Taking and Importing Marine Mammals; Taking Marine Mammals Incidental to the U.S. Navy Training Activities in the Gulf of Alaska Study Area

DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

50 CFR Part 218

[Docket No. 220726-0163]

RIN 0648-BK46

Taking and Importing Marine Mammals; Taking Marine Mammals Incidental to the U.S. Navy Training Activities in the Gulf of Alaska Study Area

AGENCY:

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

ACTION:

Proposed rule; request for comments and information.

SUMMARY:

NMFS has received a request from the U.S. Navy (Navy) to take marine mammals incidental to training activities conducted in the Gulf of Alaska (GOA) Study Area (hereafter referred to as the GOA Study Area). Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments on its proposal to issue regulations and a subsequent Letter of Authorization (LOA) to the Navy to incidentally take marine mammals during the specified activities. NMFS will consider public comments prior to issuing any final rule and making final decisions on the issuance of the requested LOA. Agency responses to public comments will be provided in the notice of the final decision. The Navy's activities qualify as military readiness activities pursuant to the MMPA, as amended by the National Defense Authorization Act for Fiscal Year 2004 (2004 NDAA).

DATES:

Comments and information must be received no later than September 26, 2022.

ADDRESSES:

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

https://www.regulations.gov

and enter NOAA-NMFS-2022-0060 in the Search box. Click on the “Comment” icon, complete the required fields, and enter or attach your comments.

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

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). Attachments to electronic comments will be accepted in Microsoft Word, Excel, or Adobe PDF file formats only.

A copy of the Navy's application and other supporting documents and documents cited herein may be obtained online at:

https://www.fisheries.noaa.gov/action/incidental-take-authorization-us-navy-training-activities-gulf-alaska-temporary-maritime-0.

In case of problems accessing these documents, please use the contact listed here (see

FOR FURTHER INFORMATION CONTACT

).

FOR FURTHER INFORMATION CONTACT:

Leah Davis, Office of Protected Resources, NMFS, (301) 427-8401.

SUPPLEMENTARY INFORMATION:

Purpose of Regulatory Action

These proposed regulations, issued under the authority of the MMPA (16 U.S.C. 1361

et seq.

), would provide the framework for authorizing the take of marine mammals incidental to the Navy's training activities (which qualify as military readiness activities), including the use of sonar and other transducers, and in-air detonations at or near the surface (within 10 m above the water surface) in the GOA Study Area. The GOA Study Area is comprised of three areas: the Temporary Maritime Activities Area (TMAA), a warning area, and the Western Maneuver Area (WMA) (see Figure 1). The TMAA and WMA are temporary areas established within the GOA for ships, submarines, and aircraft to conduct training activities. The warning area overlaps and extends slightly beyond the northern corner of the TMAA. The WMA is located south and west of the TMAA and provides additional surface, sub-surface, and airspace in which to maneuver in support of activities occurring within the TMAA. The use of sonar and other transducers, and explosives would not occur within the WMA.

NMFS received an application from the Navy requesting 7-year regulations and an authorization to incidentally take individuals of multiple species of marine mammals (“Navy's rulemaking/LOA application” or “Navy's application”). Take is anticipated to occur by Level A harassment and Level B harassment incidental to the Navy's training activities. No lethal take is anticipated or proposed for authorization.

Background

The MMPA prohibits the “take” of marine mammals, with certain exceptions. Sections 101(a)(5)(A) and (D) of the MMPA 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 proposed or, if the taking is limited to harassment, the public is provided with notice of the proposed incidental take authorization and provided the opportunity to review and submit comments.

An authorization for incidental takings shall be granted if NMFS finds that the taking will have a negligible impact on the species or stocks and will not have an unmitigable adverse impact on the availability of the species or stocks for taking for subsistence uses (where relevant). Further, NMFS must prescribe the permissible methods of taking and other means of effecting the least practicable adverse impact on the affected species or stocks and their habitat, paying particular attention to rookeries, mating grounds, and areas of similar significance, and on the availability of such species or stocks for taking for certain subsistence uses (referred to in this rule as “mitigation measures”); and requirements pertaining to the monitoring and reporting of such takings. The MMPA defines “take” to mean to harass, hunt, capture, or kill, or attempt to harass, hunt, capture, or kill any marine mammal. The Preliminary Analysis and Negligible Impact Determination section below discusses the definition of “negligible impact.”

The NDAA for Fiscal Year 2004 (2004 NDAA) (Pub. L. 108-136) amended section 101(a)(5) of the MMPA to remove the “small numbers” and “specified geographical region” provisions indicated above and amended the definition of “harassment” as applied to a “military readiness activity.” The definition of harassment for military readiness activities (Section 3(18)(B) of the MMPA) is (i) Any act that injures or has the significant potential to injure a marine mammal or marine mammal stock in the wild (Level A Harassment); or (ii) Any act that disturbs or is likely to disturb a marine mammal or marine mammal stock in the wild by causing disruption of natural behavioral patterns, including, but not limited to, migration, surfacing, nursing, breeding, feeding, or sheltering, to a

point where such behavioral patterns are abandoned or significantly altered (Level B harassment). In addition, the 2004 NDAA amended the MMPA as it relates to military readiness activities such that the least practicable adverse impact analysis shall include consideration of personnel safety, practicality of implementation, and impact on the effectiveness of the military readiness activity.

More recently, Section 316 of the NDAA for Fiscal Year 2019 (2019 NDAA) (Pub. L. 115-232), signed on August 13, 2018, amended the MMPA to allow incidental take rules for military readiness activities under section 101(a)(5)(A) to be issued for up to 7 years. Prior to this amendment, all incidental take rules under section 101(a)(5)(A) were limited to 5 years.

Summary and Background of Request

On October 9, 2020, NMFS received an adequate and complete application from the Navy requesting authorization for take of marine mammals, by Level A harassment and Level B harassment, incidental to training from the use of active sonar and other transducers and explosives (in-air, occurring at or above the water surface) in the TMAA over a 7-year period beginning when the current authorization expires. On March 12, 2021, the Navy submitted an updated application that provided revisions to the Northern fur seal take estimate and incorporated additional best available science. In August 2021, the Navy communicated to NMFS that it was considering an expansion of the GOA Study Area and an expansion of the Portlock Bank Mitigation Area proposed in its previous applications. On February 2, 2022, the Navy submitted a second updated application that described the addition of the WMA to the GOA Study Area (which previously just consisted of the TMAA) and the replacement of the Portlock Bank Mitigation Area with the Continental Shelf and Slope Mitigation Area. The Navy is not planning to conduct any testing activities.

On January 8, 2021 (86 FR 1483), we published a notice of receipt (NOR) of application in the

Federal Register

, requesting comments and information related to the Navy's request for 30 days. We received one comment on the NOR that was non-substantive in nature.

The following types of training, which are classified as military readiness activities pursuant to the MMPA, as amended by the 2004 NDAA, would be covered under the regulations and LOA (if issued): surface warfare (detonations at or above the water surface) and anti-submarine warfare (sonar and other transducers). The Navy is also conducting Air Warfare, Electronic Warfare, Naval Special Warfare, Strike Warfare, and Support Operations, but these activities do not involve sonar and other transducers, detonations at or above the water surface, or any other stressors that could result in the take of marine mammals. (See the 2020 GOA Draft SEIS/OEIS for more detail on those activities). The activities would not include in-water explosives, pile driving/removal, or use of air guns.

This would be the third time NMFS has promulgated incidental take regulations pursuant to the MMPA relating to similar military readiness activities in the GOA, following those effective beginning May 4, 2011 (76 FR 25479; May 4, 2011) and April 26, 2017 (82 FR 19530; April 27, 2017).

The Navy's mission is to organize, train, equip, and maintain combat-ready naval forces capable of winning wars, deterring aggression, and maintaining freedom of the seas. This mission is mandated by Federal law (10 U.S.C. 8062), which requires the readiness of the naval forces of the United States. The Navy executes this responsibility by establishing and executing training programs, including at-sea training and exercises, and ensuring naval forces have access to the ranges, operating areas (OPAREA), and airspace needed to develop and maintain skills for conducting naval activities.

The Navy has conducted training activities in the TMAA portion of the GOA Study Area since the 1990s. Since the 1990s, the Department of Defense has conducted a major joint training exercise in Alaska and off the Alaskan coast that involves the Departments of the Navy, Army, Air Force, and Coast Guard participants reporting to a unified or joint commander who coordinates the activities. These activities are planned to demonstrate and evaluate the ability of the services to engage in a conflict and successfully carry out plans in response to a threat to national security. The Navy's planned activities for the period of this proposed rule would be a continuation of the types and level of training activities that have been ongoing for more than a decade. While the specified activities have not changed, there are changes in the platforms and systems used in those activities, as well as changes in the bins (source classifications) used to analyze the activities. (For example, two new sonar bins were added (MF12 and ASW1) and another bin was eliminated (HF6). This was due to changes in platforms and systems.) Further, the Navy expanded the GOA Study Area to include the WMA, though the vast majority of the training activities would still occur only in the TMAA.

The Navy's rulemaking/LOA application reflects the most up-to-date compilation of training activities deemed necessary by senior Navy leadership to accomplish military readiness requirements. The types and numbers of activities included in the proposed rule account for fluctuations in training in order to meet evolving or emergent military readiness requirements. These proposed regulations would become effective in December of 2022 and would cover training activities that would occur for a 7-year period following the expiration of the current MMPA authorization for the GOA, which expired on April 26, 2022.

Description of the Specified Activity

The Navy requests authorization to take marine mammals incidental to conducting training activities. The Navy has determined that acoustic and explosives stressors are most likely to result in impacts on marine mammals that could rise to the level of harassment, and NMFS concurs with this determination. Detailed descriptions of these activities are provided in Chapter 2 of the 2020 GOA Draft Supplemental Environmental Impact Statement (SEIS)/Overseas EIS (OEIS) (2020 GOA DSEIS/OEIS) (

https://www.goaeis.com/

) and in the Navy's rulemaking/LOA application (

https://www.fisheries.noaa.gov/action/incidental-take-authorization-us-navy-training-activities-gulf-alaska-temporary-maritime-0

) and are summarized here.

Dates and Duration

Training activities would be conducted intermittently in the GOA Study Area over a maximum time period of up to 21 consecutive days annually from April to October to support a major joint training exercise in Alaska and off the Alaskan coast that involves the Departments of the Navy, Army, Air Force, and Coast Guard. The participants report to a unified or joint commander who coordinates the activities planned to demonstrate and evaluate the ability of the services to engage in a conflict and carry out plans in response to a threat to national security. The specified activities would occur over a maximum time period of up to 21 consecutive days each year during the 7-year period of validity of the regulations. The proposed number of training activities are described in the Detailed Description of Proposed Activities section (Table 3) of this proposed rule.

Geographical Region

The GOA Study Area (see Figure 1 below and Figure ES-1 of the 2022 Supplement to the 2020 GOA DSEIS/OEIS) is entirely at sea and is comprised of the TMAA and a warning area in the Gulf of Alaska, and the WMA. The term “at-sea” refers to training activities in the Study Area (both the TMAA and WMA) that occur (1) on the ocean surface, (2) beneath the ocean surface, and (3) in the air above the ocean surface. Navy training activities occurring on or over the land outside the GOA Study Area are not included in this proposed rule, and are covered under separate environmental documentation prepared by the U.S. Air Force and the U.S. Army. As depicted in Figure 1 of this proposed rule, the TMAA is a polygon roughly resembling a rectangle oriented from northwest to southeast, approximately 300 nmi (556 km) in length by 150 nmi (278 km) in width, located south of Montague Island and east of Kodiak Island. The GOA Study Area boundary was intentionally designed to avoid ESA-designated Steller sea lion critical habitat. The WMA is located south and west of the TMAA, and provides an additional 185,806 nmi

2

of surface, sub-surface, and airspace training to support activities occurring within the TMAA (Figure 1). The boundary of the WMA follows the bottom of the slope at the 4,000 m contour line, and was configured to avoid overlap and impacts to ESA-designated critical habitat, biologically important areas (BIAs), migration routes, and primary fishing grounds. The WMA provides additional airspace and sea space for aircraft and vessels to maneuver during training activities for increased training complexity. The TMAA and WMA are temporary areas established within the GOA for ships, submarines, and aircraft to conduct training activities.

Additional detail can be found in Chapter 2 of the Navy's rulemaking/LOA application.

BILLING CODE 3510-22-P

EP11AU22.000

BILLING CODE 3510-22-C

Primary Mission Areas

The Navy categorizes many of its training activities into functional warfare areas called primary mission areas. The Navy's planned activities for the GOA Study Area generally fall into the following six primary mission areas: Air Warfare; Surface Warfare; Anti-Submarine Warfare; Electronic Warfare; Naval Special Warfare; and Strike Warfare. Most activities conducted in the GOA are categorized under one of these primary mission areas; activities that do not fall within one of these areas are listed as “support operations” or “other training activities.” Each warfare community (aviation, surface, and subsurface) may train in some or all of these primary mission areas. A description of the sonar, munitions, targets, systems, and other materials used during training activities within these primary mission areas is provided in Appendix A (

Navy Activities Descriptions

) of the 2020 GOA DSEIS/OEIS and section ES.2.2 (

Proposed Activities in the Western Maneuver Area

) of the 2022 Supplement to the 2020 GOA DSEIS/OEIS.

The Navy describes and analyzes the effects of its training activities within the 2020 GOA DSEIS/OEIS and 2022 Supplement to the 2020 GOA DSEIS/OEIS. In its assessment, the Navy concluded that of the activities to be conducted within the GOA Study Area, sonar use and in-air explosives occurring at or above the water surface were the stressors resulting in impacts on marine mammals that could rise to the level of harassment as defined under the MMPA. (The Navy is not proposing to conduct any activities that use in-water or underwater explosives.) Further, these activities are limited to the TMAA. No activities involving sonar use or explosives would occur in the WMA or the portion of the warning area that extends beyond the TMAA. Therefore, the Navy's rulemaking/LOA application provides the Navy's assessment of potential effects from sonar use and explosives occurring at or above the water surface in terms of the various warfare mission areas they are associated with. Those mission areas include the following:

• surface warfare (in-air detonations at or above the water surface);

1

and

1

Defined herein as being within 10 meters of the ocean surface.

• anti-submarine warfare (sonar and other transducers).

The Navy's activities in Air Warfare, Electronic Warfare, Naval Special Warfare, Strike Warfare, Support Operations, and Other Training Activities do not involve sonar and other transducers, detonations at or near the surface, or any other stressors that could result in harassment, serious injury, or mortality of marine mammals. Therefore, the activities in these warfare areas are not discussed further in this proposed rule, but are analyzed fully in the 2020 GOA DSEIS/OEIS and 2022 Supplement to the 2020 GOA DSEIS/OEIS. The specific acoustic sources analyzed in this proposed rule are contained in the 2020 GOA DSEIS/OEIS and are presented in the following sections based on the primary mission areas.

Surface Warfare

The mission of surface warfare (named anti-surface warfare in the 2011 GOA Final Environmental Impact Statement (FEIS)/Overseas Environmental Impact Statement (OEIS) and 2016 GOA Final Supplemental Environmental Impact Statement (FSEIS)/OEIS, but since changed by the Navy to “Surface Warfare”) is to obtain control of sea space from which naval forces may operate, which entails offensive action against surface targets while also defending against enemy forces. In surface warfare, aircraft use guns, air-launched cruise missiles, or other precision-guided munitions; ships employ naval guns and surface-to-surface missiles; and submarines attack surface ships using anti-ship cruise missiles.

Anti-Submarine Warfare

The mission of anti-submarine warfare is to locate, neutralize, and defeat hostile submarine forces that threaten Navy surface forces. Anti-submarine warfare can involve various assets such as aircraft, ships, and submarines which all search for hostile submarines. These forces operate together or independently to gain early warning and detection, and to localize, track, target, and attack submarine threats.

Anti-submarine warfare training addresses basic skills such as detecting and classifying submarines, as well as evaluating sounds to distinguish between enemy submarines and friendly submarines, ships, and marine life. These integrated anti-submarine warfare training exercises are conducted in coordinated, at-sea training events involving submarines, ships, and aircraft.

Overview of the Major Training Exercise Within the GOA Study Area

The training activities in the GOA Study Area are considered to be a major training exercise (MTE). A MTE, for purposes of this rulemaking, is comprised of several unit-level activities conducted by several units operating together, commanded and controlled by a single Commander, and potentially generating more than 100 hours of active sonar. These exercises typically employ an exercise scenario developed to train and evaluate the exercise participants in tactical and operational tasks. In a MTE, most of the activities being directed and coordinated by the Commander in charge of the exercise are identical in nature to the activities conducted during individual, crew, and smaller unit-level training events. In a MTE, however, these disparate training tasks are conducted in concert, rather than in isolation. At most, only one MTE would occur in the GOA Study Area per year (over a maximum of 21 days).

Description of Stressors

The Navy uses a variety of sensors, platforms, weapons, and other devices, including ones used to ensure the safety of Sailors and Marines, to meet its mission. Training with these systems may introduce sound and energy into the environment. The proposed training activities were evaluated to identify specific components that could act as stressors by having direct or indirect impacts on the environment. This analysis included identification of the spatial variation of the identified stressors. The following subsections describe the acoustic and explosive stressors for marine mammals and their habitat (including prey species) within the GOA Study Area. Each description contains a list of activities that may generate the stressor. Stressor/resource interactions that were determined to have de minimis or no impacts (

e.g.,

vessel noise, aircraft noise, weapons noise, and high-altitude (greater than 10 m above the water surface) explosions) were not carried forward for analysis in the Navy's rulemaking/LOA application. The Navy fully considered the possibility of vessel strike, conducted an analysis, and determined that requesting take of marine mammals by vessel strike was not warranted. Although the Navy did not request take for vessel strike, NMFS also fully analyzed the potential for vessel strike of marine mammals as part of this rulemaking. Therefore, this stressor is discussed in detail below. No Sinking Exercise (SINKEX) events are proposed in the GOA Study Area for this rulemaking, nor is establishment and use of a Portable Undersea Tracking Range (PUTR) proposed. NMFS reviewed the Navy's analysis and conclusions on de minimis and no-impact sources, included in Section 3.8.3 (

Environmental Consequences

) of

the 2020 GOA DSEIS/OEIS and finds them complete and supportable.

Acoustic Stressors

Acoustic stressors include acoustic signals emitted into the water for a specific purpose, such as sonar, other transducers (devices that convert energy from one form to another—in this case, into sound waves), incidental sources of broadband sound produced as a byproduct of vessel movement, aircraft transits, and use of weapons or other deployed objects. Explosives also produce broadband sound but are characterized separately from other acoustic sources due to their unique hazardous characteristics. Characteristics of each of these sound sources are described in the following sections.

In order to better organize and facilitate the analysis of approximately 300 sources of underwater sound used by the Navy, including sonar and other transducers and explosives, a series of source classifications, or source bins, were developed. The source classification bins do not include the broadband noise produced incidental to vessel movement, aircraft transits, and weapons firing. Noise produced from vessel movement, aircraft transits, and use of weapons or other deployed objects is not carried forward because those activities were found to have de minimis or no impacts, as described above.

The use of source classification bins provides the following benefits:

• Provides the ability for new sensors or munitions to be covered under existing authorizations, as long as those sources fall within the parameters of a “bin;”

• Improves efficiency of source utilization data collection and reporting requirements anticipated under the MMPA authorizations;

• Ensures a precautionary approach to all impact estimates, as all sources within a given class are modeled as the most impactful source (highest source level, longest duty cycle, or largest net explosive weight) within that bin;

• Allows analyses to be conducted in a more efficient manner, without any compromise of analytical results; and

• Provides a framework to support the reallocation of source usage (hours/explosives) between different source bins, as long as the total numbers of takes remain within the overall analyzed and authorized limits. This flexibility is required to support evolving Navy training and testing requirements, which are linked to real world events.

Sonar and Other Transducers

Active sonar and other transducers emit non-impulsive sound waves into the water to detect objects, navigate safely, and communicate. Passive sonars differ from active sound sources in that they do not emit acoustic signals; rather, they only receive acoustic information about the environment, or listen. In this proposed rule, the terms sonar and other transducers will be used to indicate active sound sources unless otherwise specified.

The Navy employs a variety of sonars and other transducers to obtain and transmit information about the undersea environment. Some examples are mid-frequency hull-mounted sonars used to find and track enemy submarines; high-frequency small object detection sonars used to detect mines; high-frequency underwater modems used to transfer data over short ranges; and extremely high-frequency (greater than 200 kilohertz (kHz)) doppler sonars used for navigation, like those used on commercial and private vessels. The characteristics of these sonars and other transducers, such as source level, beam width, directivity, and frequency, depend on the purpose of the source. Higher frequencies can carry more information or provide more information about objects off which they reflect, but attenuate more rapidly. Lower frequencies attenuate less rapidly, so they may detect objects over a longer distance, but with less detail.

Propagation of sound produced underwater is highly dependent on environmental characteristics such as bathymetry, bottom type, water depth, temperature, and salinity. The sound received at a particular location will be different than near the source due to the interaction of many factors, including propagation loss; how the sound is reflected, refracted, or scattered; the potential for reverberation; and interference due to multi-path propagation. In addition, absorption greatly affects the distance over which higher-frequency sounds propagate. The effects of these factors are explained in Appendix B (

Acoustic and Explosive Concepts

) of the 2020 GOA DSEIS/OEIS. Because of the complexity of analyzing sound propagation in the ocean environment, the Navy relies on acoustic models in its environmental analyses that consider sound source characteristics and varying ocean conditions across the TMAA. As noted above, the Navy does not propose to use sonar and other transducers within the WMA.

The sound sources and platforms typically used in naval activities analyzed in the Navy's rulemaking/LOA application are described in Appendix A (

Navy Activities Descriptions

) of the 2020 GOA DSEIS/OEIS. Sonars and other transducers used to obtain and transmit information underwater during Navy training activities generally fall into several categories of use described below.

Anti-Submarine Warfare

Sonar used during anti-submarine warfare would impart the greatest amount of acoustic energy of any category of sonar and other transducers analyzed in this proposed rule. Types of sonars used to detect potential enemy vessels include hull-mounted, towed, line array, sonobuoy, and helicopter dipping sonars. In addition, acoustic targets and decoys (countermeasures) may be deployed to emulate the sound signatures of vessels or repeat received signals.

Most anti-submarine warfare sonars are mid-frequency (1-10 kHz) because mid-frequency sound balances sufficient resolution to identify targets with distance over which threats can be identified. However, some sources may use higher or lower frequencies. Duty cycles can vary widely, from rarely used to continuously active. For example, anti-submarine warfare sonars can be wide angle in a search mode or highly directional in a track mode.

Most anti-submarine warfare activities involving submarines or submarine targets would occur in waters greater than 600 feet (ft; 183 m) deep due to safety concerns about running aground at shallower depths.

Navigation and Safety

Similar to commercial and private vessels, Navy vessels employ navigational acoustic devices, including speed logs, Doppler sonars for ship positioning, and fathometers. These may be in use at any time for safe vessel operation. These sources are typically highly directional to obtain specific navigational data.

Communication

Sound sources used to transmit data (such as underwater modems), provide location (pingers), or send a single brief release signal to bottom-mounted devices (acoustic release) may be used throughout the TMAA. These sources typically have low duty cycles and are usually only used when it is desirable to send a detectable acoustic message.

Classification of Sonar and Other Transducers

Sonars and other transducers are grouped into classes that share an

attribute, such as frequency range or purpose. As detailed below, classes are further sorted by bins based on the frequency or bandwidth; source level; and, when warranted, the application for which the source would be used. Unless stated otherwise, a reference distance of 1 meter (m) is used for sonar and other transducers.

• Frequency of the non-impulsive acoustic source:

○ Low-frequency sources operate below 1 kHz;

○ Mid-frequency sources operate at and above 1 kHz, up to and including 10 kHz;

○ High-frequency sources operate above 10 kHz, up to and including 100 kHz; and

○ Very-high-frequency sources operate above 100 kHz but below 200 kHz.

• Sound pressure level:

○ Greater than 160 decibels (dB) referenced to 1 micropascal (re: 1 µPa), but less than 180 dB re: 1 µPa;

○ Equal to 180 dB re: 1 µPa and up to and including 200 dB re: 1 µPa; and

○ Greater than 200 dB re: 1 µPa.

• Application for which the source would be used:

○ Sources with similar functions that have similar characteristics, such as pulse length (duration of each pulse), beam pattern, and duty cycle.

The bins used for classifying active sonars and transducers that are quantitatively analyzed in the TMAA are shown in Table 1. While general parameters or source characteristics are shown in the table, actual source parameters are classified.

Table 1—Sonar and Other Transducers Quantitatively Analyzed in the TMAA

For annual training activities

Source class category

Bin

Description

Units

Annual

7-Year total

Mid-Frequency (MF) Tactical and non-tactical sources that produce signals from 1 to 10 kHz

MF1

Hull-mounted surface ship sonars (

e.g.,

AN/SQS-53C and AN/SQS-60)

H

271

1,897

MF3

Hull-mounted submarine sonars (

e.g.,

AN/BQQ-10)

H

25

175

MF4

Helicopter-deployed dipping sonars (

e.g.,

AN/AQS-22)

H

27

189

MF5

Active acoustic sonobuoys (

e.g.,

DICASS)

I

126

882

MF6

Active underwater sound signal devices (

e.g.,

MK 84)

I

14

98

MF11

Hull-mounted surface ship sonars with an active duty cycle greater than 80%

H

42

294

MF12

Towed array surface ship sonars with an active duty cycle greater than 80%

H

14

98

High-Frequency (HF) Tactical and non-tactical sources that produce signals greater than 10 kHz but less than 100 kHz

HF1

Hull-mounted submarine sonars (

e.g.,

AN/BQQ-10)

H

12

84

Anti-Submarine Warfare (ASW) Tactical sources used during ASW training activities

ASW1

MF systems operating above 200 dB

H

14

98

ASW2

MF Multistatic Active Coherent sonobuoy (

e.g.,

AN/SSQ-125)

H

42

294

ASW3

MF towed active acoustic countermeasure systems

(e.g.,

AN/SLQ-25)

H

273

1,911

ASW4

MF expendable active acoustic device countermeasures (

e.g.,

MK3)

I

7

49

Notes:

H = hours, I = count (

e.g.,

number of individual pings or individual sonobuoys), DICASS = Directional Command Activated Sonobuoy System.

Explosive Stressors

The near-instantaneous rise from ambient to an extremely high peak pressure is what makes an explosive shock wave potentially damaging. Farther from an explosive, the peak pressures decay and the explosive waves propagate as an impulsive, broadband sound. Several parameters influence the effect of an explosive: the weight of the explosive in the warhead, the type of explosive material, the boundaries and characteristics of the propagation medium, and the detonation depth in water. The net explosive weight, which is the explosive power of a charge expressed as the equivalent weight of trinitrotoluene (TNT), accounts for the first two parameters. The effects of these factors are explained in Appendix B (

Acoustic and Explosive Concepts

) of the 2020 GOA DSEIS/OEIS.

Explosive Use

Explosive detonations during training activities are from the use of explosive bombs, and naval gun shells; however, no in-water explosive detonations are included as part of the training activities. For purposes of the analysis in this proposed rule, detonations occurring in air at a height of 33 ft (10 m) or less above the water surface, and detonations occurring directly on the water surface, were modeled to detonate at a depth of 0.3 ft (0.1 m) below the water surface since there is currently no other identified methodology for modeling potential effects to marine

mammals that are underwater as a result of detonations occurring in-air at or above the surface of the ocean (within 10 m above the surface). This conservative approach over-estimates the potential underwater impacts due to low-altitude and surface explosives by assuming that all explosive energy is released and remains under the water surface.

Explosive stressors resulting from the detonation of some munitions, such as missiles and gun rounds used in air-air and surface-air scenarios, occur at high altitude. The resulting sound energy from those detonations in air would not impact marine mammals. The explosive energy released by detonations in air has been well studied, and basic methods are available to estimate the explosive energy exposure with distance from the detonation (

e.g.,

U.S. Department of the Navy (1975)). In air, the propagation of impulsive noise from an explosion is highly influenced by atmospheric conditions, including temperature and wind. While basic estimation methods do not consider the unique environmental conditions that may be present on a given day, they do allow for approximation of explosive energy propagation under neutral atmospheric conditions. Explosions that occur during Air Warfare would typically be at a sufficient altitude that a large portion of the sound refracts upward due to cooling temperatures with increased altitude. Based on an understanding of the explosive energy released by detonations in air, detonations occurring in air at altitudes greater than 10 m above the surface of the ocean are not likely to result in acoustic impacts on marine mammals; therefore, these types of explosive activities will not be discussed further in this document. (Note that most of these in-air detonations would occur at altitudes substantially greater than 10 m above the surface of the ocean, as described in further detail in section 3.0.4.2.2 (

Explosions in Air

) of the 2020 GOA DSEIS/OEIS.) Activities such as air-surface bombing or surface-surface gunnery scenarios may involve the use of explosive munitions that detonate upon impact with targets at or above the water surface (within 10 m above the surface). For these activities, acoustic effects modeling was undertaken as described below.

In order to organize and facilitate the analysis of explosives, explosive classification bins were developed. The use of explosive classification bins provides the same benefits as described for acoustic source classification bins in the

Acoustic Stressors

section, above.

The explosive bin types and the number of explosives detonating at or above the water surface in the TMAA are shown in Table 2.

Table 2—Explosive Sources Quantitatively Analyzed That Detonate At or Above the Water Surface in the TMAA

Explosives

(source class and net explosive weight (NEW))

(lb.) *

Number of explosives with the specified activity

(annually)

Number of explosives with the specified activity

(7-year total)

E5 (>5-10 lb. NEW)

56

392

E9 (>100-250 lb. NEW)

64

448

E10 (>250-500 lb. NEW)

6

42

E12 (>650-1,000 lb. NEW)

2

14

* All of the E5, E9, E10, and E12 explosives would occur in-air, at or above the surface of the water, and would also occur offshore away from the continental shelf and slope beyond the 4,000-meter isobath.

Propagation of explosive pressure waves in water is highly dependent on environmental characteristics such as bathymetry, bottom type, water depth, temperature, and salinity, which affect how the pressure waves are reflected, refracted, or scattered; the potential for reverberation; and interference due to multi-path propagation. In addition, absorption greatly affects the distance over which higher-frequency components of explosive broadband noise can propagate. Appendix B (

Acoustic and Explosive Concepts

) of the 2020 GOA DSEIS/OEIS explains the characteristics of explosive detonations and how the above factors affect the propagation of explosive energy in the water. Because of the complexity of analyzing sound propagation in the ocean environment, the Navy relies on acoustic models in its environmental analyses that consider sound source characteristics and varying ocean conditions across the TMAA.

For in-air explosives detonating at or above the water surface, the model estimating acoustic impacts assumes that all acoustic energy from the detonation is underwater with no loss of sound or energy into the air. Important considerations must be factored into the analysis of results with these modeling assumptions, given that the peak pressure and sound from a detonation in air significantly decreases across the air-water interface as it is partially reflected by the water's surface and partially transmitted underwater, as detailed in the following paragraphs.

Detonation of an explosive in air creates a supersonic high pressure shock wave that expands outward from the point of detonation (Kinney and Graham, 1985; Swisdak, 1975). The near-instantaneous rise from ambient to an extremely high peak pressure is what makes the explosive shock wave potentially injurious to an animal experiencing the rapid pressure change (U.S. Department of the Navy, 2017a). As the shock wave-front travels away from the point of detonation, it slows and begins to behave as an acoustic wave-front travelling at the speed of sound. Whereas a shock wave from a detonation in-air has an abrupt peak pressure, that same pressure disturbance when transmitted through the water surface results in an underwater pressure wave that begins and ends more gradually compared with the in-air shock wave, and diminishes with increasing depth and distance from the source (Bolghasi

et al.,

2017; Chapman and Godin, 2004; Cheng and Edwards, 2003; Moody, 2006; Richardson

et al.,

1995; Sawyers, 1968; Sohn

et al.,

2000; Swisdak, 1975; Waters and Glass, 1970; Woods

et al.,

2015). The propagation of the shock wave in-air and then transitioning underwater is very different from a detonation occurring deep underwater where there is little interaction with the surface. In the case of an underwater detonation occurring just below the surface, a portion of the energy from the detonation would be released into the air (referred to as surface blow off), and at greater depths a pulsating, air-filled cavitation bubble would form, collapse, and reform around the detonation point (Urick, 1983). The Navy's acoustic effects model for analyzing underwater impacts on marine species does not account for the loss of energy due to surface blow-

off or cavitation at depth. Both of these phenomena would diminish the magnitude of the acoustic energy received by an animal under real-world conditions (U.S. Department of the Navy, 2018b).

To more completely analyze the results predicted by the Navy's acoustic effects model from detonations occurring in-air above the ocean surface, it is necessary to consider the transfer of energy across the air-water interface. Much of the scientific literature on the transferal of shock wave impulse across the air-water interface has focused on energy from sonic booms created by fast moving aircraft flying at low altitudes above the ocean (Chapman and Godin, 2004; Cheng and Edwards, 2003; Moody, 2006; Sawyers, 1968; Waters and Glass, 1970). The shock wave created by a sonic boom is similar to the propagation of a pressure wave generated by an explosion (although having a significantly slower rise in peak pressure) and investigations of sonic booms are somewhat informative. Waters and Glass (1970) were also investigating sonic booms, but their methodology involved actual in-air detonations. In those experiments, they detonated blasting caps elevated 30 ft (9 m) above the surface in a flooded quarry and measured the resulting pressure at and below the surface to determine the penetration of the shock wave across the air-water interface. Microphones above the water surface recorded the peak pressure in-air, and hydrophones at various shallow depths underwater recorded the unreflected remainder of the pressure wave after transition across the air-water interface. The peak pressure measurements were compared and the results supported the theoretical expectations for the penetration of a pressure wave from air into water, including the predicted exponential decay of energy with distance from the source underwater. In effect, the air-water interface acted as a low-pass filter eliminating the high-frequency components of the shock wave. At incident angles greater than 14 degrees perpendicular to the surface, most of the shock wave from the detonation was reflected off the water surface, which is consistent with results from similar research (Cheng and Edwards, 2003; Moody, 2006; Yagla and Stiegler, 2003). Given that marine mammals spend, on average, up to 90 percent of their time underwater (Costa, 1993; Costa and Block, 2009), and the shock wave from a detonation is only a few milliseconds in duration, marine mammals are unlikely to be exposed in-air when surfaced.

Vessel Strike

NMFS also considered the chance that a vessel utilized in training activities could strike a marine mammal in the GOA Study Area, including both the TMAA and WMA portions of the Study Area. Vessel strikes have the potential to result in incidental take from serious injury and/or mortality. Vessel strikes are not specific to any particular training activity, but rather are a limited, sporadic, and incidental result of Navy vessel movement within a study area. Vessel strikes from commercial, recreational, and military vessels are known to seriously injure and occasionally kill cetaceans (Abramson

et al.,

2011; Berman-Kowalewski

et al.,

2010; Calambokidis, 2012; Douglas

et al.,

2008; Laggner, 2009; Lammers

et al.,

2003; Van der Hoop

et al.,

2012; Van der Hoop

et al.,

2013), although reviews of the literature on ship strikes mainly involve collisions between commercial vessels and whales (Jensen and Silber, 2003; Laist

et al.,

2001). Vessel speed, size, and mass are all important factors in determining both the potential likelihood and impacts of a vessel strike to marine mammals (Conn and Silber, 2013; Gende

et al.,

2011; Silber

et al.,

2010; Vanderlaan and Taggart, 2007; Wiley

et al.,

2016). For large vessels, speed and angle of approach can influence the severity of a strike.

Navy vessels transit at speeds that are optimal for fuel conservation and to meet training requirements. Vessels used as part of the proposed specified activities include ships, submarines, unmanned vessels, and boats ranging in size from small, 22 ft (7 m) rigid hull inflatable boats to aircraft carriers with lengths up to 1,092 ft (333 m). The average speed of large Navy ships ranges between 10 and 15 knots (kn; 19-28 km/hr), and submarines generally operate at speeds in the range of 8 to 13 kn (15 to 24 km/hr), while a few specialized vessels can travel at faster speeds. Small craft (for purposes of this analysis, less than 18 m in length) have much more variable speeds (0 to 50+ kn (0 to 93+ km/hr)), dependent on the activity), but generally range from 10 to 14 kn (19-26 km/hr). From unpublished Navy data, average median speed for large Navy ships in the other Navy ranges from 2011-2015 varied from 5 to 10 kn (9 to 19 km/hr) with variations by ship class and location (

i.e.,

slower speeds close to the coast). Similar patterns would occur in the GOA Study Area. A full description of Navy vessels that are used during training activities can be found in Section 1.2.1 and Section 2.4.2.1 of the 2011 GOA FEIS/OEIS.

While these speeds are representative of most events, some vessels need to temporarily operate outside of these parameters for certain times or during certain activities. For example, to produce the required relative wind speed over the flight deck, an aircraft carrier engaged in flight operations must adjust its speed through the water accordingly. Also, there are other instances, such as launch and recovery of a small rigid hull inflatable boat; vessel boarding, search, and seizure training events; or retrieval of a target when vessels would be dead in the water or moving slowly ahead to maintain steerage.

Large Navy vessels (greater than 18 m in length) within the offshore areas of range complexes operate differently from commercial vessels in ways that may reduce potential whale collisions. Surface ships operated by or for the Navy have multiple personnel assigned to stand watch at all times when a ship or surfaced submarine is moving through the water (underway). A primary duty of personnel standing watch on surface ships is to detect and report all objects and disturbances sighted in the water that may indicate a threat to the vessel and its crew, such as debris, a periscope, surfaced submarine, or surface disturbance. Per vessel safety requirements, personnel standing watch also report any marine mammals sighted in the path of the vessel as a standard collision avoidance procedure. All vessels proceed at a safe speed so they can take proper and effective action to avoid a collision with any sighted object or disturbance, and can be stopped within a distance appropriate to the prevailing circumstances and conditions.

Detailed Description of Proposed Activities

Proposed Training Activities

The Navy proposes to conduct a single carrier strike group (CSG) exercise which would last for a maximum of 21 consecutive days in a year. The CSG exercise is comprised of several individual training activities. Table 3 lists and describes those individual activities that may result in takes of marine mammals. The events listed would occur intermittently during the 21 days and could be simultaneous and in the same general area within the TMAA or could be independent and spatially separate from other ongoing activities. The table is organized according to primary mission areas and includes the activity name, associated stressor(s), description and duration of the activity, sound source bin, the areas

where the activities are conducted in the GOA Study Area, the maximum number of events per year in the 21-day period, and the maximum number of events over 7 years. Not all sound sources are used with each activity. The “Annual # of Events” column indicates the maximum number of times that activity could occur during any single year. The “7-Year # of Events” is the maximum number of times an activity would occur over the 7-year period of the proposed regulations if the training occurred each year and at the maximum levels requested. The events listed would occur intermittently during the exercise over a maximum of 21 days. The maximum number of activities may not occur in some years, and historically, training has occurred only every other year. However, to conduct a conservative analysis, NMFS analyzed the maximum times these activities could occur over one year and 7 years. The 2020 GOA DSEIS/OEIS includes more detailed activity descriptions. (Note the Navy proposes no low-frequency active sonar (LFAS) use for the activities in this rulemaking.)

Table 3—Proposed Training Activities Analyzed for the 7-Year Period in the GOA Study Area

Stressor

category

Activity

Description

Source bin

Annual # of events

7-year # of events

Surface Warfare

Explosive

Gunnery Exercise, Surface-to-Surface (GUNEX-S-S)

Surface ship crews fire inert small-caliber, inert medium-caliber, or large-caliber explosive rounds at surface targets

E5

6

42

Explosive

Bombing Exercise (Air-to-Surface) (BOMBEX [A-S])

Fixed-wing aircraft conduct bombing exercises against stationary floating targets, towed targets, or maneuvering targets

E9, E10, E12

18

126

Anti-Submarine Warfare (ASW)

Acoustic

Tracking Exercise—Helicopter (TRACKEX—Helo)

Helicopter crews search for, track, and detect submarines

MF4, MF5, MF6

22

154

Acoustic

Tracking Exercise—Maritime Patrol Aircraft (TRACKEX—MPA)

Maritime patrol aircraft crews search for, track, and detect submarines

MF5, MF6, ASW2

13

91

Acoustic

Tracking Exercise—Ship (TRACKEX—Ship)

Surface ship crews search for, track, and detect submarines

ASW1, ASW3, MF1, MF11, MF12

2

14

Acoustic

Tracking Exercise—Submarine (TRACKEX—Sub)

Submarine crews search for, track, and detect submarines

ASW4, HF1, MF3

2

14

Notes:

S-S = Surface to Surface, A-S = Air to Surface.

Standard Operating Procedures

For training to be effective, personnel must be able to safely use their sensors and weapon systems as they are intended to be used in military missions and combat operations and to their optimum capabilities. Standard operating procedures applicable to training have been developed through years of experience, and their primary purpose is to provide for safety (including public health and safety) and mission success. Because standard operating procedures are essential to safety and mission success, the Navy considers them to be part of the proposed specified activities, and has included them in the analysis. In many cases, there are benefits to natural and cultural resources resulting from standard operating procedures. Standard operating procedures that are recognized as having a potential benefit to marine mammals during training activities are noted below and discussed in more detail within the 2020 GOA DSEIS/OEIS.

• Vessel Safety;

• Weapons Firing Procedures;

• Target Deployment and Retrieval Safety; and

• Towed In-Water Device Procedures.

Standard operating procedures (which are implemented regardless of their secondary benefits) are different from mitigation measures (which are designed entirely for the purpose of avoiding or reducing impacts). Information on mitigation measures is provided in the Proposed Mitigation Measures section below. Additional information on standard operating procedures is presented in Section 2.3.2 (Standard Operating Procedures) in the 2020 GOA DSEIS/OEIS.

Description of Marine Mammals and Their Habitat in the Area of the Specified Activities

Marine mammal species and their associated stocks that have the potential to occur in the GOA Study Area are presented in Table 4 along with each stock's ESA and MMPA statuses, abundance estimate and associated coefficient of variation value, minimum abundance estimate, and expected occurrence in the GOA Study Area. The Navy requested authorization to take individuals of 16 marine mammal species by Level A harassment and Level B harassment, and NMFS has conservatively analyzed and proposes to authorize incidental take of two additional species. The Navy does not request authorization for any serious injuries or mortalities of marine mammals, and NMFS agrees that serious injury and mortality is unlikely to occur from the Navy's activities. NMFS recently designated critical habitat under the Endangered Species Act (ESA) for humpback whales in the TMAA portion of the GOA Study Area, and this designated critical habitat is considered below (86 FR 21082; April 21, 2021). The WMA portion of the GOA Study Area does not overlap ESA-designated critical habitat for humpback whales or any other species.

Information on the status, distribution, abundance, population trends, habitat, and ecology of marine mammals in the GOA Study Area may be found in Chapter 4 of the Navy's rulemaking/LOA application. NMFS has reviewed this information and found it

to be accurate and complete. Additional information on the general biology and ecology of marine mammals is included in the 2020 GOA DSEIS/OEIS. Table 4 incorporates the best available science, including data from the 2020 U.S. Pacific and the Alaska Marine Mammal Stock Assessment Reports (SARs; Carretta

et al.,

2021; Muto

et al.,

2021), 2021 draft U.S. Pacific and Alaska Marine Mammal SARs, as well as monitoring data from the Navy's marine mammal research efforts.

To better define marine mammal occurrence in the TMAA, the portion of the GOA Study Area where take of marine mammals is anticipated to occur, four regions within the TMAA were defined (and are depicted in Figure 3-1 of the Navy's rulemaking/LOA application), consistent with the survey strata used by Rone

et al.

(2017) during the most recent marine mammal surveys in the TMAA. The four regions are: inshore, slope, seamount, and offshore.

Species Not Included in the Analysis

There has been no change in the species unlikely to be present in the GOA Study Area since the last MMPA rulemaking process (82 FR 19530; April 27, 2017). The species carried forward for analysis are those likely to be found in the GOA Study Area based on the most recent data available and do not include species that may have once inhabited or transited the area but have not been sighted in recent years (

e.g.,

species which were extirpated from factors such as 19th and 20th century commercial exploitation). Several species and stocks that may be present in the northeast Pacific Ocean generally have an extremely low probability of presence in the GOA Study Area. These species and stocks are considered extralimital (may be sightings, acoustic detections, or stranding records, but the GOA Study Area is outside the species' range of normal occurrence) or rare (occur in the GOA Study Area sporadically, but sightings are rare). These species and stocks include the Eastern North Pacific Northern Resident and the West Coast Transient stocks of killer whale (

Orcinus orca

), beluga whale (

Delphinapterus leucas

), false killer whale (

Pseudorca crassidens

), short-finned pilot whale (

Globicephala macrorhynchus

), northern right whale dolphin (

Lissodelphis borealis

), and Risso's dolphin (

Grampus griseus

).

The Eastern North Pacific Northern Resident and the West Coast Transient stocks of killer whale are considered extralimital in the GOA Study Area. Given the paucity of any beluga whale sightings in the GOA (Laidre

et al.

2000), the occurrence of this species within the GOA Study Area is considered extralimital. The GOA Study Area is also outside of the normal range of the false killer whale's distribution in the Pacific Ocean, and despite rare stranding or sighting reports, the GOA Study Area is outside of the normal range of the short-finned pilot whale as well. There are two sighting records of northern right whale dolphins in the Gulf of Alaska, but these are considered extremely rare (U.S. Department of the Navy 2006; NOAA 2012) and extralimital in the GOA Study Area. There are a few records of Risso's dolphins near the GOA Study Area; however, their occurrence within the GOA Study Area is rare, and therefore Risso's dolphin is considered extralimital. NMFS agrees with the Navy's assessment that these species are unlikely to occur in the GOA Study Area and they are not discussed further.

One species of marine mammal, the Northern sea otter, occurs in the Gulf of Alaska but is managed by the U.S. Fish and Wildlife Service and is not considered further in this document.

Table 4—Marine Mammal Occurrence Within the GOA Study Area

Common name

Scientific name

Stock

ESA

status, MMPA status, strategic

(Y/N)

1

Stock abundance

(CV, Nmin, year of most recent

abundance survey)

2

PBR

Annual M/SI

3

Occurrence in GOA study area

4

Order Cetacea—Suborder Mysticeti (baleen whales)

Family Balaenidae (right whales):

North Pacific right whale

Eubalaena japonica

Eastern North Pacific

E, D, Y

31 (0.226, 26, 2008)

5

0.05

0

Rare.

Family Balaenopteridae (rorquals):

Humpback whale

Megaptera novaeangliae

Central North Pacific

6

-, -, Y

10,103 (0.3, 7,891, 2006)

83

26

Seasonal; highest likelihood June to September.

California, Oregon, and Washington

6

-, -, Y

4,973 (0.05, 4,776, 2018)

28.7

≥48.6

Seasonal; highest likelihood June to September.

Western North Pacific

E, D, Y

1,107 (0.3, 865, 2006)

3

2.8

Seasonal; highest likelihood June to September.

Blue whale

Balaenoptera musculus

Eastern North Pacific

E, D, Y

1,898 (0.085, 1,767, 2018)

4.1

≥19.4

Seasonal; highest likelihood June to December.

Central North Pacific

E, D, Y

133 (1.09, 63, 2010)

0.1

0

Seasonal; highest likelihood June to December.

Fin whale

Balaenoptera physalus

Northeast Pacific

E, D, Y

3,168 (0.26, 2,554, 2013)

7

5.1

0.6

Likely.

Sei whale

Balaenoptera borealis

Eastern North Pacific

8

E, D, Y

519 (0.4, 374, 2014)

0.75

≥0.2

Rare.

Minke whale

Balaenoptera acutorostrata

Alaska

-, -, N

UNK

UND

0

Likely.

Family Eschrichtiidae (gray whale):

Gray whale

Eschrichtius robustus

Eastern North Pacific

-, -, N

26,960 (0.05, 25,849, 2016)

801

131

Likely: Highest numbers during seasonal migrations (fall, winter, spring).

Western North Pacific

E, D, Y

290 (N/A, 271, 2016)

0.12

UNK

Rare: Individuals migrate through GOA.

Order Cetacea—Suborder Odontoceti (toothed whales)

Family Physeteridae (sperm whale):

Sperm whale

Physeter macrocephalus

North Pacific

E, D, Y

345 (0.43, 244, 2015)

9

UND

3.5

Likely; More likely in waters >1,000 m depth, most often >2,000 m.

Family Delphinidae (dolphins)

:

Killer whale

Orcinus orca

Eastern North Pacific Alaska Resident

-, -, N

10

2,347 (N/A, 2,347, 2012)

24

1

Likely.

Eastern North Pacific Offshore

-, -, N

300 (0.1, 276, 2012)

2.8

0

Likely.

AT1 Transient

-, D, Y

10

7 (N/A, 7, 2018)

0.01

0

Rare; more likely inside Prince William Sound and Kenai Fjords.

Eastern North Pacific GOA, Aleutian Island, and Bering Sea Transient

-, -, N

10

587 (N/A, 587, 2012)

5.87

0.8

Likely.

Pacific white-sided dolphin

Lagenorhynchus obliquidens

North Pacific

-, -, N

26,880 (N/A, N/A, 1990)

UND

0

Likely.

Family Phocoenidae (porpoises):

Harbor porpoise

Phocoena phocoena

GOA

-, -, Y

31,046 (0.21, N/A, 1998)

UND

72

Rare; Inshore and Slope Regions, if present.

Southeast Alaska

-, -, Y

1,354 (0.12, 1,224, 2012)

12

34

Rare.

Dall's porpoise

Phocoenoides dalli

Alaska

-, -, N

83,400 (0.097, 3,110, 2015)

UND

37

Likely.

Family Ziphiidae (beaked whales):

Cuvier's beaked whale

Ziphius cavirostris

Alaska

-, -, N

UNK

UND

0

Likely.

Baird's beaked whale

Berardius bairdii

Alaska

-, -, N

UNK

UND

0

Likely.

Stejneger's beaked whale

Mesoplodon stejnegeri

Alaska

-, -, N

UNK

UND

0

Likely.

Order Carnivora—Suborder Pinnipedia

8

Family Otarieidae (fur seals and sea lions):

Steller sea lion

Eumetopias jubatus

Eastern U.S

-, -, N

11

43,201 (N/A, 43,201, 2017)

2,592

112

Rare.

Western U.S.

E, D, Y

11

52,932 (N/A, 52,932, 2013)

318

254

Likely; Inshore region.

California sea lion

Zalophus californianus

U.S

-, -, N

257,606 (N/A, 233,515, 2014)

14,011

>320

Rare (highest likelihood April and May).

Northern fur seal

Callorhinus ursinus

Eastern Pacific

-, D, Y

626,618 (0.2, 530,376, 2019)

11,403

373

Likely.

California

-, D, N

14,050 (N/A, 7,524, 2013)

451

1.8

Rare.

Family Phocidae (true seals):

Northern elephant seal

Mirounga angustirostris

California Breeding

-, -, N

187,386 (N/A, 85,369, 2013)

5,122

5.3

Seasonal (highest likelihood July-September).

Harbor seal

Phoca vitulina

N Kodiak

-, -, N

8,677 (N/A, 7,609, 2017)

228

38

Likely; Inshore region.

S Kodiak

-, -, N

26,448 (N/A, 22,351, 2017)

939

127

Likely; Inshore region.

Prince William Sound

-, -, N

44,756 (N/A, 41,776, 2015)

1,253

413

Likely; Inshore region.

Cook Inlet/Shelikof

-, -, N

28,411 (N/A, 26,907, 2018)

807

107

Likely; Inshore region.

Ribbon seal

Histriophoca fasciata

Unidentified

-, -, N

184,697 (N/A, 163,086, 2013)

9,785

163

Rare.

Notes:

CV = coefficient of variation, ESA = Endangered Species Act, GOA = Gulf of Alaska, m = meter(s), MMPA = Marine Mammal Protection Act, N/A = not available, U.S. = United States, M/SI = mortality and serious injury, UNK = unknown, UND = undetermined.

1

Endangered Species Act (ESA) status: Endangered (E), Threatened (T)/MMPA status: Depleted (D). A dash (-) indicates that the species is not listed under the ESA or designated as depleted under the MMPA. Under the MMPA, a strategic stock is one for which the level of direct human-caused mortality exceeds potential biological removal (PBR) or which is determined to be declining and likely to be listed under the ESA within the foreseeable future. Any species or stock listed under the ESA is automatically designated under the MMPA as depleted and as a strategic stock.

2

The stocks and stock abundance number are as provided in Carretta

et al.,

2021 and Muto

et al.,

2021. Nmin is the minimum estimate of stock abundance. In some cases, CV is not applicable. NMFS marine mammal stock assessment reports online at:

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

.

3

These values, found in NMFS' SARs, represent annual levels of human-caused mortality and serious injury (M/SI) from all sources combined (

e.g.,

commercial fisheries, ship strike). Annual mortality and serious injury (M/SI) often cannot be determined precisely and is in some cases presented as a minimum value or range. A CV associated with estimated mortality due to commercial fisheries is presented in some cases.

4

RARE: The distribution of the species is near enough to the GOA Study Area that the species could occur there, or there are a few confirmed sightings. LIKELY: Year-round sightings or acoustic detections of the species in the GOA Study Area, although there may be variation in local abundance over the year. SEASONAL: Species absence and presence as documented by surveys or acoustic monitoring. Regions within the GOA Study Area follow those presented in Rone

et al.

(2015); Rone

et al.

(2009); Rone

et al.

(2014); Rone

et al.

(2017): inshore, slope, seamount, and offshore.

5

See SAR for more details

6

Humpback whales in the Central North Pacific stock and the California, Oregon, and Washington stock are from three Distinct Population Segments based on animals identified in breeding areas in Hawaii, Mexico, and Central America (Carretta

et al.,

2021; Muto

et al.,

2021; National Marine Fisheries Service, 2016c).

7

The SAR reports this stock abundance assessment as provisional and notes that it is an underestimate for the entire stock because it is based on surveys which covered only a small portion of the stock's range.

8

This analysis assumes that these individuals are from the Eastern North Pacific stock; however, they are not discussed in the West Coast or the Alaska Stock Assessment Reports (Carretta

et al.,

2021; Muto

et al.,

2021).

9

The SAR reports that this is an underestimate for the entire stock because it is based on surveys of a small portion of the stock's extensive range and it does not account for animals missed on the trackline or for females and juveniles in tropical and subtropical waters.

10

Stock abundance is based on counts of individual animals identified from photo-identification catalogues. Surveys for abundance estimates of these stocks are conducted infrequently.

11

Stock abundance is the best estimate of pup and non-pup counts, which have not been corrected to account for animals at sea during abundance surveys.

Below, we consider additional information about the marine mammals in the area of the specified activities that informs our analysis, such as identifying known areas of important habitat or behaviors, or where Unusual Mortality Events (UME) have been designated.

Critical Habitat

On April 21, 2021 (86 FR 21082), NMFS published a final rule designating critical habitat for the endangered Western North Pacific DPS, the endangered Central America DPS, and the threatened Mexico DPS of humpback whales, including specific marine areas located off the coasts of California, Oregon, Washington, and Alaska. Based on consideration of national security, economic impacts, and data deficiency in some areas, NMFS excluded certain areas from the designation for each DPS.

NMFS identified prey species, primarily euphausiids and small pelagic schooling fishes (see the final rule for particular prey species identified for each DPS; 86 FR 21082; April 21, 2021) of sufficient quality, abundance, and accessibility within humpback whale feeding areas to support feeding and population growth, as an essential habitat feature. NMFS, through a critical habitat review team (CHRT), also considered inclusion of migratory corridors and passage features, as well as sound and the soundscape, as essential habitat features. However, NMFS did not include either, as the CHRT concluded that the best available science did not allow for identification of any consistently used migratory corridors or definition of any physical, essential migratory or passage conditions for whales transiting between or within habitats of the three DPSs. The best available science also currently does not enable NMFS to identify a sound-related habitat feature that is essential to the conservation of humpback whales.

NMFS considered the co-occurrence of this designated humpback whale critical habitat and the GOA Study Area. Figure 4-1 of the Navy's rulemaking/LOA application shows the overlap of the humpback whale critical habitat with the TMAA. As shown in the Navy's rulemaking/LOA application, the TMAA overlaps with humpback whale critical habitat Unit 5 (destination for whales from the Hawaii, Mexico, and Western North Pacific DPSs; Calambokidis

et al.,

2008) and Unit 8 (destination for whales from the Hawaii and Mexico DPSs (Baker

et al.,

1986, Calambokidis

et al.,

2008); Western North Pacific DPS whales have not been photo-identified in this specific area, but presence has been inferred based on available data indicating that humpback whales from Western North Pacific wintering areas occur in the Gulf of Alaska (NMFS 2020, Table C5)). Approximately 4 percent of the humpback whale critical habitat in the GOA region overlaps with the TMAA, and approximately 2 percent of critical habitat in both the GOA and U.S. west coast regions combined overlaps with the TMAA. The WMA portion of the GOA Study Area does not overlap ESA-designated critical habitat for humpback whales.

As noted above in the

Geographical Region

section, the TMAA boundary was intentionally designed to avoid ESA-designated Western DPS (MMPA Western U.S. stock) Steller sea lion critical habitat.

Biologically Important Areas

BIAs include areas of known importance for reproduction, feeding, or migration, or areas where small and resident populations are known to occur (Van Parijs, 2015). Unlike ESA critical habitat, these areas are not formally designated pursuant to any statute or law, but are a compilation of the best available science intended to inform impact and mitigation analyses. An interactive map of BIAs may be found here:

https://cetsound.noaa.gov/biologically-important-area-map

.

The WMA does not overlap with any known BIAs. BIAs in the GOA that overlap portions of the TMAA include the following feeding and migration areas: North Pacific right whale feeding BIA (June-September); Gray whale migratory corridor BIA (November-January, southbound; March-May, northbound) (Ferguson

et al.,

2015). Fin whale feeding areas (east, west, and southwest of Kodiak Island) occur to the west of the TMAA and gray whale feeding areas occur both east (Southeast Alaska) and west (Kodiak Island) of the TMAA; however, these feeding areas are located well outside of (> 20 nmi (37 km)) the TMAA and beyond the Navy's estimated range to effects for take by Level A harassment and Level B harassment.

A portion of the North Pacific right whale feeding BIA overlaps with the western side of the TMAA by approximately 2,051 square kilometers (km

2

; approximately 1.4 percent of the TMAA, and 7 percent of the feeding BIA). A small portion of the gray whale migration corridor BIA also overlaps with the western side of the TMAA by approximately 1,582 km

2

(approximately 1 percent of the TMAA, and 1 percent of the migration corridor BIA). To mitigate impacts to marine mammals in these BIAs, the Navy would implement several procedural mitigation measures and mitigation areas (described in the Proposed Mitigation Measures section).

Unusual Mortality Events (UMEs)

A UME is defined under Section 410(6) of the MMPA as a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response. There is one UME that is applicable to our evaluation of the Navy's activities in the GOA Study Area. The gray whale UME along the west coast of North America is active and involves ongoing investigations in the GOA that inform our analysis are discussed below.

Gray Whale UME

Since January 1, 2019, elevated gray whale strandings have occurred along the west coast of North America, from Mexico to Canada. As of June 3, 2022, there have been a total of 578 strandings along the coasts of the United States, Canada, and Mexico, with 278 of those strandings occurring along the U.S. coast. Of the strandings on the U.S. coast, 118 have occurred in Alaska, 66 in Washington, 14 in Oregon, and 80 in California. Full or partial necropsy examinations were conducted on a subset of the whales. Preliminary findings in several of the whales have shown evidence of emaciation. These findings are not consistent across all of the whales examined, so more research is needed. As part of the UME investigation process, NOAA has assembled an independent team of scientists to coordinate with the Working Group on Marine Mammal Unusual Mortality Events to review the data collected, sample stranded whales, consider possible causal-linkages between the mortality event and recent ocean and ecosystem perturbations, and determine the next steps for the investigation. Please refer to:

https://www.fisheries.noaa.gov/national/marine-life-distress/2019-2022-gray-whale-unusual-mortality-event-along-west-coast-and

for more information on this UME.

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. Current data indicate that 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) recommended that marine mammals be divided into functional hearing groups based on directly measured or estimated hearing ranges on the basis of available behavioral response data, audiograms derived using auditory evoked potential techniques, anatomical modeling, and other data. Note that no direct measurements of hearing ability have been successfully completed for mysticetes (

i.e.,

low-frequency cetaceans). Subsequently, NMFS (2018) described generalized hearing ranges for these marine mammal hearing groups. Generalized hearing ranges were chosen based on the approximately 65 dB threshold from the normalized composite audiograms, with the exception for lower limits for low-frequency cetaceans where the lower bound was deemed to be biologically implausible and the lower bound from Southall

et al.

(2007) retained. The functional groups and the associated frequencies are indicated below (note that these frequency ranges correspond to the range for the composite group, with the entire range not necessarily reflecting the capabilities of every species within that group):

• Low-frequency cetaceans (mysticetes): generalized hearing is estimated to occur between approximately 7 Hz and 35 kHz;

• Mid-frequency cetaceans (larger toothed whales, beaked whales, and most delphinids): generalized hearing is estimated to occur between approximately 150 Hz and 160 kHz;

• High-frequency cetaceans (porpoises, river dolphins, and members of the genera Kogia and Cephalorhynchus; including two members of the genus Lagenorhynchus, on the basis of recent echolocation data and genetic data): generalized hearing is estimated to occur between approximately 275 Hz and 160 kHz;

• Pinnipeds in water; Phocidae (true seals): generalized hearing is estimated to occur between approximately 50 Hz to 86 kHz; and

• Pinnipeds in water; Otariidae (eared seals): generalized hearing is estimated to occur between 60 Hz and 39 kHz.

The pinniped functional hearing group was modified from Southall

et al.

(2007) on the basis of data indicating that phocid species have consistently demonstrated an extended frequency range of hearing compared to otariids, especially in the higher frequency range (Hemilä

et al.,

2006; Kastelein

et al.,

2009; Reichmuth and Holt, 2013).

For more details concerning these groups and associated frequency ranges, please see NMFS (2018) for a review of the available information.

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

This section includes a discussion of the ways that components of the specified activity may impact marine mammals and their habitat. The Estimated Take of Marine Mammals section later in this rule includes a quantitative analysis of the number of instances of take that could occur from these activities. The Preliminary Analysis and Negligible Impact Determination section considers the content of this section, the Estimated Take of Marine Mammals section, and the Proposed Mitigation Measures section to draw conclusions regarding the likely impacts of these activities on the reproductive success or survivorship of individuals and whether those impacts on individuals are likely to adversely affect the species through effects on annual rates of recruitment or survival.

The Navy has requested authorization for the take of marine mammals that may occur incidental to training activities in the GOA Study Area. The Navy analyzed potential impacts to marine mammals in its rulemaking/LOA application. NMFS carefully reviewed the information provided by the Navy along with independently reviewing applicable scientific research and literature and other information to evaluate the potential effects of the Navy's activities on marine mammals, which are presented in this section. (As noted above, activities that would result in take of marine mammals would only occur in the TMAA portion of the GOA Study Area.)

Other potential impacts to marine mammals from training activities in the GOA Study Area were analyzed in the Navy's rulemaking/LOA application as well as in the 2020 GOA DSEIS/OEIS and 2022 Supplement to the 2020 GOA DSEIS/OEIS, in consultation with NMFS as a cooperating agency, and determined to be unlikely to result in marine mammal take. These include incidental take from vessel strike and serious injury or mortality from explosives. Therefore, the Navy did not request authorization for incidental take of marine mammals by vessel strike or serious injury or mortality from explosives from its proposed specified activities. NMFS has carefully considered the information in the 2020 GOA DSEIS/OEIS, the 2022 Supplement to the 2020 GOA DSEIS/OEIS, and all other pertinent information and agrees that incidental take is unlikely to occur from these sources. NMFS conducted a detailed analysis of the potential for vessel strike, and based on that analysis,

NMFS does not anticipate vessel strikes of large whales or smaller marine mammals in the GOA Study Area. In this proposed rule, NMFS analyzes the potential effects of the Navy's activities on marine mammals in the GOA Study Area, focusing primarily on the activity components that may cause the take of marine mammals: exposure to acoustic or explosive stressors including non-impulsive (sonar and other transducers) and impulsive (explosives) stressors.

For the purpose of MMPA incidental take authorizations, NMFS' effects assessments serve four primary purposes: (1) to determine whether the specified activities would have a negligible impact on the affected species or stocks of marine mammals (based on whether it is likely that the activities would adversely affect the species or stocks through effects on annual rates of recruitment or survival); (2) to determine whether the specified activities would have an unmitigable adverse impact on the availability of the species or stocks for subsistence uses; (3) to prescribe the permissible methods of taking (

i.e.,

Level B harassment (behavioral disturbance and temporary threshold shift (TTS)), Level A harassment (permanent threshold shift (PTS) and non-auditory injury), serious injury, or mortality), including identification of the number and types of take that could occur by harassment, serious injury, or mortality, and to prescribe means of effecting the least practicable adverse impact on the species or stocks and their habitat (

i.e.,

mitigation measures); and (4) to prescribe requirements pertaining to monitoring and reporting.

In this section, NMFS provides a description of the ways marine mammals potentially could be affected by these activities in the form of mortality, physical trauma, sensory impairment (permanent and temporary threshold shifts and acoustic masking), physiological responses (particularly stress responses), behavioral disturbance, or habitat effects. The Estimated Take of Marine Mammals section discusses how the potential effects on marine mammals from non-impulsive and impulsive sources relate to the MMPA definitions of Level A Harassment and Level B Harassment, and quantifies those effects that rise to the level of a take. The Preliminary Analysis and Negligible Impact Determination section assesses whether the proposed authorized take would have a negligible impact on the affected species and stocks.

Potential Effects of Underwater Sound

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 possibly result in one or more of the following: temporary or permanent hearing impairment, non-auditory physical or physiological effects, behavioral response, stress, and masking (Richardson

et al.,

1995; Gordon

et al.,

2004; Nowacek

et al.,

2007; Southall

et al.,

2007; Götz

et al.,

2009, Southall

et al.,

2019a). 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 can occur after exposure to noise, and occurs almost exclusively for noise within an animal's hearing range. Note that in the following discussion, we refer in many cases to a review article concerning studies of noise-induced hearing loss conducted from 1996-2015 (

i.e.,

Finneran, 2015). For study-specific citations, please see that work. We first describe general manifestations of acoustic effects before providing discussion specific to the Navy's activities.

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 responsiveness. 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 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 also describe more severe potential effects (

i.e.,

certain non-auditory physical or physiological effects). 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) 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).

Acoustic Sources

Direct Physiological Effects

Non-impulsive sources of sound can cause direct physiological effects including noise-induced loss of hearing sensitivity (or “threshold shift”), nitrogen decompression, acoustically-induced bubble growth, and injury due to sound-induced acoustic resonance. Only noise-induced hearing loss is anticipated to occur due to the Navy's activities. Acoustically-induced (or mediated) bubble growth and other pressure-related physiological impacts are addressed below, but are not expected to result from the Navy's activities. Separately, an animal's behavioral reaction to an acoustic exposure might lead to physiological effects that might ultimately lead to injury or death, which is discussed later in the

Stranding and Mortality

subsection.

Hearing Loss—Threshold Shift

Marine mammals exposed to high-intensity sound, or to lower-intensity sound for prolonged periods, can experience hearing threshold shift, which is the loss of hearing sensitivity at certain frequency ranges after cessation of sound (Finneran, 2015). Threshold shift can be permanent (PTS), in which case the loss of hearing sensitivity is not fully recoverable, or temporary (TTS), in which case the animal's hearing threshold would recover over time (Southall

et al.,

2007). 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). While there is no simple functional relationship between TTS and PTS or other auditory injury (

e.g.,

neural degeneration), as TTS increases, the likelihood that additional exposure sound pressure level (SPL) or duration will result in PTS or other injury also increases (see also the 2020 GOA DSEIS/OEIS for additional discussion). Exposure thresholds for the onset of PTS or other auditory injury are defined by the amount of sound energy that results in 40 dB of TTS. This value is informed by experimental data, and is used as a proxy for the onset of auditory injury;

i.e.,

it is assumed that exposures beyond those capable of causing 40 dB of TTS have the potential to result in PTS or other auditory injury (

e.g.,

loss of cochlear neuron synapses, even in the absence of PTS). In severe cases of PTS, there can be total or partial deafness, while in most cases the animal has an impaired ability to hear sounds in specific frequency ranges (Kryter, 1985).

When PTS occurs, there is physical damage to the sound receptors in the ear (

i.e.,

tissue damage), whereas TTS represents primarily tissue fatigue and is reversible (Southall

et al.,

2007). PTS is permanent (

i.e.,

there is incomplete recovery back to baseline/pre-exposure levels), but also can occur in a specific frequency range and amount as mentioned above for TTS. In addition, other investigators have suggested that TTS is within the normal bounds of physiological variability and tolerance and does not represent physical injury (

e.g.,

Ward, 1997). Therefore, NMFS does not consider TTS to constitute auditory injury.

The following physiological mechanisms are thought to play a role in inducing auditory threshold shift: effects to sensory hair cells in the inner ear that reduce their sensitivity; modification of the chemical environment within the sensory cells; residual muscular activity in the middle ear; displacement of certain inner ear membranes; increased blood flow; and post-stimulatory reduction in both efferent and sensory neural output (Southall

et al.,

2007). The amplitude, duration, frequency, temporal pattern, and energy distribution of sound exposure all can affect the amount of associated threshold shift and the frequency range in which it occurs. Generally, the amount of threshold shift, and the time needed to recover from the effect, increase as amplitude and duration of sound exposure increases. Human non-impulsive noise exposure guidelines are based on the assumption that exposures of equal energy (the same sound exposure level (SEL)) produce equal amounts of hearing impairment regardless of how the sound energy is distributed in time (NIOSH, 1998). Previous marine mammal TTS studies have also generally supported this equal energy relationship (Southall

et al.,

2007). However, some more recent studies concluded that for all noise exposure situations the equal energy relationship may not be the best indicator to predict TTS onset levels (Mooney

et al.,

2009a and 2009b; Kastak

et al.,

2007). These studies highlight the inherent complexity of predicting TTS onset in marine mammals, as well as the importance of considering exposure duration when assessing potential impacts. Generally, with sound exposures of equal energy, those that were quieter (lower SPL) with longer duration were found to induce TTS onset at lower levels than those of louder (higher SPL) and shorter duration. Less threshold shift will occur from intermittent sounds than from a continuous exposure with the same energy (some recovery can occur between intermittent exposures) (Kryter

et al.,

1966; Ward, 1997; Mooney

et al.,

2009a, 2009b; Finneran

et al.,

2010). For example, one short but loud (higher SPL) sound exposure may induce the same impairment as one longer but softer (lower SPL) sound, which in turn may cause more impairment than a series of several intermittent softer sounds with the same total energy (Ward, 1997). Additionally, though TTS is temporary, very prolonged or repeated exposure to sound strong enough to elicit TTS, or shorter-term exposure to sound levels well above the TTS threshold can cause PTS, at least in terrestrial mammals (Kryter, 1985; Lonsbury-Martin

et al.,

1987).

PTS is considered auditory injury (Southall

et al.,

2007). Irreparable damage to the inner or outer cochlear hair cells may cause PTS; however, other mechanisms are also involved, such as exceeding the elastic limits of certain tissues and membranes in the middle and inner ears and resultant changes in the chemical composition of the inner ear fluids (Southall

et al.,

2007).

The NMFS Acoustic Technical Guidance (NMFS, 2018), which was used in the assessment of effects for this rule, compiled, interpreted, and synthesized the best available scientific information for noise-induced hearing effects for marine mammals to derive updated thresholds for assessing the impacts of noise on marine mammal hearing. More recently, Southall

et al.

(2019a) evaluated Southall

et al.

(2007) and used updated scientific information to propose revised noise exposure criteria to predict onset of auditory effects in marine mammals (

i.e.,

PTS and TTS onset). Southall

et al.

(2019a) note that the quantitative processes described and the resulting exposure criteria (

i.e.,

thresholds and auditory weighting functions) are largely identical to those in Finneran (2016) and NMFS (2018). They only differ in that the Southall

et al.

(2019a) exposure criteria are more broadly applicable as they include all marine mammal species (rather than only those under NMFS jurisdiction) for all noise exposures (both in air and underwater for amphibious species) and, while the hearing group compositions are identical, they renamed the hearing groups. Southall

et al.

(2021) updated the behavioral response severity criteria laid out in Southall

et al.

(2007) and included recommendations on how to present and score behavioral responses in future work.

Many studies have examined noise-induced hearing loss in marine mammals (see Finneran (2015) and Southall

et al.

(2019a) for summaries), however for cetaceans, published data on the onset of TTS are limited to the captive bottlenose dolphin, beluga, harbor porpoise, and Yangtze finless porpoise, and for pinnipeds in water, measurements of TTS are limited to harbor seals, elephant seals, and California sea lions. These studies examine hearing thresholds measured in marine mammals before and after exposure to intense sounds. The difference between the pre-exposure and post-exposure thresholds can then be used to determine the amount of threshold shift at various post-exposure times. NMFS has reviewed the available studies, which are summarized below (see also the 2020 GOA DSEIS/OEIS which includes additional discussion on TTS studies related to sonar and other transducers).

• The method used to test hearing may affect the resulting amount of measured TTS, with neurophysiological measures producing larger amounts of TTS compared to psychophysical measures (Finneran

et al.,

2007; Finneran, 2015).

• The amount of TTS varies with the hearing test frequency. As the exposure SPL increases, the frequency at which the maximum TTS occurs also increases (Kastelein

et al.,

2014b). For high-level exposures, the maximum TTS typically occurs one-half to one octave above the exposure frequency (Finneran

et al.,

2007; Mooney

et al.,

2009a; Nachtigall

et al.,

2004; Popov

et al.,

2011; Popov

et al.,

2013; Schlundt

et al.,

2000;

Kastelein

et al.,

2021b; Kastelien

et al.,

2022). The overall spread of TTS from tonal exposures can therefore extend over a large frequency range (

i.e.,

narrowband exposures can produce broadband (greater than one octave) TTS).

• The amount of TTS increases with exposure SPL and duration and is correlated with SEL, especially if the range of exposure durations is relatively small (Kastak

et al.,

2007; Kastelein

et al.,

2014b; Popov

et al.,

2014). As the exposure duration increases, however, the relationship between TTS and SEL begins to break down. Specifically, duration has a more significant effect on TTS than would be predicted on the basis of SEL alone (Finneran

et al.,

2010a; Kastak

et al.,

2005; Mooney

et al.,

2009a). This means if two exposures have the same SEL but different durations, the exposure with the longer duration (thus lower SPL) will tend to produce more TTS than the exposure with the higher SPL and shorter duration. In most acoustic impact assessments, the scenarios of interest involve shorter duration exposures than the marine mammal experimental data from which impact thresholds are derived; therefore, use of SEL tends to over-estimate the amount of TTS. Despite this, SEL continues to be used in many situations because it is relatively simple, more accurate than SPL alone, and lends itself easily to scenarios involving multiple exposures with different SPL.

• Gradual increases of TTS may not be directly observable with increasing exposure levels, before the onset of PTS (Reichmuth

et al.,

2019). Similarly, PTS can occur without measurable behavioral modifications (Reichmuth

et al.,

2019).

• The amount of TTS depends on the exposure frequency. Sounds at low frequencies, well below the region of best sensitivity, are less hazardous than those at higher frequencies, near the region of best sensitivity (Finneran and Schlundt, 2013). The onset of TTS—defined as the exposure level necessary to produce 6 dB of TTS (

i.e.,

clearly above the typical variation in threshold measurements)—also varies with exposure frequency. At low frequencies, onset-TTS exposure levels are higher compared to those in the region of best sensitivity. For example, for harbor porpoises exposed to one-sixth octave noise bands at 16 kHz (Kastelein

et al.,

2019f), 32 kHz (Kastelein

et al.,

2019d), 63 kHz (Kastelein

et al.,

2020a), and 88.4 kHz (Kastelein

et al.,

2020b), less susceptibility to TTS was found as frequency increased, whereas exposure frequencies below ~6.5 kHz showed an increase in TTS susceptibility as frequency increased and approached the region of best sensitivity. Kastelein

et al.

(2020b) showed a much higher onset of TTS for a 88.5 kHz exposure as compared to lower exposure frequencies (

i.e.,

16 kHz (Kastelein

et al.,

2019) 1.5 kHz and 6.5 kHz (Kastelein

et al.,

2020a)). For the 88.4 kHz test frequency, a 185 dB re 1 micropascal squared per second (µPa

2

-s) exposure resulted in 3.6 dB of TTS, and a 191 dB re 1 µPa

2

-s exposure produced 5.2 dB of TTS at 100 kHz and 5.4 dB of TTS at 125 kHz. Together, these new studies demonstrate that the criteria for high-frequency (HF) cetacean auditory impacts is likely to be conservative.

• TTS can accumulate across multiple exposures, but the resulting TTS will be less than the TTS from a single, continuous exposure with the same SEL (Finneran

et al.,

2010a; Kastelein

et al.,

2014b; Kastelein

et al.,

2015b; Mooney

et al.,

2009b). This means that TTS predictions based on the total, cumulative SEL will overestimate the amount of TTS from intermittent exposures such as sonars and impulsive sources. The importance of duty cycle in predicting the likelihood of TTS is demonstrated further in Kastelein

et al.

(2021b). The authors found that reducing the duty cycle of a sound generally reduced the potential for TTS in California sea lions, and that, further, California sea lions are more susceptible to TTS than previously believed at the 2 and 4 kHz frequencies tested.

• The amount of observed TTS tends to decrease with increasing time following the exposure; however, the relationship is not monotonic (

i.e.,

increasing exposure does not always increase TTS). The time required for complete recovery of hearing depends on the magnitude of the initial shift; for relatively small shifts recovery may be complete in a few minutes, while large shifts (

e.g.,

approximately 40 dB) may require several days for recovery. Recovery times are consistent for similar-magnitude TTS, regardless of the type of fatiguing sound exposure (impulsive, continuous noise band, or sinusoidal wave; (Kastelein

et al.,

2019e)). Under many circumstances TTS recovers linearly with the logarithm of time (Finneran

et al.,

2010a, 2010b; Finneran and Schlundt, 2013; Kastelein

et al.,

2012a; Kastelein

et al.,

2012b; Kastelein

et al.,

2013a; Kastelein

et al.,

2014b; Kastelein

et al.,

2014c; Popov

et al.,

2011; Popov

et al.,

2013; Popov

et al.,

2014). This means that for each doubling of recovery time, the amount of TTS will decrease by the same amount (

e.g.,

6 dB recovery per doubling of time). Please see Section 3.8.3.1.1.2 of the 2020 GOA DSEIS/OEIS for discussion of additional threshold shift literature.

Nachtigall

et al.

(2018) and Finneran (2018) describe the measurements of hearing sensitivity of multiple odontocete species (bottlenose dolphin, harbor porpoise, beluga, and false killer whale) when a relatively loud sound was preceded by a warning sound. These captive animals were shown to reduce hearing sensitivity when warned of an impending intense sound. Based on these experimental observations of captive animals, the authors suggest that wild animals may dampen their hearing during prolonged exposures or if conditioned to anticipate intense sounds. Another study showed that echolocating animals (including odontocetes) might have anatomical specializations that might allow for conditioned hearing reduction and filtering of low-frequency ambient noise, including increased stiffness and control of middle ear structures and placement of inner ear structures (Ketten

et al.,

2021). Finneran recommends further investigation of the mechanisms of hearing sensitivity reduction in order to understand the implications for interpretation of existing TTS data obtained from captive animals, notably for considering TTS due to short duration, unpredictable exposures.

Marine mammal hearing plays a critical role in communication with conspecifics and in 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, similar to those discussed in auditory masking below. 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 takes place 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 if it were in the same frequency band as the necessary vocalizations and of a severity that impeded communication. Animals exposed to high levels of sound that would be expected to result in this physiological response would also be expected to have behavioral responses of a

comparatively more severe or sustained nature, which is potentially more significant than simple existence of a TTS. However, it is important to note that TTS could occur due to longer exposures to sound at lower levels so that a behavioral response may not be elicited.

Depending on the degree and frequency range, the effects of PTS on an animal could also range in severity, although it is considered generally more serious than TTS because it is a permanent condition. Of note, reduced hearing sensitivity as a simple function of aging has been observed in marine mammals, as well as humans and other taxa (Southall

et al.,

2007), so we can infer that strategies exist for coping with this condition to some degree, though likely not without some cost to the animal.

Acoustically-Induced Bubble Formation Due to Sonars and Other Pressure-Related Impacts

One theoretical cause of injury to marine mammals is rectified diffusion (Crum and Mao, 1996), the process of increasing the size of a bubble by exposing it to a sound field. This process could be facilitated if the environment in which the ensonified bubbles exist is supersaturated with gas. Repetitive diving by marine mammals can cause the blood and some tissues to accumulate gas to a greater degree than is supported by the surrounding environmental pressure (Ridgway and Howard, 1979). The deeper and longer dives of some marine mammals (for example, beaked whales) are theoretically predicted to induce greater supersaturation (Houser

et al.,

2001b). If rectified diffusion were possible in marine mammals exposed to high-level sound, conditions of tissue supersaturation could theoretically speed the rate and increase the size of bubble growth. Subsequent effects due to tissue trauma and emboli would presumably mirror those observed in humans suffering from decompression sickness.

It is unlikely that the short duration (in combination with the source levels) of sonar pings would be long enough to drive bubble growth to any substantial size, if such a phenomenon occurs. However, an alternative but related hypothesis has also been suggested: stable bubbles could be destabilized by high-level sound exposures such that bubble growth then occurs through static diffusion of gas out of the tissues. In such a scenario the marine mammal would need to be in a gas-supersaturated state for a long enough period of time for bubbles to become of a problematic size. Recent research with

ex vivo

supersaturated bovine tissues suggested that, for a 37 kHz signal, a sound exposure of approximately 215 dB referenced to (re) 1 μPa would be required before microbubbles became destabilized and grew (Crum

et al.,

2005). Assuming spherical spreading loss and a nominal sonar source level of 235 dB re: 1 μPa at 1 m, a whale would need to be within 10 m (33 ft) of the sonar dome to be exposed to such sound levels. Furthermore, tissues in the study were supersaturated by exposing them to pressures of 400-700 kilopascals for periods of hours and then releasing them to ambient pressures. Assuming the equilibration of gases with the tissues occurred when the tissues were exposed to the high pressures, levels of supersaturation in the tissues could have been as high as 400-700 percent. These levels of tissue supersaturation are substantially higher than model predictions for marine mammals (Houser

et al.,

2001; Saunders

et al.,

2008). It is improbable that this mechanism is responsible for stranding events or traumas associated with beaked whale strandings because both the degree of supersaturation and exposure levels observed to cause microbubble destabilization are unlikely to occur, either alone or in concert.

Yet another hypothesis (decompression sickness) has speculated that rapid ascent to the surface following exposure to a startling sound might produce tissue gas saturation sufficient for the evolution of nitrogen bubbles (Jepson

et al.,

2003; Fernandez

et al.,

2005; Fernández

et al.,

2012). In this scenario, the rate of ascent would need to be sufficiently rapid to compromise behavioral or physiological protections against nitrogen bubble formation. Alternatively, Tyack

et al.

(2006) studied the deep diving behavior of beaked whales and concluded that: “Using current models of breath-hold diving, we infer that their natural diving behavior is inconsistent with known problems of acute nitrogen supersaturation and embolism.” Collectively, these hypotheses can be referred to as “hypotheses of acoustically mediated bubble growth.”

Although theoretical predictions suggest the possibility for acoustically mediated bubble growth, there is considerable disagreement among scientists as to its likelihood (Piantadosi and Thalmann, 2004; Evans and Miller, 2003; Cox

et al.,

2006; Rommel

et al.,

2006). Crum and Mao (1996) hypothesized that received levels would have to exceed 190 dB in order for there to be the possibility of significant bubble growth due to supersaturation of gases in the blood (

i.e.,

rectified diffusion). Work conducted by Crum

et al.

(2005) demonstrated the possibility of rectified diffusion for short duration signals, but at SELs and tissue saturation levels that are highly improbable to occur in diving marine mammals. To date, energy levels (ELs) predicted to cause in vivo bubble formation within diving cetaceans have not been evaluated (NOAA, 2002b). Jepson

et al.

(2003, 2005) and Fernandez

et al.

(2004, 2005, 2012) concluded that in vivo bubble formation, which may be exacerbated by deep, long-duration, repetitive dives may explain why beaked whales appear to be relatively vulnerable to MF/HF sonar exposures. It has also been argued that traumas from some beaked whale strandings are consistent with gas emboli and bubble-induced tissue separations (Jepson

et al.,

2003); however, there is no conclusive evidence of this (Rommel

et al.,

2006). Based on examination of sonar-associated strandings, Bernaldo de Quiros

et al.

(2019) list diagnostic features, the presence of all of which suggest gas and fat embolic syndrome for beaked whales stranded in association with sonar exposure.

As described in additional detail in the Nitrogen Decompression subsection of the 2020 GOA DSEIS/OEIS, marine mammals generally are thought to deal with nitrogen loads in their blood and other tissues, caused by gas exchange from the lungs under conditions of high ambient pressure during diving, through anatomical, behavioral, and physiological adaptations (Hooker

et al.,

2012). Although not a direct injury, variations in marine mammal diving behavior or avoidance responses have been hypothesized to result in nitrogen off-gassing in super-saturated tissues, possibly to the point of deleterious vascular and tissue bubble formation (Hooker

et al.,

2012; Jepson

et al.,

2003; Saunders

et al.,

2008) with resulting symptoms similar to decompression sickness, however the process is still not well understood.

Fahlman

et al.

(2021) explained how stress can have a critical role in causing the gas emboli present in stranded cetaceans. The authors review decompression theory and the mechanisms dolphins have evolved to prevent high N

2

levels and gas emboli in normal conditions, and describe how, in times of high stress, the selective gas exchange hypothesis states that this mechanism can break down. In addition, circulating microparticles may be a useful biomarker for decompression stress in cetaceans. Velazquez-Wallraf

et al.

(2021) found that individual variation also has an essential role in

this condition. To validate decompression sickness observations in certain stranded cetaceans found coincident with naval activities, the study used rabbits as an experimental pathological model and found that rabbit mortalities during or immediately following decompression showed systematically distributed gas bubbles (microscopic and macroscopic), as well as emphysema and hemorrhages in multiple organs, similar to observations in the stranded cetacean mortalities. Similar findings were not found in almost half the rabbits that survived at least one hour after decompression, revealing individual variation has an essential role in this condition.

In 2009, Hooker

et al.

tested two mathematical models to predict blood and tissue tension N

2

(P

N2

) using field data from three beaked whale species: northern bottlenose whales, Cuvier's beaked whales, and Blainville's beaked whales. The researchers aimed to determine if physiology (body mass, diving lung volume, and dive response) or dive behavior (dive depth and duration, changes in ascent rate, and diel behavior) would lead to differences in P

N2

levels and thereby decompression sickness risk between species. In their study, they compared results for previously published time depth recorder data (Hooker and Baird, 1999; Baird

et al.,

2006, 2008) from Cuvier's beaked whale, Blainville's beaked whale, and northern bottlenose whale. They reported that diving lung volume and extent of the dive response had a large effect on end-dive P

N2

. Also, results showed that dive profiles had a larger influence on end-dive P

N2

than body mass differences between species. Despite diel changes (

i.e.,

variation that occurs regularly every day or most days) in dive behavior, P

N2

levels showed no consistent trend. Model output suggested that all three species live with tissue P

N2

levels that would cause a significant proportion of decompression sickness cases in terrestrial mammals. The authors concluded that the dive behavior of Cuvier's beaked whale was different from both Blainville's beaked whale and northern bottlenose whale, and resulted in higher predicted tissue and blood N

2

levels (Hooker

et al.,

2009). They also suggested that the prevalence of Cuvier's beaked whales stranding after naval sonar exercises could be explained by either a higher abundance of this species in the affected areas or by possible species differences in behavior and/or physiology related to MF active sonar (Hooker

et al.,

2009).

Bernaldo de Quiros

et al.

(2012) showed that, among stranded whales, deep diving species of whales had higher abundances of gas bubbles compared to shallow diving species. Kvadsheim

et al.

(2012) estimated blood and tissue P

N2

levels in species representing shallow, intermediate, and deep diving cetaceans following behavioral responses to sonar and their comparisons found that deep diving species had higher end-dive blood and tissue N

2

levels, indicating a higher risk of developing gas bubble emboli compared with shallow diving species. Fahlmann

et al.

(2014) evaluated dive data recorded from sperm, killer, long-finned pilot, Blainville's beaked and Cuvier's beaked whales before and during exposure to low-frequency (1-2 kHz), as defined by the authors, and mid-frequency (2-7 kHz) active sonar in an attempt to determine if either differences in dive behavior or physiological responses to sonar are plausible risk factors for bubble formation. The authors suggested that CO

2

may initiate bubble formation and growth, while elevated levels of N

2

may be important for continued bubble growth. The authors also suggest that if CO

2

plays an important role in bubble formation, a cetacean escaping a sound source may experience increased metabolic rate, CO

2

production, and alteration in cardiac output, which could increase risk of gas bubble emboli. However, as discussed in Kvadsheim

et al.

(2012), the actual observed behavioral responses to sonar from the species in their study (sperm, killer, long-finned pilot, Blainville's beaked, and Cuvier's beaked whales) did not imply any significantly increased risk of decompression sickness due to high levels of N

2.

Therefore, further information is needed to understand the relationship between exposure to stimuli, behavioral response (discussed in more detail below), elevated N

2

levels, and gas bubble emboli in marine mammals. The hypotheses for gas bubble formation related to beaked whale strandings is that beaked whales potentially have strong avoidance responses to MF active sonars because they sound similar to their main predator, the killer whale (Cox

et al.,

2006; Southall

et al.,

2007; Zimmer and Tyack, 2007; Baird

et al.,

2008; Hooker

et al.,

2009). Further investigation is needed to assess the potential validity of these hypotheses.

To summarize, while there are several hypotheses, there is little data directly connecting intense, anthropogenic underwater sounds with non-auditory physical effects in marine mammals. The available data do not support identification of a specific exposure level above which non-auditory effects can be expected (Southall

et al.,

2007) or any meaningful quantitative predictions of the numbers (if any) of marine mammals that might be affected in these ways. In addition, such effects, if they occur at all, would be expected to be limited to situations where marine mammals are exposed to high powered sounds at very close range over a prolonged period of time, which is not expected to occur based on the speed of the vessels operating sonar in combination with the speed and behavior of marine mammals in the vicinity of sonar.

Injury Due to Sonar-Induced Acoustic Resonance

An object exposed to its resonant frequency will tend to amplify its vibration at that frequency, a phenomenon called acoustic resonance. Acoustic resonance has been proposed as a potential mechanism by which a sonar or sources with similar operating characteristics could damage tissues of marine mammals. In 2002, NMFS convened a panel of government and private scientists to investigate the potential for acoustic resonance to occur in marine mammals (NOAA, 2002). They modeled and evaluated the likelihood that Navy mid-frequency sonar (2-10 kHz) caused resonance effects in beaked whales that eventually led to their stranding. The workshop participants concluded that resonance in air-filled structures was not likely to have played a primary role in the Bahamas stranding in 2000. They listed several reasons supporting this finding including (among others): tissue displacements at resonance are estimated to be too small to cause tissue damage; tissue-lined air spaces most susceptible to resonance are too large in marine mammals to have resonant frequencies in the ranges used by mid-frequency or low-frequency sonar; lung resonant frequencies increase with depth, and tissue displacements decrease with depth so if resonance is more likely to be caused at depth it is also less likely to have an affect there; and lung tissue damage has not been observed in any mass, multi-species stranding of beaked whales. The frequency at which resonance was predicted to occur in the animals' lungs was 50 Hz, well below the frequencies used by the mid-frequency sonar systems associated with the Bahamas event. The workshop participants focused on the March 2000 stranding of beaked whales in the Bahamas as high-quality data were available, but the workshop report notes that the results apply to other sonar-related stranding events. For the reasons given by the

2002 workshop participants, we do not anticipate injury due to sonar-induced acoustic resonance from the Navy's planned activities.

Physiological Stress

There is growing interest in monitoring and assessing the impacts of stress responses to sound in marine animals. Classic stress responses begin when an animal's central nervous system perceives a potential threat to its homeostasis. That perception triggers stress responses regardless of whether a stimulus actually threatens the animal; the mere perception of a threat is sufficient to trigger a stress response (Moberg, 2000; Sapolsky

et al.,

2005; Seyle, 1950). Once an animal's central nervous system perceives a threat, it mounts a biological response or defense that consists of a combination of the four general biological defense responses: behavioral responses, autonomic nervous system responses, neuroendocrine responses, or immune responses.

According to Moberg (2000), in the case of many stressors, an animal's first and sometimes most economical (in terms of biotic costs) response is behavioral avoidance of the potential stressor or avoidance of continued exposure to a stressor. An animal's second line of defense to stressors involves the sympathetic part of the autonomic nervous system and the classical “fight or flight” response which includes the cardiovascular system, the gastrointestinal system, the exocrine glands, and the adrenal medulla to produce changes in heart rate, blood pressure, and gastrointestinal activity that humans commonly associate with “stress.” These responses have a relatively short duration and may or may not have significant long-term effect on an animal's welfare.

An animal's third line of defense to stressors involves its neuroendocrine systems or sympathetic nervous systems; the system that has received the most study has been the hypothalmus-pituitary-adrenal system (also known as the HPA axis in mammals or the hypothalamus-pituitary-interrenal axis in fish and some reptiles). Unlike stress responses associated with the autonomic nervous system, virtually all neuro-endocrine 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 (Moberg, 1987; Rivier and Rivest, 1991), altered metabolism (Elasser

et al.,

2000), reduced immune competence (Blecha, 2000), and behavioral disturbance (Moberg, 1987; Blecha, 2000). Increases in the circulation of glucocorticosteroids (cortisol, corticosterone, and aldosterone in marine mammals; see Romano

et al.,

2004) have been equated with stress for many years.

The primary distinction between stress (which is adaptive and does not normally place an animal at risk) and distress is the biotic 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 biotic functions, which impairs those functions that experience the diversion. For example, when a stress response diverts energy away from growth in young animals, those animals may experience stunted growth. When a stress response diverts energy from a fetus, an animal's reproductive success and its fitness will suffer. In these cases, the animals will have entered a pre-pathological or pathological state which is called “distress” (Seyle, 1950) or “allostatic loading” (McEwen and Wingfield, 2003). This pathological state of distress will last until the animal replenishes its energetic reserves sufficiently to restore normal function. Note that these examples involved a long-term (days or weeks) stress response exposure to stimuli.

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

et al.,

1996; Hood

et al.,

1998; Jessop

et al.,

2003; Krausman

et al.,

2004; Lankford

et al.,

2005; Reneerkens

et al.,

2002; Thompson and Hamer, 2000). However, it should be noted (and as is described in additional detail in the 2020 GOA DSEIS/OEIS) that our understanding of the functions of various stress hormones (for example, cortisol), is based largely upon observations of the stress response in terrestrial mammals. Atkinson

et al.,

2015 note that the endocrine response of marine mammals to stress may not be the same as that of terrestrial mammals because of the selective pressures marine mammals faced during their evolution in an ocean environment. For example, due to the necessity of breath-holding while diving and foraging at depth, the physiological role of epinephrine and norepinephrine (the catecholamines) in marine mammals might be different than in other mammals.

Marine mammals naturally experience stressors within their environment and as part of their life histories. Changing weather and ocean conditions, exposure to disease and naturally occurring toxins, lack of prey availability, and interactions with predators all contribute to the stress a marine mammal experiences (Atkinson

et al.,

2015). Breeding cycles, periods of fasting, and social interactions with members of the same species are also stressors, although they are natural components of an animal's life history. Anthropogenic activities have the potential to provide additional stressors beyond those that occur naturally (Fair

et al.,

2014; Meissner

et al.,

2015; Rolland

et al.,

2012). Anthropogenic stressors potentially include such things as fishery interactions, pollution, tourism, and ocean noise.

Acoustically induced stress in marine mammals is not well understood. There are ongoing efforts to improve our understanding of how stressors impact marine mammal populations (

e.g.,

King

et al.,

2015; New

et al.,

2013a; New

et al.,

2013b; Pirotta

et al.,

2015a), however little data exist on the consequences of sound-induced stress response (acute or chronic). Factors potentially affecting a marine mammal's response to a stressor include the individual's life history stage, sex, age, reproductive status, overall physiological and behavioral plasticity, and whether they are naïve or experienced with the sound (

e.g.,

prior experience with a stressor may result in a reduced response due to habituation (Finneran and Branstetter, 2013; St. Aubin and Dierauf, 2001). Stress responses due to exposure to anthropogenic sounds or other stressors and their effects on marine mammals have 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).

Other research has also investigated the impact from vessels (both whale-watching and general vessel traffic noise), and demonstrated impacts do occur (Bain, 2002; Erbe, 2002; Lusseau, 2006; Williams

et al.,

2006; Williams

et al.,

2009; Noren

et al.,

2009; Read

et al.,

2014; Rolland

et al.,

2012; Skarke

et al.,

2014; Williams

et al.,

2013; Williams

et al.,

2014a; Williams

et al.,

2014b; Pirotta

et al.,

2015b). This body of research has generally investigated impacts associated with the presence of chronic stressors, which differ significantly from the proposed Navy training activities in the GOA Study Area. For example, in an analysis of energy costs to killer whales, Williams

et al.

(2009) suggested that whale-watching in Canada's Johnstone Strait resulted in lost feeding opportunities due to vessel disturbance, which could carry higher costs than other measures of behavioral change might suggest. Ayres

et al.

(2012) reported on research in the Salish Sea (Washington state) involving the measurement of southern resident killer whale fecal hormones to assess two potential threats to the species recovery: lack of prey (salmon) and impacts to behavior from vessel traffic. Ayres

et al.

(2012) suggested that the lack of prey overshadowed any population-level physiological impacts on southern resident killer whales from vessel traffic. In a conceptual model developed by the Population Consequences of Acoustic Disturbance (PCAD) working group, serum hormones were identified as possible indicators of behavioral effects that are translated into altered rates of reproduction and mortality (NRC, 2005). The Office of Naval Research hosted a workshop (Effects of Stress on Marine Mammals Exposed to Sound) in 2009 that focused on this topic (ONR, 2009). Ultimately, the PCAD working group issued a report (Cochrem, 2014) that summarized information compiled from 239 papers or book chapters relating to stress in marine mammals and concluded that stress responses can last from minutes to hours and, while we typically focus on adverse stress responses, stress response is part of a natural process to help animals adjust to changes in their environment and can also be either neutral or beneficial.

Most sound-induced stress response studies in marine mammals have focused on acute responses to sound either by measuring catecholamines or by measuring heart rate as an assumed proxy for an acute stress response. Belugas demonstrated no catecholamine response to the playback of oil drilling sounds (Thomas

et al.,

1990) but showed a small but statistically significant increase in catecholamines following exposure to impulsive sounds produced from a seismic water gun (Romano

et al.,

2004). A bottlenose dolphin exposed to the same seismic water gun signals did not demonstrate a catecholamine response, but did demonstrate a statistically significant elevation in aldosterone (Romano

et al.,

2004), albeit the increase was within the normal daily variation observed in this species (St. Aubin

et al.,

1996). Increases in heart rate were observed in bottlenose dolphins to which known calls of other dolphins were played, although no increase in heart rate was observed when background tank noise was played back (Miksis

et al.,

2001). Unfortunately, in this study, it cannot be determined whether the increase in heart rate was due to stress or an anticipation of being reunited with the dolphin to which the vocalization belonged. Similarly, a young beluga's heart rate was observed to increase during exposure to noise, with increases dependent upon the frequency band of noise and duration of exposure, and with a sharp decrease to normal or below normal levels upon cessation of the exposure (Lyamin

et al.,

2011). Spectral analysis of heart rate variability corroborated direct measures of heart rate (Bakhchina

et al.,

2017). This response might have been in part due to the conditions during testing, the young age of the animal, and the novelty of the exposure; a year later the exposure was repeated at a slightly higher received level and there was no heart rate response, indicating the beluga whale may have acclimated to the noise exposure. Kvadsheim

et al.

(2010) measured the heart rate of captive hooded seals during exposure to sonar signals and found an increase in the heart rate of the seals during exposure periods versus control periods when the animals were at the surface. When the animals dove, the normal dive-related bradycardia (decrease in heart rate) was not impacted by the sonar exposure. Elmegaard

et al.

(2021) found that sonar sweeps did not elicit a startle response in captive harbor porpoises, but initial exposures induced bradycardia, whereas impulse exposures induced startle responses without a change in heart rate. The authors suggested that the parasympathetic cardiac dive response may override any transient sympathetic response, or that diving mammals may not have the cardiac startle response seen in terrestrial mammals in order to maintain volitional cardiovascular control at depth. Similarly, Thompson

et al.

(1998) observed a rapid but short-lived decrease in heart rates in harbor and grey seals exposed to seismic air guns (cited in Gordon

et al.,

2003). Williams

et al.

(2017) monitored the heart rates of narwhals released from capture and found that a profound dive bradycardia persisted, even though exercise effort increased dramatically as part of their escape response following release. Thus, although some limited evidence suggests that tachycardia might occur as part of the acute stress response of animals that are at the surface, the dive bradycardia persists during diving and might be enhanced in response to an acute stressor. Yang

et al.

(2021) measured cortisol concentrations in two bottlenose dolphins and found significantly higher concentrations after exposure to 140 dB re 1 µPa impulsive noise playbacks. Two out of six tested indicators of immune system function underwent acoustic dose-dependent changes, suggesting that repeated exposures or sustained stress response to impulsive sounds may increase an affected individual's susceptibility to pathogens. However, exposing dolphins to a different acoustic stressor yielded contrasting results. Houser

et al.

(2020) measured cortisol and epinephrine obtained from 30 bottlenose dolphins exposed to simulated U.S. Navy mid-frequency sonar and found no correlation between SPL and stress hormone levels. In the same experiment (Houser

et al.,

2013b), behavioral responses were shown to increase in severity with increasing received SPLs. These results suggest that behavioral reactions to sonar signals are not necessarily indicative of a hormonal stress response. Houser

et al.

(2020) notes that additional research is needed to determine the relationship between behavioral responses and physiological responses.

Despite the limited amount of data available on sound-induced stress responses for marine mammals exposed to anthropogenic sounds, studies of other marine animals and terrestrial animals would also lead us to expect that some marine mammals experience physiological stress responses and, perhaps, physiological responses that would be classified as “distress” upon exposure to high-frequency, mid-frequency, and low-frequency sounds. For example, Jansen (1998) reported on the relationship between acoustic exposures and physiological responses that are indicative of stress responses in humans

(e.g.,

elevated respiration and increased heart rates). Jones (1998) reported on reductions in human performance when faced with acute,

repetitive exposures to acoustic disturbance. Trimper

et al.

(1998) reported on the physiological stress responses of osprey to low-level aircraft noise while Krausman

et al.

(2004) reported on the auditory and physiological stress responses of endangered Sonoran pronghorn to military overflights. However, take due to aircraft noise is not anticipated as a result of the Navy's activities. Smith

et al.

(2004a, 2004b) identified noise-induced physiological transient stress responses in hearing-specialist fish (

i.e.,

goldfish) that accompanied short- and long-term hearing losses. Welch and Welch (1970) reported physiological and behavioral stress responses that accompanied damage to the inner ears of fish and several mammals.

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, or navigation) (Richardson

et al.,

1995; Erbe and Farmer, 2000; Tyack, 2000; 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. As described in detail in the 2020 GOA DSEIS/OEIS, 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. Masking these acoustic signals can disturb the behavior of individual animals, groups of animals, or entire populations. Masking can lead to behavioral changes including vocal changes (

e.g.,

Lombard effect, increasing amplitude, or changing frequency), cessation of foraging, and leaving an area, to both signalers and receivers, in an attempt to compensate for noise levels (Erbe

et al.,

2016).

In humans, significant masking of tonal signals occurs as a result of exposure to noise in a narrow band of similar frequencies. As the sound level increases, though, the detection of frequencies above those of the masking stimulus decreases also. This principle is expected to apply to marine mammals as well because of common biomechanical cochlear properties across taxa.

Under certain circumstances, marine mammals experiencing significant masking could also be impaired from maximizing their performance fitness in survival and reproduction. Therefore, when the coincident (masking) sound is man-made, it may be considered harassment when disrupting natural behavioral patterns to the point where the behavior is abandoned or significantly altered. It is important to distinguish TTS and PTS, which persist after the sound exposure, from masking, which only occurs during the sound exposure. Because masking (without resulting in threshold shift) is not associated with abnormal physiological function, it is not considered a physiological effect, but rather a potential behavioral effect.

Richardson

et al.

(1995b) argued that the maximum radius of influence of an industrial noise (including broadband low-frequency sound transmission) on a marine mammal is the distance from the source to the point at which the noise can barely be heard. This range is determined by either the hearing sensitivity (including critical ratios, or the lowest signal-to-noise ratio in which animals can detect a signal, Finneran and Branstetter, 2013; Johnson

et al.,

1989; Southall

et al.,

2000) of the animal or the background noise level present. Industrial masking is most likely to affect some species' ability to detect communication calls and natural sounds (

i.e.,

surf noise, prey noise,

etc.;

Richardson

et al.,

1995).

The frequency range of the potentially masking sound is important in determining 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; Matthews

et al.,

2016) 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 can be reduced 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).

The echolocation calls of toothed whales are subject to masking by high-frequency sound. Human data indicate low-frequency sound can mask high-frequency sounds (

i.e.,

upward masking). Studies on captive odontocetes by Au

et al.

(1974, 1985, 1993) indicate that some species may use various processes to reduce masking effects (

e.g.,

adjustments in echolocation call intensity or frequency as a function of background noise conditions). There is also evidence that the directional hearing abilities of odontocetes are useful in reducing masking at the high-frequencies these cetaceans use to echolocate, but not at the low-to-moderate frequencies they use to communicate (Zaitseva

et al.,

1980). A study by Nachtigall and Supin (2018) showed that false killer whales adjust their hearing to compensate for ambient sounds and the intensity of returning echolocation signals.

Impacts on signal detection, measured by masked detection thresholds, are not the only important factors to address when considering the potential effects of masking. As marine mammals use sound to recognize conspecifics, prey, predators, or other biologically significant sources (Branstetter

et al.,

2016), it is also important to understand the impacts of masked recognition thresholds (often called “informational masking”). Branstetter

et al.,

2016 measured masked recognition thresholds for whistle-like sounds of bottlenose dolphins and observed that they are approximately 4 dB above detection thresholds (energetic masking) for the same signals. Reduced ability to recognize a conspecific call or the acoustic signature of a predator could have severe negative impacts. Branstetter

et al.,

2016 observed that if “quality communication” is set at 90 percent recognition the output of communication space models (which are based on 50 percent detection) would likely result in a significant decrease in communication range.

As marine mammals use sound to recognize predators (Allen

et al.,

2014; Cummings and Thompson, 1971; Curé

et al.,

2015; Fish and Vania, 1971), the presence of masking noise may also prevent marine mammals from responding to acoustic cues produced by their predators, particularly if it occurs in the same frequency band. For example, harbor seals that reside in the coastal waters off British Columbia are frequently targeted by mammal-eating killer whales. The seals acoustically discriminate between the calls of mammal-eating and fish-eating killer whales (Deecke

et al.,

2002), a capability that should increase survivorship while reducing the energy required to attend to all killer whale calls. Similarly, sperm whales (Curé

et al.,

2016; Isojunno

et al.,

2016), long-finned pilot whales (Visser

et al.,

2016), and humpback whales (Curé

et al.,

2015) changed their behavior in response to killer whale vocalization playbacks; these findings indicate that some recognition of predator cues could be missed if the killer whale vocalizations were masked. The potential effects of masked predator acoustic cues depends on the duration of the masking noise and the likelihood of a marine mammal encountering a predator during the time that detection and recognition of predator cues are impeded.

Redundancy and context can also facilitate detection of weak signals. These phenomena may help marine mammals detect weak sounds in the presence of natural or manmade noise. Most masking studies in marine mammals present the test signal and the masking noise from the same direction. The dominant background noise may be highly directional if it comes from a particular anthropogenic source such as a ship or industrial site. Directional hearing may significantly reduce the masking effects of these sounds by improving the effective signal-to-noise ratio.

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

e.g.,

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

Impaired Communication

In addition to making it more difficult for animals to perceive and recognize acoustic cues in their environment, anthropogenic sound presents separate challenges for animals that are vocalizing. When they vocalize, animals are aware of environmental conditions that affect the “active space” (or communication space) of their vocalizations, which is the maximum area within which their vocalizations can be detected before it drops to the level of ambient noise (Brenowitz, 2004; Brumm

et al.,

2004; Lohr

et al.,

2003). Animals are also aware of environmental conditions that affect whether listeners can discriminate and recognize their vocalizations from other sounds, which is more important than simply detecting that a vocalization is occurring (Brenowitz, 1982; Brumm

et al.,

2004; Dooling, 2004, Marten and Marler, 1977; Patricelli

et al.,

2006). Anthropogenic sounds that reduce the signal-to-noise ratio of animal vocalizations, increase the masked auditory thresholds of animals listening for such vocalizations, or reduce the active space of an animal's vocalizations, impair communication between animals. Most species that vocalize have evolved with an ability to make adjustments to their vocalizations to increase the signal-to-noise ratio, active space, and recognizability/distinguishability of their vocalizations in the face of temporary changes in background noise (Brumm

et al.,

2004; Patricelli

et al.,

2006). Vocalizing animals can make adjustments to vocalization characteristics such as the frequency structure, amplitude, temporal structure, and temporal delivery (repetition rate), or may cease to vocalize.

Many animals will combine several of these strategies to compensate for high levels of background noise. Although the fitness consequences of vocal adjustments are not directly known in all instances, like most other trade-offs animals must make, some of these strategies probably come at a cost (Patricelli

et al.,

2006). Shifting songs and calls to higher frequencies may also impose energetic costs (Lambrechts, 1996). For example, in birds, vocalizing more loudly in noisy environments may have energetic costs that decrease the net benefits of vocal adjustment and alter a bird's energy budget (Brumm, 2004; Wood and Yezerinac, 2006).

Marine mammals are also known to make vocal changes in response to anthropogenic noise. In cetaceans, vocalization changes have been reported from exposure to anthropogenic noise sources such as sonar, vessel noise, and seismic surveying (see the following for examples: Gordon

et al.,

2003; Di Iorio and Clark, 2010; Hatch

et al.,

2012; Holt

et al.,

2008; Holt

et al.,

2011; Lesage

et al.,

1999; McDonald

et al.,

2009; Parks

et al.,

2007, Risch

et al.,

2012, Rolland

et al.,

2012), as well as changes in the natural acoustic environment (Caruso

et al.,

2020; Dunlop

et al.,

2014; Helble

et al.,

2020). Vocal changes can be temporary, or can be persistent. For example, model simulation suggests that the increase in starting frequency for the North Atlantic right whale upcall over the last 50 years resulted in increased detection ranges between right whales. The frequency shift, coupled with an increase in call intensity by 20 dB, led to a call detectability range of less than 3 km to over 9 km (Tennessen and Parks, 2016). Holt

et al.

(2008) measured killer whale call source levels and background noise levels in the one to 40 kHz band and reported that the whales increased their call source levels by one dB SPL for every one dB SPL increase in background noise level. Similarly, another study on St. Lawrence River belugas reported a similar rate of increase in vocalization activity in response to passing vessels (Scheifele

et al.,

2005). Di Iorio and Clark (2010) showed that blue whale calling rates vary in association with seismic sparker survey activity, with whales calling more on days with surveys than on days without surveys. They suggested that the whales called more during seismic survey periods as a way to compensate for the elevated noise conditions.

In some cases, these vocal changes may have fitness consequences, such as an increase in metabolic rates and oxygen consumption, as observed in bottlenose dolphins when increasing their call amplitude (Holt

et al.,

2015). A switch from vocal communication to physical, surface-generated sounds such as pectoral fin slapping or breaching was observed for humpback whales in the presence of increasing natural background noise levels, indicating that adaptations to masking may also move beyond vocal modifications (Dunlop

et al.,

2010).

While these changes all represent possible tactics by the sound-producing animal to reduce the impact of masking, the receiving animal can also reduce masking by using active listening strategies such as orienting to the sound source, moving to a quieter location, or reducing self-noise from hydrodynamic flow by remaining still. The temporal structure of noise (

e.g.,

amplitude modulation) may also provide a considerable release from masking through comodulation masking release (a reduction of masking that occurs when broadband noise, with a frequency spectrum wider than an animal's auditory filter bandwidth at the

frequency of interest, is amplitude modulated) (Branstetter and Finneran, 2008; Branstetter

et al.,

2013). Signal type (

e.g.,

whistles, burst-pulse, sonar clicks) and spectral characteristics (

e.g.,

frequency modulated with harmonics) may further influence masked detection thresholds (Branstetter

et al.,

2016; Cunningham

et al.,

2014).

Masking Due to Sonar and Other Transducers

The functional hearing ranges of mysticetes, odontocetes, and pinnipeds underwater overlap the frequencies of the sonar sources used in the Navy's low-frequency active sonar (LFAS)/mid-frequency active sonar (MFAS)/high-frequency active sonar (HFAS) training exercises (though the Navy proposes no LFAS use for the activities in this rulemaking). Additionally, almost all affected species' vocal repertoires span across the frequencies of these sonar sources used by the Navy. The closer the characteristics of the masking signal to the signal of interest, the more likely masking is to occur. Masking by mid-frequency active sonar (MFAS) with relatively low-duty cycles is not anticipated (or would be of very short duration) for most cetaceans as sonar signals occur over a relatively short duration and narrow bandwidth (overlapping with only a small portion of the hearing range). While dolphin whistles and MFAS are similar in frequency, masking is not anticipated (or would be of very short duration) due to the low-duty cycle of most sonars.

As described in the 2020 GOA DSEIS/OEIS, newer high-duty cycle or continuous active sonars have more potential to mask vocalizations. These sonars transmit more frequently (greater than 80 percent duty cycle) than traditional sonars, but at a substantially lower source level. HFAS, such as pingers that operate at higher repetition rates (

e.g.,

2-10 kHz with harmonics up to 19 kHz, 76 to 77 pings per minute) (Culik

et al.,

2001), also operate at lower source levels and have faster attenuation rates due to the higher frequencies used. These lower source levels limit the range of impacts, however compared to traditional sonar systems, individuals close to the source are likely to experience masking at longer time scales. The frequency range at which high-duty cycle systems operate overlaps the vocalization frequency of many mid-frequency cetaceans. Continuous noise at the same frequency of communicative vocalizations may cause disruptions to communication, social interactions, acoustically mediated cooperative behaviors, and important environmental cues. There is also the potential for the mid-frequency sonar signals to mask important environmental cues (

e.g.,

predator or conspecific acoustic cues), possibly affecting survivorship for targeted animals. Masking due to high duty cycle sonars is likely analogous to masking produced by other continuous sources (

e.g.,

vessel noise and low-frequency cetaceans), and would likely have similar short-term consequences, though longer in duration due to the duration of the masking noise. A study by von Benda-Beckmann

et al.

(2021) modeled the effect of pulsed and continuous 1-2 kHz active sonar on sperm whale echolocation clicks, and found that the presence of upper harmonics in the sonar signal increased masking of clicks produced in the search phase of foraging compared to buzz clicks produced during prey capture. Different levels of sonar caused intermittent to continuous masking (120 to 160 dB re 1 μPa2, respectively), but varied based on click level, whale orientation, and prey target strength. Continuous active sonar resulted in a greater percentage of time that echolocation clicks were masked compared to pulsed active sonar. Other short-term consequences may include changes to vocalization amplitude and frequency (Brumm and Slabbekoorn, 2005; Hotchkin and Parks, 2013) and behavioral impacts such as avoidance of the area and interruptions to foraging or other essential behaviors (Gordon

et al.,

2003; Isojunno

et al.,

2021). Long-term consequences could include changes to vocal behavior and vocalization structure (Foote

et al.,

2004; Parks

et al.,

2007), abandonment of habitat if masking occurs frequently enough to significantly impair communication (Brumm and Slabbekoorn, 2005), a potential decrease in survivorship if predator vocalizations are masked (Brumm and Slabbekoorn, 2005), and a potential decrease in recruitment if masking interferes with reproductive activities or mother-calf communication (Gordon

et al.,

2003).

Masking Due to Vessel Noise

Masking is more likely to occur in the presence of broadband, relatively continuous noise sources such as vessels. Several studies have shown decreases in marine mammal communication space and changes in behavior as a result of the presence of vessel noise. For example, right whales were 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) as well as increasing the amplitude (intensity) of their calls (Parks, 2009; Parks

et al.,

2011). Fournet

et al.

(2018) observed that humpback whales in Alaska responded to increasing ambient sound levels (natural and anthropogenic) by increasing the source levels of their calls (non-song vocalizations). Clark

et al.

(2009) also observed that right whales communication space decreased by up to 84 percent in the presence of vessels (Clark

et al.,

2009). Cholewiak

et al.

(2018) also observed loss in communication space in Stellwagen National Marine Sanctuary for North Atlantic right whales, fin whales, and humpback whales with increased ambient noise and shipping noise. Gabriele

et al.

(2018) modeled the effects of vessel traffic sound on communication space in Glacier Bay National Park in Alaska and found that typical summer vessel traffic in the National Park causes losses of communication space to singing whales (reduced by 13-28 percent), calling whales (18-51 percent), and roaring seals (32-61 percent), particularly during daylight hours and even in the absence of cruise ships. Dunlop (2019) observed that an increase in vessel noise reduced modelled communication space and resulted in significant reduction in group social interactions in Australian humpback whales. However, communication signal masking did not fully explain this change in social behavior in the model, indicating there may also be an additional effect of the physical presence of the vessel on social behavior (Dunlop, 2019). Although humpback whales off Australia did not change the frequency or duration of their vocalizations in the presence of ship noise, their source levels were lower than expected based on source level changes to wind noise, potentially indicating some signal masking (Dunlop, 2016). Multiple delphinid species have also been shown to increase the minimum or maximum frequencies of their whistles in the presence of anthropogenic noise and reduced communication space (for examples see: Holt

et al.,

2008; Holt

et al.,

2011; Gervaise

et al.,

2012; Williams

et al.,

2013; Hermannsen

et al.,

2014; Papale

et al.,

2015; Liu

et al.,

2017; Pine

et al.,

2021).

Behavioral Response/Disturbance

Behavioral responses to sound are highly variable and context-specific. Many different variables can influence an animal's perception of and response to (nature and magnitude) an acoustic event. An animal's prior experience with a sound or sound source affects whether it is less likely (habituation) or more likely (sensitization) to respond to certain sounds in the future (animals

can also be innately predisposed to respond to certain sounds in certain ways) (Southall

et al.,

2007). Related to the sound itself, the perceived nearness of the sound, bearing of the sound (approaching vs. retreating), the similarity of a sound to biologically relevant sounds in the animal's environment (

i.e.,

calls of predators, prey, or conspecifics), and familiarity of the sound may affect the way an animal responds to the sound (Southall

et al.,

2007; DeRuiter

et al.,

2013). Individuals (of different age, gender, reproductive status,

etc.

) among most populations will have variable hearing capabilities, and differing behavioral sensitivities to sounds that will be affected by prior conditioning, experience, and current activities of those individuals. Often, specific acoustic features of the sound and contextual variables (

i.e.,

proximity, duration, or recurrence of the sound, or the current behavior that the marine mammal is engaged in or its prior experience), as well as entirely separate factors such as the physical presence of a nearby vessel, may be more relevant to the animal's response than the received level alone. For example, Goldbogen

et al.

(2013) demonstrated that individual behavioral state was critically important in determining response of blue whales to sonar, noting that some individuals engaged in deep (>50 m) feeding behavior had greater dive responses than those in shallow feeding or non-feeding conditions. Some blue whales in the Goldbogen

et al.

(2013) study that were engaged in shallow feeding behavior demonstrated no clear changes in diving or movement even when received levels (RLs) were high (~160 dB re: 1µPa) for exposures to 3-4 kHz sonar signals, while others showed a clear response at exposures at lower received levels of sonar and pseudorandom noise.

Studies by DeRuiter

et al.

(2012) indicate that variability of responses to acoustic stimuli depends not only on the species receiving the sound and the sound source, but also on the social, behavioral, or environmental contexts of exposure. Another study by DeRuiter

et al.

(2013) examined behavioral responses of Cuvier's beaked whales to MF sonar and found that whales responded strongly at low received levels (RL of 89-127 dB re: 1µPa) by ceasing normal fluking and echolocation, swimming rapidly away, and extending both dive duration and subsequent non-foraging intervals when the sound source was 3.4-9.5 km away. Importantly, this study also showed that whales exposed to a similar range of received levels (78-106 dB re: 1 µPa) from distant sonar exercises (118 km away) did not elicit such responses, suggesting that context may moderate reactions.

Ellison

et al.

(2012) outlined an approach to assessing the effects of sound on marine mammals that incorporates contextual-based factors. The authors recommend considering not just the received level of sound, but also the activity the animal is engaged in at the time the sound is received, the nature and novelty of the sound (

i.e.,

is this a new sound from the animal's perspective), and the distance between the sound source and the animal. They submit that this “exposure context,” as described, greatly influences the type of behavioral response exhibited by the animal. Forney

et al.

(2017) also point out that an apparent lack of response (

e.g.,

no displacement or avoidance of a sound source) may not necessarily mean there is no cost to the individual or population, as some resources or habitats may be of such high value that anima

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