects on fish abundance may be transitory, primarily occurring during the sound exposure itself. In some cases, effects on catch rates are variable within a study, which may be more broadly representative of temporary displacement of fish in response to airgun noise (

i.e.,

catch rates may increase in some locations and decrease in others) than any long-term damage to the fish themselves (Streever

et al.,

2016).

SPLs of sufficient strength have been known to cause injury to fish and fish mortality and, in some studies, fish auditory systems have been damaged by airgun noise (McCauley

et al.,

2003; Popper

et al.,

2005; Song

et al.,

2008). However, in most fish species, hair cells in the ear continuously regenerate and loss of auditory function likely is restored when damaged cells are replaced with new cells. Halvorsen

et al.

(2012) showed that a TTS of 4-6 dB was recoverable within 24 hours for one species. Impacts would be most severe when the individual fish is close to the source and when the duration of exposure is long; both of which are conditions unlikely to occur for this survey that is necessarily transient in any given location and likely result in brief, infrequent noise exposure to prey species in any given area. For seismic surveys, the sound source is constantly moving, and most fish would likely avoid the sound source prior to receiving sound of sufficient intensity to cause physiological or anatomical damage. In addition, ramp-up may allow certain fish species the opportunity to move further away from the sound source.

A comprehensive review (Carroll

et al.,

2017) found that results are mixed as to the effects of airgun noise on the prey of marine mammals. While some studies suggest a change in prey distribution and/or a reduction in prey abundance following the use of seismic airguns, others suggest no effects or even positive effects in prey abundance. As one specific example, Paxton

et al.

(2017), which describes findings related to the effects of a 2014 seismic survey on a reef off of North Carolina, showed a 78 percent decrease in observed nighttime abundance for certain species. It is important to note that the evening hours during which the decline in fish habitat use was recorded (via video recording) occurred on the same day that the seismic survey passed, and no subsequent data is presented to support an inference that the response was long-lasting. Additionally, given that the finding is based on video images, the lack of recorded fish presence does not support a conclusion that the fish actually moved away from the site or suffered any serious impairment. In summary, this particular study corroborates prior studies indicating that a startle response or short-term displacement should be expected.

Available data suggest that cephalopods are capable of sensing the particle motion of sounds and detect low frequencies up to 1-1.5 kHz, depending on the species, and so are likely to detect airgun noise (Kaifu

et al.,

2008; Hu

et al.,

2009; Mooney

et al.,

2010; Samson

et al.,

2014). Auditory injuries (lesions occurring on the statocyst sensory hair cells) have been reported upon controlled exposure to low-frequency sounds, suggesting that cephalopods are particularly sensitive to low-frequency sound (André

et al.,

2011; Solé

et al.,

2013). Behavioral responses, such as inking and jetting, have also been reported upon exposure to low-frequency sound (McCauley

et al.,

2000; Samson

et al.,

2014). Similar to fish, however, the transient nature of the survey leads to an expectation that effects will be largely limited to behavioral reactions and would

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