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

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URL: https://www.frixlaw.com/law-library/documents/fr%3A2026-03691

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** February 24, 2026
- **Citation:** 91 FR 9014

## Text

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 l

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2026-03691. Public record. Not legal advice.