# Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to U.S. Navy Operations of Surveillance Towed Array Sensor System Low Frequency Active Sonar in the Western and Central North Pacific Ocean and Eastern Indian Ocean

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URL: https://www.frixlaw.com/law-library/documents/fr%3A2026-04668

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** March 10, 2026
- **Citation:** 91 FR 11618

## Text

DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
50 CFR Part 218
[Docket No. 260304-0065]
RIN 0648-BN61
Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to U.S. Navy Operations of Surveillance Towed Array Sensor System Low Frequency Active Sonar in the Western and Central North Pacific Ocean and Eastern Indian Ocean

AGENCY:

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

ACTION:

Proposed rule; proposed letter of authorization; request for comments.

SUMMARY:

NMFS has received a request from the U.S. Department of the Navy (Navy) for Incidental Take Regulations (ITR) and an associated Letter of Authorization (LOA) pursuant to the Marine Mammal Protection Act (MMPA). The requested regulations would govern the authorization of take of marine mammals incidental to training and testing activities using Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) sonar systems in the western and central North Pacific and eastern Indian oceans over the course of 7 years from August 2026 through August 2033. NMFS requests comments on this proposed rule. NMFS will consider public comments prior to making any final decision on the promulgation of the requested ITR and issuance of the LOA; agency responses to public comments will be summarized in the final rule, if issued. The Navy's activities are considered military readiness activities pursuant to the MMPA, as amended by the National Defense Authorization Act for Fiscal Year 2004 (2004 NDAA) and the NDAA for Fiscal Year 2019 (2019 NDAA).

DATES:

Comments and information must be received no later than April 9, 2026.

ADDRESSES:

A plain language summary of this proposed rule is available at:
https://www.regulations.gov/docket/NOAA-NMFS-2025-0999.
You may submit comments on this document, identified by NOAA-NMFS-2025-0999, by any of the following methods:

•
Electronic Submission:
Submit all electronic public comments via the Federal e-Rulemaking Portal. Visit
https://www.regulations.gov
and type NOAA-NMFS-2025-0999 in the Search box. Click on the “Comment” icon, complete the required fields, and enter or attach your comments.

•
Mail:
Submit written comments to Ben Laws, Incidental Take Program Supervisor, Permits and Conservation Division, Office of Protected Resources, National Marine Fisheries Service, 1315 East-West Highway, Silver Spring, MD 20910-3225.

•
Fax:
(301) 713-0376; Attn: Ben Laws.

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 at:
https://www.regulations.gov
without change. All personal identifying information (
e.g.,
name, address,
etc.
), 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 Incidental Take Authorization (ITA) application and supporting documents, as well as a list of the references cited in this document, may be obtained online at:
https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-military-readiness-activities.
In case of problems accessing these documents, please call the contact listed below (see
FOR FURTHER INFORMATION CONTACT
).

FOR FURTHER INFORMATION CONTACT:

Alyssa Clevenstine, Office of Protected Resources, NMFS, (301) 427-8401.

SUPPLEMENTARY INFORMATION:

Purpose and Need for Regulatory Action

This proposed rule, if promulgated, would provide a framework under the authority of the MMPA (16 U.S.C. 1361
et seq.
) to allow for the authorization of take of marine mammals incidental to the Navy's training and testing activities (which qualify as military readiness activities) using SURTASS LFA sonar in the western and central North Pacific Ocean and eastern Indian Ocean (see figure 2-1 of the rulemaking and LOA application (hereafter referred to as the application)). Please see the Legal Authority for the Proposed Action section for relevant definitions.

Legal Authority for the Proposed Action

The MMPA prohibits the “take” of marine mammals, with certain exceptions. Section 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361
et seq.
) directs 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, a notice of a proposed authorization is provided to the public for review and the opportunity to submit comment.

Authorization for incidental takings shall be granted if NMFS finds that the taking will have a negligible impact on the species or stock(s) and will not have an unmitigable adverse impact on the availability of the species or stock(s) for taking for subsistence uses (where relevant). Further, NMFS must prescribe the permissible methods of taking; 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 the species or stocks for taking for certain subsistence uses (collectively referred to as “mitigation”); and requirements pertaining to the monitoring and reporting of the takings. The MMPA defines “take” to mean to harass, hunt, capture, or kill, or attempt to harass, hunt, capture, or kill any marine mammal (16 U.S.C. 1362). The Preliminary Analysis and Negligible Impact Determination section discusses the definition of “negligible impact.”

The 2004 NDAA (Pub. L. 108-136) amended section 101(a)(5) of the MMPA to remove the “small numbers” and “specified geographical region” provisions (16 U.S.C. 1371(a)(5)(F)) and amended the definition of “harassment” in section 3(18)(B) of the MMPA as applied to a “military readiness activity” to read as follows: (1) 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 (2) 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) (16 U.S.C.

1362(18)(B)). The 2004 NDAA also amended the MMPA to establish in section 101(a)(5)(A)(iii) that “[f]or a military readiness activity . . . , a determination of `least practicable adverse impact' . . . shall include consideration of personnel safety, practicality of implementation, and impact on the effectiveness of the military readiness activity” (16 U.S.C. 1371(a)(5)(A)(iii)). On August 13, 2018, the 2019 NDAA (Pub. L. 115-232) amended the MMPA to allow ITRs for military readiness activities to be issued for up to 7 years (16 U.S.C. 1371(a)(5)(A)(ii)).

Summary of Major Provisions Within the Proposed Rule

The major provisions of this proposed rule are as follows:

• The proposed authorization of take of marine mammals by Level A harassment and Level B harassment;

• The proposed use of visual, passive acoustic, and active acoustic monitoring mitigation;

• The proposed implementation of geographic activity limitations including within 22 kilometers (km) (12 nautical miles (nmi)) of any emergent land and in certain offshore areas and times that are biologically important (
i.e.,
for foraging, migration, reproduction) for marine mammals;

• The proposed implementation of a Notification and Reporting Plan (for dead, live stranded, or marine mammals struck by any vessel engaged in military readiness activities); and

• The proposed implementation of a robust monitoring plan to improve our understanding of the environmental effects resulting from the Navy's training and testing activities.

This proposed rule includes an adaptive management component that allows for timely modification of mitigation, monitoring, and/or reporting measures based on new information, when appropriate.

Summary of Request

On April 6, 2025, NMFS received an application from the Navy requesting authorization to take marine mammals, by Level A and Level B harassment, incidental to training and testing activities (characterized as military readiness activities) using SURTASS LFA sonar in the western and central North Pacific Ocean and eastern Indian Ocean. The Navy is requesting one 7-year LOA for training and testing activities. In response to our comments and following an information exchange, the Navy submitted a revised application, deemed adequate and complete on July 1, 2025. The Navy's request is for take of 44 species of marine mammals by Level B harassment and a subset of those species by Level A harassment (9 species). On July 11, 2025, we published a notice of receipt (NOR) of application in the
Federal Register
(90 FR 30877), requesting comments and information related to the Navy's request for 30 days. During the 30-day public comment period on the NOR, we received one public comment from Turtle Island Restoration Network requesting that NMFS deny the Navy's ITA request and consider alternatives that prioritize avoiding critical habitats, reducing sonar intensity, or limiting operational time frames. NMFS reviewed and considered all submitted material during the drafting of this proposed rule.

NMFS has previously promulgated ITRs pursuant to the MMPA relating to similar military readiness activities using SURTASS LFA sonar. NMFS published the first rule effective August 15, 2002 through August 15, 2007 (67 FR 46712, July 16, 2002), the second rule effective from August 16, 2007 through August 15, 2012 (72 FR 46846, August 21, 2007), the third rule effective from August 15, 2012 through August 15, 2017 (77 FR 50290, August 20, 2012), and the fourth rule effective from August 12, 2019 through August 11, 2026 (84 FR 40132, August 13, 2019). Of note, on August 10, 2017, the Secretary of Defense,
1

after conferring with the Secretary of Commerce, determined that it was necessary for the national defense to exempt all military readiness activities that use SURTASS LFA sonar from compliance with the requirements of the MMPA for 2 years from August 13, 2017, through August 12, 2019, or until such time when NMFS issues regulations and a LOA under title 16, section 1371 for military readiness activities associated with the use of SURTASS LFA sonar, whichever is earlier. For this proposed rulemaking, the Navy proposes to conduct substantially similar training and testing activities using SURTASS LFA sonar that were conducted under previous rules.

1
Pursuant to Executive Order 14347, “
Restoring the United States Department of War,”
(90 FR 43893), as of September 5, 2025, the “Secretary of Defense” is authorized to use the additional secondary title of “Secretary of War.”

The Navy's application reflects the most up-to-date compilation of training and testing activities deemed necessary to accomplish military readiness requirements. The types and numbers of activities included in the proposed rule account for interannual variability in training and testing to meet evolving or emergent military readiness requirements. In this proposed rule, we have undertaken a comprehensive assessment of the impacts of all SURTASS LFA sonar training and testing activities on marine mammals likely to be present within the western and central North Pacific Ocean and eastern Indian Ocean in the area described below.

Description of Proposed Activity

Overview

The Navy requests authorization to take marine mammals incidental to conducting military readiness activities. The Navy has determined that acoustic stressors are likely to result in take of marine mammals in the form of Level A and Level B harassment. Descriptions of these activities are provided in the Navy's application (
https://www.fisheries.noaa.gov/action/incidental-take-authorization-united-states-navys-surveillance-towed-array-sensor-system-low
), with additional detail provided in chapter 2 and appendix F of the 2025 SURTASS Draft Supplemental Environmental Impact Statement/Overseas Environmental Impact Statement (2025 SURTASS Draft SEIS/OEIS) (
https://www.nepa.navy.mil/surtass-lfa/
) which are summarized here.

The Navy's statutory mission is to organize, train, equip, and maintain combat-ready naval forces for the peacetime promotion of the national security interests and prosperity of the United States, including deterring maritime aggression and maintaining freedom of navigation in ocean areas. This mission is mandated by Federal law (10 U.S.C. 8062), which requires the readiness of the naval forces of the United States. Due to the advancements and use of quieting technologies in diesel-electric and nuclear submarines, undersea submarine threats have become increasingly difficult to locate solely using passive acoustic technologies. At the same time, the distance at which submarine threats can be detected has been decreasing due to these quieting technologies, and improvements in torpedo and missile design have extended the effective range of these weapons. To meet the requirement for improved capability to detect quieter and harder-to-find foreign submarines at greater distances, the Navy developed and uses SURTASS LFA sonar.

Dates and Duration

The specified activities would occur at any time during the 7-year period of validity of the regulations, from August 12, 2026 through August 11, 2033. The

proposed number of military readiness activities are described in the Detailed Description of the Specified Activity section.

Geographic Region

The Pacific SURTASS LFA Sonar Study Area includes the western and central North Pacific Ocean and Eastern Indian Ocean, not including the Western Indian Ocean or Sea of Okhotsk (figure 1). Please refer to figure 2-1 of the application for a color map of the Study Area. The Study Area remains unchanged from the previous rulemaking (84 FR 40132, August 13, 2019) (see also the 2019 SURTASS SEIS/OEIS (U.S. Department of the Navy, 2019)).

Importantly and as described in greater detail in the Proposed Mitigation section, in areas within 22 km from any emergent land (coastal standoff range (CSR)) and in areas outside of the CSR identified as offshore biologically important areas (OBIAs), SURTASS LFA sonar training and testing would be conducted such that received levels of LFA sonar are below 180 decibels referenced to 1 microPascal (dB re 1 μPa) root-mean-square (RMS) sound pressure level (SPL). This restriction would be observed year-round for the CSR and during known periods of biological importance for OBIAs.

BILLING CODE 3510-22-P

EP10MR26.000

BILLING CODE 3510-22-C
Detailed Description of the Specified Activity

The Navy proposes to use 1,100 hours of SURTASS LFA sonar per year. The analysis for the current SURTASS LFA incidental take regulations (84 FR 40132, August 13, 2019) analyzed the use of 592 hours. The change from 592 to 1,100 hours does not reflect new or additional training requirements. Instead, it is the result of a change in how the Navy counts an “hour” of transmission. Previously, SURTASS LFA sonar hours were calculated by adding the portions of time a sonar emits sound during its duty cycle (ratio of time the signal is on compared to off), whereas other Navy sonar systems, such as mid-frequency and high-frequency active sonar (MFAS and HFAS, respectively), report hours based on “duration” time (total time the source is active, including silent periods between pings). To bring SURTASS LFA sonar in line with these other systems, the Navy

developed a conversion method that considers various factors including LFA sonar pings, wave trains, and other classified considerations. As a result, the 1,100 hours of annual SURTASS LFA training proposed are equivalent to the 592 hours under the previous counting method. The SURTASS LFA sonar transmission hours, which are classified as military readiness activities pursuant to the section 315(f) of Public Law 101-314 (16 U.S.C. 703), represent a distribution across three activities that include:

• Training (
i.e.,
contractor crew proficiency training, military crew proficiency training, active training);

• Maintenance and upgrade (
i.e.,
equipment maintenance checks and performance evaluations); and

• Exercises (
e.g.,
Valiant Shield, Rim of the Pacific (RIMPAC)).

Compared to the 2019 final rule (84 FR 40132, August 13, 2019), the geographic bounds of the Study Area and geographic mitigations remain the same, as do the operating characteristics of SURTASS LFA sonar system, which remains the only stressor with the potential to cause take during SURTASS training and testing activities. Although the sonar hours are calculated differently between the 2019 final rule and current proposed rulemaking (592 hours and 1,100 hours, respectively), the active sonar duration remains the same as analyzed for the 2019 rule. All of the acoustic thresholds and take calculation methods used here are referred to as “Phase IV” and described in the technical report “Criteria and Thresholds for U.S. Navy Acoustic and Explosive Effects Analysis (Phase 4)” (U.S. Department of the Navy, 2025) (hereafter referred to as the Criteria and Thresholds Technical Report) mirroring those used in analyses supporting the Phase IV AFTT (90 FR 50504, November 7, 2025) and HCTT (90 FR 58810, December 17, 2025) training and testing regulations. Alternatively, in the previous SURTASS LFA sonar rule (84 FR 40132, August 13, 2019), Phase III thresholds were used for acoustic injury prediction, a SURTASS-specific threshold was used to predict behavioral disturbance, and different SURTASS-specific methods and modeling were used in the calculation of take. This proposed rulemaking proposes to authorize take of marine mammals by Level A harassment that was not previously requested or authorized in the 2019 final rule. This change is due to updated marine mammal hearing thresholds, the criteria for estimating impacts to marine mammals, and the Navy's reliance on their Navy Acoustics Effects Model (NAEMO) rather than the Acoustic Integration Model (AIM), which was used to model and quantify estimated take for the 2019 rulemaking. These changes are described in the
Marine Mammal Hearing Groups
section, the Estimated Take of Marine Mammals section, and the
Navy Acoustic Effects Model
section. This proposed rule also includes five new marine mammal OBIAs for SURTASS LFA sonar, as described in the
Geographic Mitigation
section of this proposed rule.

The Navy describes and analyzes the effects of their activities within the application and provides additional details in the 2025 SURTASS Draft SEIS/OEIS. In their assessment, the Navy concluded that the transmission of acoustic signals was the only stressor likely to result in impacts on marine mammals that qualify as harassment as defined under the MMPA. Therefore, the Navy's application provides their assessment of potential effects from this stressor.

SURTASS LFA Sonar System

Sonar is an acronym for “sound navigation and ranging” and its definition includes any system that uses underwater sound or acoustics for observations and communications. The two basic types of sonar used in the SURTASS LFA sonar system are passive sonar and active sonar. Passive sonar detects sound created by a source. This is a one-way transmission of sound waves through water from the source to the receiver. Very simply, passive sonar “listens” without transmitting any sound signals. Active sonar is the transmission of sound energy for the purpose of sensing the environment by interpreting features of received signals. Active sonar detects objects by creating a sound pulse or “ping” that is transmitted from the sonar system through the water, reflects off a target object, and returns in the form of an echo to be detected by a receiver. Active sonar is a two-way transmission of sound waves through water (sound source to reflector to receiver).

SURTASS LFA sonar is a system with three components: low-frequency (less than 1,000 hertz (Hz)) active sonar system, passive sonar system, and active high-frequency/marine mammal monitoring (HF/M3) sonar (see figure 1-1 of the application). The passive component is the SURTASS receiver array while the active component includes the LFA sonar source array and HF/M3 sonar.

Although SURTASS LFA sonar vessels usually operate independently from one another, SURTASS LFA sonar vessels may operate in conjunction with other naval air, surface, or submarine assets as part of naval exercises. SURTASS LFA sonar vessels generally travel in straight lines or racetrack patterns depending on the scenario. When the SURTASS or LFA sonar arrays are deployed, a SURTASS LFA sonar vessel must maintain a speed of at least 5.6 kilometers per hour (km/hr) (3 knots), with a typical speed of 7.4 km/hr. When not towing the SURTASS or LFA sonar arrays, Tactical-Auxiliary General Ocean Surveillance (T-AGOS) vessels travel at maximum speeds of approximately 22.2 km/hr. Movements of SURTASS LFA sonar vessels are not unusual or extraordinary and are in line with routine operations of seagoing vessels.

Low-Frequency Active (LFA) Sonar

LFA sonar is employed when active sound signals are needed to detect and track underwater targets of interest. LFA sonar complements SURTASS passive activities by actively acquiring and tracking submarines when they are in quiet operating modes, measuring accurate target range, and re-acquiring lost contacts. LFA sonar consists of a vertical source array of sound-producing elements that are suspended by cable under one of the T-AGOS vessels. These elements, called projectors, are devices that produce the active sonar sound pulses or pings. To produce a ping, the projectors transform electrical energy into mechanical energy (
i.e.,
vibrations), which travel as pressure disturbances in water.

The LFA sonar source is a vertical line array consisting of up to 18 projectors. Each LFA projector transmits sonar beams that are omnidirectional (360 degrees) in the horizontal, with a narrow vertical beamwidth that can be steered above or below the horizontal. The operating features of the LFA sonar are as follows:

• The source level (SL) of an individual projector on the LFA sonar array is approximately 215 dB re 1 μPa RMS SPL or less;

• For the array SL, the effective SL of the system design was used; effective SL is a theoretical value, hypothetically measured at 1 meter (m) from the array on its horizontal axis, calculated from the formula: SEL + 20 Log10(N), where sound exposure level (SEL) = SL of an individual projector and N = number of projectors;

• The source frequency ranges from 100 to 500 Hz;

• The typical LFA sonar signal is not a constant tone but consists of various waveforms that vary in frequency and duration. A complete sequence of sound transmissions (waveforms) is referred to

as a wavetrain (also known as a ping). These wavetrains last between 6 and 100 seconds, with an average length of 60 seconds. Within each wavetrain, a variety of signal types can be used, including continuous wave and frequency-modulated signals. The duration of each continuous frequency sound transmission within the wavetrain is no longer than 10 seconds;

• The maximum duty cycle (ratio of sound “on” time to total time) is 20 percent. The typical duty cycle, based on historical SURTASS LFA sonar operational parameters (from 2003 to 2017), is 7.5-10 percent; and

• The time between wavetrain transmissions typically ranges from 6 to 15 minutes.

Compact LFA Active Component

In addition to the LFA sonar system currently deployed on the T-AGOS vessel United States Naval Ship (USNS) IMPECCABLE, the Navy developed a compact LFA (CLFA) sonar system, which is now deployed on its three smaller T-AGOS vessels (USNS ABLE, USNS EFFECTIVE, and USNS VICTORIOUS). The operational characteristics of the active component for the CLFA sonar system are comparable to the LFA sonar system and the potential impacts from the CLFA sonar system will be similar to the effects from the LFA sonar system. The CLFA sonar system consists of smaller projectors that weigh 64,410 kilograms (kg), which is 82,554 kg less than the weight of the LFA projectors on the USNS IMPECCABLE. The CLFA sonar system also consists of up to 18 projectors suspended beneath the surveillance vessel in a vertical line array, and the CLFA sonar system projectors transmit in the low-frequency band (also between 100 and 500 Hz) with the same duty cycle as described for LFA sonar. Similar to the active component of the LFA sonar system, the source level of an individual projector in the CLFA sonar array is approximately 215 dB re 1 μPa or less.

For the analysis in this rulemaking, NMFS will use the term LFA to refer to both the LFA sonar system and/or the CLFA sonar system, unless otherwise specified.

Passive Acoustic System

SURTASS is the passive, or listening, component of the system that detects returning sounds from submerged objects, such as threat submarines, through the use of hydrophones. Hydrophones transform mechanical energy (
i.e.,
received acoustic sound waves) into an electrical signal that can be analyzed by the sonar processing system. The return (received) signals, which are usually below background or ambient noise level, are processed and evaluated to identify and classify potential underwater threats. SURTASS consists of a twin-line (TL-29A), “Y” shaped horizontal line array of hydrophones with two apertures that is approximately 305 m (1,000 feet) long. The SURTASS horizontal line array can be towed in shallow, littoral environments; can provide significant directional noise rejection; and can resolve bearing ambiguities without the vessel's course having to be changed.

High-Frequency/Marine Mammal Monitoring Active (HF/M3) Sonar

The HF/M3 sonar is a Navy-developed, enhanced high-frequency (HF) commercial sonar used as a mitigation and monitoring asset to detect, locate, and track marine mammals that may pass close enough to the SURTASS LFA sonar's transmit array to enter the LFA mitigation zone (1.8 km (2,000 yards)). This intermittent source has a low-duty cycle and would operate at a SL of 220 dB re 1 µPa at 1 m RMS SPL and source frequencies between 30 and 40 kHz, with maximum pulse length of 40 milliseconds and a variable duty cycle that is nominally 3-4 percent.

Vessel Movement

The Navy currently deploys SURTASS LFA sonar on four T-AGOS vessels that are 72-86 m in length, with twin-shafted diesel electric engines capable of providing 3,200-5,000 horsepower. T-AGOS vessels have a catamaran-type split-hull shape and an enclosed propeller system, and the bridge of T-AGOS vessels are positioned forward of the centerline, offering good visibility ahead of the bow and good visibility aft to visually monitor for marine mammals. Each vessel has an observation area on the bridge that is more than 9.1 m above sea level from where Lookouts will monitor for marine mammals whenever SURTASS LFA sonar is transmitting.

NMFS considered the likelihood that vessel movement during military readiness activities could result in an incidental, but not intentional, strike of a marine mammal in the Study Area, which has the potential to result in serious injury or mortality. Vessel strikes are not specific to any specific military readiness activity but rather, a limited, sporadic, and incidental result of the Navy's vessel movement during military readiness activities within the 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; Crum
et al.,
2019; Douglas
et al.,
2008; Laggner 2009; Van der Hoop
et al.,
2012; Van der Hoop
et al.,
2013), although reviews of the literature on vessel 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 (Blondin
et al.
2025; Conn and Silber, 2013; Garrison
et al.
2025; Gende
et al.,
2011; Redfern
et al.,
2019; Silber
et al.,
2010; Szesciorka
et al.,
2019; Vanderlaan and Taggart, 2007; Wiley
et al.,
2016). For large vessels, speed and angle of approach can influence the severity of a strike.

Here, the limited number of Navy vessels operating within the Study Area, the design and operation of the T-AGOS vessels, the monitoring and mitigation utilized, and the fact that no strikes of marine mammals from vessels conducting SURTASS training and testing have occurred in the past support the conclusion that Navy vessel strikes are not expected to result from the activities covered by this proposed rule. The number of vessels used in the Navy's SURTASS testing and training activities is minimal (especially as compared to the number of commercial ships transiting in the same areas on an annual basis), rendering the base probability of a vessel strike very low. The design of the T-AGOS vessels further reduces the risk of a vessel strike, with the catamaran-type split hull shape and enclosed propeller system, a propeller system design that has been suggested as effectively reducing sharp force injuries to marine species, particularly when combined with reduced speeds (
i.e.,
5.6-7.4 km/hr) (Schoeman
et al.,
2020). Both detection and avoidance of marine mammals are more effective at slower speeds and the T-AGOS vessel's slow operational speed (averaging 7.4 km/hr), as well as the relatively slow vessel cruising speed when SURTASS LFA sonar is not in use (a maximum of approximately 22.2 km/hr), are generally below the speed at which, records suggest, that marine mammal injury or death is more common (Laist
et al.,
2001).

Surface ships operated by the Navy have multiple personnel assigned to stand watch at all times when 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 stop within a distance appropriate to the prevailing circumstances and conditions. The Navy utilizes Lookouts to avoid collisions, and Lookouts are trained to spot marine mammals so that vessels may change course or take other appropriate action to avoid collisions (for more information, see section 2.1.2 of the application).

Further, the use of HF/M3 sonar to monitor for marine mammals in tandem with Lookouts would support detection of cetaceans well in advance of any potential vessel strike during SURTASS LFA sonar training and testing activities. Proposed mitigation, monitoring, and reporting measures are described in detail later in this document (please see Proposed Mitigation Measures section, Proposed Monitoring section, and Proposed Reporting section).

Due to the reasons described above (
i.e.,
low probability of Navy vessel and marine mammal interactions, vessel design, relatively slow vessel speeds, high probability of detection and avoidance due to applied monitoring and mitigation measures), and the fact that there have been no known Navy vessel strikes in the 22-year history of SURTASS LFA sonar activities, the Navy has determined, and NMFS preliminarily concurs, that take of marine mammals by vessel strike is highly unlikely. Therefore, the Navy has not requested any take of marine mammals by vessel strike, and NMFS is not proposing to authorize take by serious injury or mortality by vessel strike.

Standard Operating Procedures

For training and testing to be effective, Navy personnel must be able to safely use their sensors, platforms, weapons, and other devices to their optimum capabilities and as intended for use in missions and combat operations. The Navy has developed standard operating procedures through decades of experience to provide for safety and mission success. Because they are essential to safety and mission success, standard operating procedures are part of the proposed activities and are considered in the environmental analysis for applicable resources (see chapter 3 (Affected Environment and Environmental Consequences) of the 2025 SURTASS Draft SEIS/OEIS). While standard operating procedures are designed for the safety of personnel and equipment and to ensure the success of training and testing activities, their implementation often yields additional benefits for environmental (
e.g.,
marine mammals), socioeconomic, public health and safety, and cultural resources.

Because standard operating procedures are essential to safety and mission success, the Navy considers them to be part of the proposed activities and has included them in the environmental analysis. Standard operating procedures that are recognized as providing a potential secondary benefit on marine mammals that apply to SURTASS LFA sonar training and testing activities include those related to the following, described in more detail in section 2.1.2 of the application:

• Vessel safety; and

• Towed in-water device safety.

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.

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

Marine mammal species and their associated stocks that have the potential to occur in the Study Area are presented in table 1 along with each stock's Endangered Species Act (ESA) and MMPA statuses, abundance estimate and associated coefficient of variation (CV) value, minimum abundance estimate, potential biological removal (PBR), annual M/SI, as applicable, and potential occurrence in the Study Area. The Navy requests authorization to take individuals of 44 species by Level B harassment and, for a subset of those species, Level A harassment (9 species), incidental to military readiness activities from the use of SURTASS LFA sonar in the Study Area. Of note, based on improvements to the Navy's density research since the 2019 SURTASS LFA Final Rule (84 FR 40132, August 13, 2019), seven more species were modeled for this proposed rulemaking than the 2019 rulemaking. Of those seven, the Navy's application includes estimated take of four species from the proposed activity that were not included in the 2019 final rule: (1) bearded seal; (2) ringed seal; (3) harbor seal; and (4) Steller sea lion. Multiple stocks of some species are affected, and independent assessments are conducted to make the necessary findings and determinations for each of these.

There are 34 stocks under NMFS' jurisdiction with confirmed or possible occurrence in the Study Area, of which 11 are listed as endangered or threatened under the ESA (16 U.S.C. 1531
et seq.
). Currently, the false killer whale (Main Hawaiian Islands Insular DPS) and Hawaiian monk seal have critical habitat designated under the ESA in the Study Area (see
Critical Habitat
section below).

The remaining species in the Central and Western Pacific (CWP) and Eastern Indian Oceans (EIO) have no stock designation (NSD) under the MMPA.

Sections 3 and 4 and appendix A (Marine Mammal Species Supplemental Information) of the application summarize available information regarding status and trends, distribution and habitat preferences, and behavior and life history of the potentially affected species. NMFS fully considered all of this information, and we refer the reader to these descriptions, instead of reprinting the information. Additional information regarding population trends and threats may be found in NMFS' Stock Assessment Reports (SARs) (
https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments
) and more general information about these species (
e.g.,
physical and behavioral descriptions) may be found on NMFS' website at:
https://www.fisheries.noaa.gov/find-species.
Additional information on the general biology and ecology of marine mammals is included in the 2025 SURTASS Draft SEIS/OEIS. Table 1 incorporates the best available science, including data from the 2023 Pacific and Alaska Marine Mammal Stock Assessment Reports (Carretta
et al.,
2024; Young
et al.,
2024) (see
https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments),
and 2024 draft SARs, as well as monitoring data from the Navy's marine mammal research efforts.

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Species Not Included in the Analysis

The species carried forward for analysis (and described in table 1) are those likely to be found in the 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). The following species were not included in the analysis.

The North Pacific right whale (NPRW) is one of the rarest marine mammals worldwide. Their historical range spanned the entire North Pacific Ocean from approximately 35 degrees north, with feeding grounds in the Bering Sea, Gulf of Alaska, Okhotsk Sea, and northwestern North Pacific, the latter of which overlaps a portion of the Study Area. Two transboundary stocks of NPRW are currently recognized: a Western North Pacific stock feeding primarily in the Sea of Okhotsk and an Eastern North Pacific (ENP) stock feeding primarily in the southeastern Bering Sea (Rosenbaum
et al.,
2000; Brownell
et al.,
2001; LeDuc
et al.,
2012; Pastene
et al.,
2022). The summer range of the ENP stock includes the Gulf of Alaska and Bering Sea but the winter calving grounds for both stocks are unknown, as they likely migrate out of the Bering Sea during winter months (Wright, 2017). Recent sightings of NPRW have occurred in the eastern Bering Sea, southeastern Bering Sea, northern Bering Sea, and British Columbia (Little, 2021). Acoustic detections of NPRW have also occurred in the southeastern Bering Sea, Gulf of Alaska, and the eastern Aleutian Islands. Young
et al.
(2024) indicate the N
min
of the ENP stock is 26 individuals based on the 20th percentile of the photo-identification estimate of 31 whales (CV = 0.226) (Wade
et al.,
2011). Despite the uncertainty in the current extent of the range of the ENP stock, and because the abundance of the ENP stock is so low and recent sightings data does not overlap the Study Area, it is considered unlikely the ENP stock would be impacted by the proposed activities.

Bowhead whales (
Balaena mysticetus
) are limited to the Arctic and sub-Arctic regions in the Northern Hemisphere, with shorter migrations than most baleen whales; the Okhotsk Sea and Bering Chukchi-Beaufort Seas stocks are confined to the Okhotsk and Bering Seas, respectively (Citta
et al.,
2023; Citta
et al.,
2015; Ivashchenko and Clapham, 2010). There has been one record of a calf in lower latitudes in Canadian waters; however, it is the only sighting of a bowhead within the eastern North Pacific (Towers
et al.,
2022). There are no known or documented sightings of this species within the Study Area.

Beluga whales (
Delphinapterus leucas
) of the Beaufort Sea, Eastern Chukchi Sea, and Cook Inlet stocks of beluga whales do not overlap the Study Area. The Sakhalin-Amur, Ulbansky, Tugursky, Udskaya, and Shelikhov populations are all found in the northern part of the Okhotsk Sea, whereas the Anadyr population and the Bristol Bay and Eastern Bering Sea stocks are all found within the northern parts of the Bering Sea (Hobbs
et al.,
2019). While both of these seas border the Study Area, both seas occur outside the Study Area, and there have been no documented sightings of belugas within the Study Area.

The distribution of shallow-water porpoise species, such as the Indo-Pacific finless porpoise (
Neophocaena phocaenoides
) and narrow-ridged finless porpoise (
N. asiaeorientalis
) are in shallow nearshore waters, where SURTASS LFA sonar is highly unlikely to be detectable.

Freshwater dolphin species, such as the Ganges River dolphin (
Platanista gangetica gangetica
), the Indus River dolphin (
P. gangetica minor
), and the baiji/Chinese river dolphin (
Lipotes vexillifer
), are restricted to riverine waters of the Ganges, Indus, and Yangtze Rivers, respectively. These river dolphins occur only in the main channels of these rivers, well inshore of where SURTASS LFA sonar would be detectable.

Inshore and coastal delphinid species, such as the Irrawaddy dolphin (
Orcaella brevirostris
), Australian snubfin dolphin (
O. heinsohni
), Indian Ocean humpback dolphin (
Sousa plumbea
), Indo-Pacific humpbacked dolphin (
S. chinensis
), Australian humpback dolphin (
S. sahulensis
), and Taiwanese humpbacked dolphin (
S. chinensis taiwanensis
), all occur in shallow, coastal waters near shore, where SURTASS LFA sonar is unlikely to be detectable.

Two species of marine mammal, sea otters (
Enhydra lutris
) and dugongs (
Dugong dugon
), occur in the Study Area but are managed by the U.S. Fish and Wildlife Service (U.S. FWS) and thus are not considered further in this analysis.

NMFS standardly considers 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 have been designated. For SURTASS LFA sonar training and testing activities, the Navy coordinated with NMFS to develop a comprehensive method for consideration of these types of important areas globally and specifically within the Study Area (for the purposes of identifying OBIAs, discussed further in the Offshore Biologically Important Areas for SURTASS LFA Sonar section). This method and these areas are summarized in the Proposed Mitigation Measures section and described in detail in appendix F of the 2025 SURTASS Draft SEIS/OEIS. Further, we note here that the OBIA identification criteria include consideration of the hearing sensitivity of the affected species, given the low frequency (100-500 Hz) of the LFA sonar signal, which most odontocetes and pinnipeds hear with significantly reduced sensitivity (9-65 dB lower, or more). Additional details regarding marine mammal hearing sensitivity are included in the
Marine Mammal Hearing Groups
section.

Below, we describe the critical habitat for the two species in the Study Area for which it has been designated under the ESA, as well two National Marine Sanctuaries that include marine mammal resources, although both critical habitat and Sanctuaries are considered through the referenced OBIA process. Further, we briefly describe biologically important areas (BIAs) for cetaceans identified and scored in Kratofil
et al.
(2023), which are not explicitly addressed through the OBIA process, but which NMFS addresses in the context of mitigation in the
Geographic Mitigation
section.

Critical Habitat

Currently, the false killer whale (Main Hawaiian Islands Insular DPS) and Hawaiian monk seal have ESA-designated critical habitat in the Study Area.

False Killer Whale (Main Hawaiian Island Insular DPS)

Critical habitat for the ESA-listed Main Hawaiian Islands insular false killer whale DPS was finalized in July 2018 (83 FR 35062, July 24, 2018) designating waters from the 45 m depth contour to the 3,200 m depth contour around the main Hawaiian Islands from Niihau east to Hawaii. This designation does not include most bays, harbors, or coastal in-water structures. NMFS excluded 14 areas. The total area designated was approximately 45,504 square kilometers (km
2
) (13,267 nmi
2
) of marine habitat. Critical habitat for the main Hawaiian Islands insular DPS of

false killer whale overlaps the Study Area.

Main Hawaiian Islands insular false killer whales are island-associated whales that rely entirely on the productive submerged habitat of the main Hawaiian Islands to support all of their life-history stages. Island-associated marine habitat for Main Hawaiian Islands insular false killer whale is the only essential feature of the critical habitat. The following characteristics of this habitat support insular false killer whales' ability to travel, forage, communicate, and move freely around and among the waters surrounding the main Hawaiian Islands: (1) adequate space for movement and use within shelf and slope habitat; (2) prey species of sufficient quantity, quality, and availability to support individual growth, reproduction, and development, as well as overall population growth; (3) waters free of pollutants of a type and amount harmful to Main Hawaiian Islands insular false killer whales; and (4) sound levels that would not significantly impair false killer whales' use or occupancy.

Hawaiian Monk Seal

Critical habitat for Hawaiian monk seals was designated in 1986 (51 FR 16047, April 30, 1986) and later revised in 1988 (53 FR 18988, May 26, 1988) and in 2015 (80 FR 50925, August 21, 2015). In the Northwestern Hawaiian Islands Hawaiian monk seal critical habitat includes all beach areas, sand spits and islets, including all beach crest vegetation to its deepest extent inland as well as the seafloor and marine habitat 10 m in height above the seafloor from the shoreline out to the 200 m depth contour around Kure Atoll (Hōlanikū), Midway Atoll (Kuaihelani), Pearl and Hermes Reef (Manawai), Lisianski Island (Kapou), Laysan Island (Kamole), Maro Reef (Kamokuokamohoali'i), Gardner Pinnacles ('Ōnūnui), French Frigate Shoals (Lalo), Necker Island (Mokumanamana) and Nihoa Island. In the main Hawaiian Islands, Hawaiian monk seal critical habitat includes the seafloor and marine habitat to 10 m above the seafloor from the 200 m depth contour through the shoreline and extending into terrestrial habitat 5 m inland from the shoreline between identified boundary points around Kaula Island (includes marine habitat only), Ni'ihau (includes marine habitat from 10 to 200 m in depth), Kaua'i, Ōahu, Maui Nui (including Kaho'olawe, Lāna'i, Maui, and Moloka'i), and Hawaii Island. Critical habitat for the Hawaiian monk seal overlaps the Study Area.

The essential features of Hawaiian monk seal critical habitat are: (1) terrestrial areas and adjacent shallow, sheltered aquatic areas with characteristics preferred by monk seals for pupping and nursing; (2) marine areas from 0 to 200 m in depth that support adequate prey quality and quantity for juvenile and adult monk seal foraging; and (3) significant areas used by monk seals for hauling out, resting or molting.

Biologically Important Areas

Ferguson
et al.
(2015) identified BIAs within U.S. waters, which represent areas and times in which cetaceans are known to concentrate for reproduction, feeding, and migration, or areas where small and resident populations are known to occur. Harrison
et al.
(2023) identified a new scoring system, described below, and the BIAs in Hawaiian waters were updated (Kratofil
et al.,
2023). 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 the BIAs is available at:
https://oceannoise.noaa.gov/biologically-important-areas.
A summary of all of the BIAs in the Study Area is included below.

Kratofil
et al.
(2023) delineates and scores BIAs for cetaceans in the Hawaii region following standardized protocols. Experts identified an overall Importance Score for each BIA that considers: (1) “Intensity,” meaning the intensity and characteristics underlying an area's identification as a BIA; and (2) “Data Support,” meaning the quantity, quality, and type of information, and associated uncertainties, upon which the BIA delineation and scoring depend. Importance Scores range from 1 to 3, with a higher score representing an area of higher intensity and data support. Each BIA is also scored for boundary uncertainty and spatiotemporal variability (dynamic, ephemeral, or static). Additionally, hierarchical BIAs are identified for some species and stocks where a higher intensity score is appropriate for a smaller core area(s) (child BIA) within a larger BIA unit (parent BIA).

The Study Area overlaps BIAs in Hawaii for small and resident populations of the following species: spinner dolphin, short-finned pilot whale, rough-toothed dolphin, pygmy killer whale, pantropical spotted dolphin, melon-headed whale, false killer whale, dwarf sperm whale, goose-beaked whale, common bottlenose dolphin, and Blainville's beaked whale, and the updated BIAs for humpback whale reproduction (Kratofil
et al.,
2023). Table 2 describes each BIA that overlaps the Study Area and the scores for the above criteria. We note that the BIAs for small and resident populations of spinner dolphin, melon-headed whale, and dwarf sperm whale are all fully contained within OBIAs. The BIAs for small and resident populations of short-finned pilot whale, rough-toothed dolphin, pygmy killer whale, goose-beaked whale, and common bottlenose dolphin, and the reproductive BIA for humpback whale, are mostly contained within the OBIAs. The BIAs for small and resident populations of pantropical spotted dolphin, false killer whale, and Blainville's beaked whale are partially contained within the OBIAs described in the
Geographic Mitigation
section and proposed for implementation in this rule.

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National Marine Sanctuaries

Under Title III of the Marine Protection, Research, and Sanctuaries Act of 1972 (also known as the National Marine Sanctuaries Act (NMSA)), NOAA has authority to establish as national marine sanctuaries (NMS) areas of the marine environment with special conservation, recreational, ecological, historical, cultural, archaeological, scientific, educational, or aesthetic qualities. Sanctuary regulations prohibit destroying, causing the loss of, or injuring any sanctuary resource managed under the law or regulations for that sanctuary (15 CFR part 922).

NMS are managed on a site-specific basis, and each sanctuary has site-specific regulations. Most, but not all sanctuaries have site-specific regulatory exemptions from the prohibitions for certain military activities. Separately, section 304(d) of the NMSA requires Federal agencies to consult with the Office of National Marine Sanctuaries (ONMS) whenever their Proposed Activities are likely to destroy, cause the loss of, or injure a sanctuary resource. There are two designated NMSs within the Study Area that contain areas or resources important to marine mammals (see chapter 3 of the 2025 SURTASS Draft SEIS/OEIS):

• Hawaiian Islands Humpback Whale NMS; and

• Papahānaumokuākea NMS.

Hawaiian Islands Humpback Whale NMS is a single-species managed sanctuary, composed of 3,540 km
2
of the submerged lands and waters off the coast of Maui, Lāna'i, and Moloka'i; and smaller areas off the north shore of Kaua'i, off Hawaii's west coast, and off the north and southeast coasts of Oahu. Hawaiian Islands Humpback Whale NMS is entirely within the Study Area and constitutes one of the world's most important Hawaii humpback whale DPS habitats (81 FR 62259, September 8, 2016) and is a primary region for humpback reproduction in the U.S. (National Marine Sanctuaries Program, 2002). Scientists estimate that more than 50 percent of the entire North Pacific humpback whale population migrates to Hawaiian waters each winter to mate, calve, and nurse their young. The North Pacific humpback whale population has been split into two DPSs. The Hawaii humpback whale DPS migrates to Hawaiian waters each winter and is not listed under the ESA. In addition to protection under the MMPA, the Hawaii humpback whale DPS is protected in sanctuary waters by the Hawaiian Islands Humpback Whale NMS. The sanctuary was created to protect humpback whales and shallow, protected waters important for calving and nursing (Office of National Marine Sanctuaries, 2010).

Papahānaumokuākea NMS, the largest NMS, consists of approximately 1,508,849 km
2
of Pacific Ocean waters surrounding the Northwestern Hawaiian Islands and the submerged lands thereunder. The sanctuary comprises several interconnected ecosystems, such as coral islands surrounded by shallow reefs, low-light mesophotic reefs with extensive algal beds, open ocean waters connected to the greater North Pacific Ocean, deep-water habitats such as abyssal plains 4,999 m below sea level, and deep reef habitat characterized by seamounts, banks, and shoals. The 2,172 km stretch of coral islands, seamounts, banks, and shoals supports a diversity of coral, fish, birds, and marine mammals, many of which are unique to the Hawaiian Island chain. Many of the islands and shallow water environments are important habitats for rare species such as the endangered Hawaiian monk seal, while the waters are also important for humpback whale breeding and calving.

Unusual Mortality Events

An unusual mortality event (UME) is defined under section 410(9) of the MMPA as a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response (16 U.S.C. 1421h(9)). There are no active UMEs in the Study Area.

Marine Mammal Hearing Groups

Hearing is the most important sensory modality for marine mammals underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Not all marine mammal species have equal hearing capabilities (
e.g.,
Richardson
et al.,
1995, Wartzok and Ketten, 1999, Au and Hastings, 2008; Erbe
et al.,
2025). To reflect this, Southall
et al.
(2007) and Southall
et al.
(2019c) recommended that marine mammals be divided into hearing groups based on directly measured (behavioral or auditory evoked potential techniques) or estimated hearing ranges (
e.g.,
behavioral response data, anatomical modeling). NMFS (2024) generalized hearing ranges were chosen based on the approximately 65-dB threshold from the composite audiograms, previous analysis in NMFS (2018), and/or data from Southall
et al.
(2007) and Southall
et al.
(2019c). We note that the names of two hearing groups and the generalized hearing ranges of all marine mammal hearing groups have been recently updated (NMFS, 2024) as reflected below in table 3.

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Of particular relevance to the assessment of the impacts of the Navy's SURTASS LFA sonar training and testing are the auditory weighting functions shown in figure 2 and figure 3 (NMFS, 2024), which illustrate the significantly reduced sensitivity of most marine mammal taxa to frequencies in the 100-500 Hz range (
i.e.,
0.1-0.5 kHz as indicated on the x-axis of these figures in NMFS (2024)), such as SURTASS LFA sonar. Specifically, the HF cetacean weighting function curve shows approximately 17-40 dB reduced sensitivity in that frequency range (
i.e.,
the sound would be perceived as that much lower level than a sound in the most noise susceptible portion of their hearing range) (figure 2), the underwater pinniped weighting function curves (PW, OW) show 9-30 dB reductions (figure 3), and the VHF cetacean weighting function curve shows a 47-65 dB reduction at frequencies from 200 to 500 Hz (
i.e.,
generalized hearing range for this hearing group starts at 200 Hz) and suggest even further reduced sensitivity (figure 2). Even the LF cetacean species have somewhat reduced sensitivity in the 100 to 500 Hz range (0.5-6 dB) (figure 2).

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

Of note, the Navy adjusted the LF cetacean hearing group using data from recent hearing measurements in minke whales (Houser
et al.,
2024). These data support separating mysticetes (the LF cetaceans marine mammal hearing group in table 3) into two hearing groups, which the Navy designates as “very low-frequency (VLF) cetaceans” and “low-frequency (LF) cetaceans,”

which follows the recommendations of Southall
et al.,
(2019c). Within the Navy's adjusted hearing groups, the VLF cetacean group contains the larger mysticetes (
i.e.,
blue, pygmy blue, fin, right, and bowhead whales) and the LF cetacean group contains the mysticete species not included in the VLF group (
e.g.,
minke, humpback, gray, pygmy right whales). Although there have been no direct measurements of hearing sensitivity in the larger mysticetes included in Navy's VLF hearing group, an audible frequency range of approximately 10 Hz to 30 kHz has been estimated from measured vocalization frequencies, observed responses to playback of sounds, and anatomical analyses of the auditory system. The upper frequency limit of hearing in Navy's LF hearing group has been estimated as 64 kHz, based on direct measurements of auditory evoked potentials in minke whales (Houser
et al.,
2024).

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

This section provides a discussion of the ways in which components of the specified activity may impact marine mammals and their habitat. The Estimated Take of Marine Mammals section later in this document includes a quantitative analysis of the number of individuals that are expected to be taken by this activity. 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 or stock 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 and testing activities in the Study Area. The Navy analyzed potential impacts to marine mammals from acoustic sources in the application. NMFS carefully reviewed the information provided by the Navy and concurs with their synthesis of science, 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 (see appendix D in the 2025 SURTASS Draft SEIS/OEIS for additional information).

Potential impacts to marine mammals from training and testing activities in the Study Area were analyzed in the 2025 SURTASS Draft SEIS/OEIS, in consultation with NMFS as a cooperating agency, and stressors other than acoustic sources were determined to be unlikely to result in marine mammal take. Therefore, the Navy has not requested authorization for take of marine mammals incidental to other components of their proposed Specified Activities, and we agree that incidental take is unlikely to occur from those components. In this proposed rule, NMFS analyzes the potential effects on marine mammals from the activity components that may result in take of marine mammals: exposure to acoustic stressors (
i.e.,
sonar).

For the purpose of MMPA ITAs, 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 harassment and temporary threshold shift (TTS)), Level A harassment (auditory injury (AUD INJ), 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 other 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 may generally be affected by acoustic stressors in the form of mortality, physical injury, sensory impairment (permanent and temporary threshold shifts and acoustic masking), physiological responses (particular 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 sources relate to the MMPA definitions of Level A Harassment and Level B Harassment and quantifies those effects that do not qualify as a take under the MMPA. 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 on Marine Mammals

The marine soundscape is composed of both ambient and anthropogenic sounds. Ambient sound is defined as the all-encompassing sound in a given place and is usually a composite of sound from many sources both near and far (American National Standards Institute, 1995). The sound level of an area is defined by the total acoustical energy being generated by known and unknown sources, which may include physical (
e.g.,
waves, wind, precipitation, earthquakes, ice, atmospheric sound), biological (
e.g.,
sounds produced by marine mammals, fish, and invertebrates), and anthropogenic sound (
e.g.,
vessels, dredging, aircraft, construction).

The sum of the various natural and anthropogenic sound sources at any given location and time—which comprise “ambient” or “background” sound—depends not only on the source levels (as determined by current weather conditions and levels of biological and shipping activity) but also on the ability of sound to propagate through the environment. In turn, sound propagation is dependent on the spatially and temporally varying properties of the water column and sea floor and is frequency-dependent. As a result of the dependence on a large number of varying factors, ambient sound levels can be expected to vary widely over both coarse and fine spatial and temporal scales. Sound levels at a given frequency and location can vary by 10-20 dB from day to day (Richardson
et al.,
1995). The result is that, depending on the source type and its intensity, sound from the specified activities may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals.

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, other auditory injury, non-auditory physical or physiological effects, behavioral disturbance, stress, and masking (Richardson
et al.,
1995; Gordon
et al.,
2003; Nowacek
et al.,
2007; Southall

et

al.,

2007; Götz
et al.,
2009, Southall
et al.,
2019a; Erbe
et al.,
2025). 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 auditory injury, as can longer exposures to lower level sounds. Temporary or permanent loss of hearing can occur after exposure to noise and occur almost exclusively for noise within an animal's hearing range.

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 high-level sound sources can range in severity from effects such as behavioral disturbance or tactile perception to physical discomfort, physiological damage, and injury of the auditory system (Yelverton
et al.,
1973). Non-auditory physiological effects or injuries that theoretically might occur in marine mammals exposed to high levels of underwater sound or as a secondary effect of extreme behavioral responses (
e.g.,
change in dive profile as a result of an avoidance response) caused by exposure to sound include neurological effects, bubble formation, resonance effects, and other types of organ or tissue damage (Cox
et al.,
2006; Southall
et al.,
2007; Zimmer and Tyack, 2007; Tal
et al.,
2015).

Hearing

Marine mammals have adapted hearing based on their biology and habitat. Amphibious marine mammals (
e.g.,
pinnipeds that spend time on land and underwater) have modified ears that allow them to hear both in-air and in-water, while fully aquatic marine mammals (
e.g.,
cetaceans that are always underwater) have specialized ear adaptations for in-water hearing (Wartzok and Ketten, 1999). These adaptations explain the variation in hearing ability and sensitivity among marine mammals and have led to the characterization of marine mammal functional hearing groups based on those sensitivities (see
Marine Mammal Hearing Groups
section).

The hearing sensitivity of marine mammals is also directional, meaning the angle between an animal's position and the location of a sound source impacts the animal's hearing threshold, thereby impacting an animal's ability to perceive the sound emanating from that source. This directionality is likely useful for determining the general location of a sound, whether for detection of prey, predators, or members of the same species, and can be dependent upon the frequency of the sound (Accomando
et al.,
2020; Au and Moore, 1984; Byl
et al.,
2016; Byl
et al.
2019; Kastelein
et al.,
2005; Kastelein
et al.,
2019; Popov and Supin, 2009).

Acoustic Signaling

An acoustic signal refers to the sound waves used to communicate underwater, and marine mammals use a variety of acoustic signals for socially important functions, such as communicating, as well as biologically important functions, such as echolocating (Richardson
et al.,
1995; Wartzok and Ketten, 1999; Erbe
et al.,
2025). Acoustic signals used for communication are lower frequency (
i.e.,
20 Hz to 30 kHz) than those signals used for echolocation, which are high-frequency (approximately 10-200 kHz peak frequency) signals used by odontocetes to sense their underwater environment. Lower frequency vocalizations used for communication may have a specific, prominent fundamental frequency (Brady
et al.,
2021) or have a wide frequency range, depending on the functional hearing group and whether the marine mammal is vocalizing in-water or in-air. Acoustic signals used for echolocation are high-frequency, high-energy sounds with patterns and peak frequencies that are often species-specific (Baumann-Pickering
et al.,
2013).

Marine mammal species typically produce sounds at frequencies within their own hearing range, though auditory and vocal ranges do not perfectly align (
e.g.,
odontocetes may hear only a portion of the frequencies of an echolocation click). Because determining a species vocal range is easier than determining a species' hearing range, vocal ranges are often used to infer a species' hearing range when species-specific hearing data are not available (
e.g.,
large whale species). Table 3, figure 2, and figure 3 in the
Marine Mammal Hearing Groups
section summarize the available data on marine mammal hearing groups, which is relevant given the significantly reduced sensitivity of most marine mammal taxa (9-65 dB and above for all but LF species) to the SURTASS LFA sonar source.

Hearing Loss and Auditory Injury

Marine mammals, like all mammals, lose their ability to hear over time due to age-related degeneration of auditory pathways and sensory cells of the inner ear. This natural, age-related hearing loss is distinct from acute noise-induced hearing loss (Møller, 2013). Noise-induced hearing loss can be temporary (
i.e.,
TTS) or result in a permanent (permanent threshold shift (PTS)), with higher-level sound exposures more likely to cause PTS or other AUD INJ. For marine mammals, AUD INJ is considered to be possible when sound exposures are sufficient to produce 40 dB of TTS measured approximately 4 minutes after exposure (NMFS, 2024; U.S. Department of the Navy, 2025). Numerous studies have directly examined noise-induced hearing loss in marine mammals by measuring an animal's hearing threshold before and after exposure to intense or long duration sounds. The difference between the post-exposure and pre-exposure hearing thresholds is then used to determine the amount of TTS (in dB) that was produced as a result of the sound exposure (see appendix D of the 2025 SURTASS Draft SEIS/OEIS for additional details). The Navy used these studies to generate exposure functions, which are predictions of the onset of TTS or PTS or other AUD INJ based on sound frequency, level, and type (non-impulsive or impulsive), for each marine mammal hearing group (NMFS, 2024; U.S. Department of the Navy, 2025).

TTS can last from minutes or hours to days (
i.e.,
however, there is complete recovery back to baseline/pre-exposure hearing threshold), can occur within a specific frequency range (
i.e.,
an animal might have a temporary loss of hearing sensitivity within only 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 AUD INJ (
e.g.,
neural degeneration), as TTS increases, the likelihood that additional exposure to increased SPL or duration will result in PTS or other injury also increases (see appendix D of the 2025 SURTASS Draft SEIS/OEIS for additional discussion). Exposure thresholds for the occurrence of AUD INJ, which include the potential for PTS, as well as situations when AUD INJ occurs without PTS, can therefore be defined based on a specific amount of TTS; that is, although an exposure has been shown to produce only TTS, we assume that any additional TTS exposure may result in some AUD INJ. The specific upper limit of TTS is based on experimental data showing amounts of TTS that have not resulted in AUD INJ. In other words, we do not need to know the exact functional relationship between TTS and AUD INJ, we need to know only the upper limit for TTS to determine when some AUD INJ is possible. In most cases of AUD INJ, the animal has an impaired ability to hear sounds in specific frequency ranges (Kryter, 1985; Finneran, 2015).

The following physiological mechanisms are thought to play a role in inducing auditory threshold shift: (1) effects to sensory hair cells in the inner ear that reduce their sensitivity; (2) modification of the chemical environment within the sensory cells; (3) displacement of certain inner ear membranes; (4) increased blood flow; and (5) 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/or duration of sound exposure increases. Human non-impulsive noise exposure guidelines are based on the assumption that exposures of equal energy (the same cumulative 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; Finneran, 2015; Southall
et al.,
2019). Cumulative SEL is used to predict TTS in marine mammals and is considered a good predictor of TTS for shorter duration exposures than longer duration exposures. The amount of TTS increases with exposure SPL and duration, and is correlated with cumulative SEL, but duration of the exposure has a more significant effect on TTS than would be predicted based on cumulative SEL alone (
e.g.,
Finneran
et al.,
2010b; Kastak
et al.,
2007; Kastak
et al.,
2005; Kastelein
et al.,
2014a; Mooney
et al.,
2009a; Popov
et al.,
2014; Gransier and Kastelein, 2024). 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, TTS increases with cumulative SEL in a non-linear fashion, where lower SEL exposures will elicit a steady rate of TTS increase while higher SEL exposures will either increase TTS more rapidly or plateau (Finneran, 2015; U.S. Department of the Navy, 2025). Additionally, with sound exposures of equal energy, those that had lower SPL with longer duration were found to induce TTS onset at lower levels than those of 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; Kastelein
et al.,
2014; Kastelein
et al.,
2015). For example, one short, higher SPL sound exposure may induce the same impairment as one longer 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 loud enough to elicit TTS, or shorter-term exposure to sound levels well above the TTS threshold, can cause AUD INJ, at least in terrestrial mammals (Kryter, 1985; Lonsbury-Martin
et al.,
1987).

Although TTS increases non-linearly in marine mammals, recovery from TTS typically occurs in a linear fashion with the logarithm of time (Finneran, 2015; Finneran
et al.,
2010a; Finneran
et al.,
2010b; Finneran and Schlundt, 2013; Kastelein
et al.,
2012a; Kastelein
et al.,
2012b; Kastelein
et al.,
2013a; Kastelein
et al.,
2014a; Kastelein
et al.,
2014b; Kastelein
et al.,
2014c; Popov
et al.,
2014; Popov
et al.,
2013; Popov
et al.,
2011; Muslow
et al.,
2023; Finneran
et al.,
2023). Considerable variation has been measured in individuals of the same species in both the amount of TTS incurred from similar cumulative SELs (Kastelein
et al.,
2012a; Popov
et al.,
2013) and the time-to-recovery from TTS (Finneran, 2015; Kastelein
et al.,
2019e). Many of these studies relied on continuous sound exposures, but intermittent, impulsive sound exposures have also been tested. Few studies (Finneran
et al.,
2002; Lucke
et al.,
2009; Sills
et al.,
2020; Muslow
et al.,
2023) using impulsive sounds have produced enough TTS to make predictions about hearing loss due to this source type (see U.S. Department of the Navy, 2025). In general, predictions of TTS based on cumulative SEL for this type of sound exposure are likely to overestimate TTS because some recovery from TTS may occur in the quiet periods between impulsive or intermittent sounds, especially when the duty cycle is low. Peak SPL (unweighted) is also used to predict TTS due to impulsive sounds (Southall
et al.,
2007; Southall
et al.,
2019c; U.S. Department of the Navy, 2025).

In some cases, associated with terrestrial mammal noise studies, intense noise exposures have caused AUD INJ (
e.g.,
loss of cochlear neuron synapses), despite thresholds eventually returning to normal (
i.e.,
it is possible to have AUD INJ without a resulting PTS (
e.g.,
Kujawa and Liberman, 2006, 2009; Fernandez
et al.,
2015; Ryan
et al.,
2016; Houser, 2021). In these situations, however, threshold shifts were 30-50 dB measured 24 hours after the exposure (
i.e.,
there is no evidence that an exposure resulting in less than 40 dB TTS measured a few minutes after exposure can produce AUD INJ). Therefore, an exposure producing 40 dB of TTS, measured a few minutes after exposure, can also be used as an upper limit to prevent AUD INJ (
i.e.,
it is assumed that exposures beyond those capable of causing 40 dB of TTS have the potential to result in INJ (which may or may not result in PTS)).

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 inner ears and resultant changes in the chemical composition of the inner ear fluids (Southall
et al.,
2007). When AUD INJ occurs, there is physical damage (
i.e.,
typically mechanical) to the sound receptors in the ear, whereas TTS represents primarily tissue fatigue (
i.e.,
typically metabolic) and is fully reversible (Southall
et al.,
2007). AUD INJ 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 less than 40 dB of TTS to constitute AUD

INJ. The NMFS Acoustic Updated Technical Guidance (NMFS, 2024), which was used in the assessment of effects for this proposed 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.

While many studies have examined noise-induced hearing loss in marine mammals (see Finneran (2015) and Southall
et al.
(2019a) for summaries), published data on the onset of TTS for cetaceans 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 (
Mirounga
species), California sea lions (
Zalophus californianus
), and bearded seals. These studies examine hearing thresholds measured in marine mammals before and after exposure to intense sounds, which 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 2025 SURTASS Draft SEIS/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). 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 cumulative 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 cumulative SEL begins to break down. Specifically, duration has a more significant effect on TTS than would be predicted on the basis of cumulative SEL alone (Finneran
et al.,
2010a; Kastak
et al.,
2005; Mooney
et al.,
2009a). This means if two exposures have the same cumulative 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 cumulative SEL tends to over-estimate the amount of TTS. Despite this, cumulative 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 (Finneran, 2015).

• Gradual increases of TTS over multiple exposures 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 highest susceptibility, are less hazardous than those at higher frequencies, near the region of highest susceptibility (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 the low frequency end of a species' hearing curve (
i.e.,
audiogram), onset-TTS exposure levels are higher compared to those in the region of best sensitivity.

• TTS can accumulate across multiple intermittent exposures, but the resulting TTS will be less than the TTS from a single, continuous exposure with the same cumulative 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 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 or greater) may require several days for recovery. 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).

Nachtigall
et al.
(2018) and Finneran (2018) describe the measurements of hearing sensitivity of multiple odontocete species (
i.e.,
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. Finneran (2018) 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. The fact that 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 is potentially more significant than the 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 AUD INJ on an animal could also range in severity, although it is considered generally more serious than TTS because it is a permanent condition (Reichmuth
et al.,
2019). Of note, reduced hearing sensitivity as a simple function of aging has been observed in marine mammals as well as in humans and other taxa (Southall
et al.,
2007). We can infer that strategies exist for coping with this condition to some degree, though likely not without some cost to the animal.

As the amount of research on hearing sensitivity has grown, so, too, has the understanding that marine mammals may be able to self-mitigate or protect against noise-induced hearing loss. An animal may learn to reduce or suppress their hearing sensitivity when warned of an impending intense sound exposure, or if the duty cycle of the sound source is predictable, as has been demonstrated in some odontocete species (Finneran, 2018; Finneran
et al.,
2024; Nachtigall and Supin, 2013, 2014, 2015; Nachtigall
et al.,
2016a, 2016b, 2016c, 2018). This has been shown with several species, including the false killer whale (Nachtigall and Supin, 2013), bottlenose dolphin (Finneran, 2018; Nachtigall and Supin, 2014, 2015; Nachtigall
et al.,
2016c), beluga whale (Nachtigall
et al.,
2016a), and harbor porpoise (Nachtigall
et al.,
2016b; Kastelein
et al.,
2020). Additionally, Finneran
et al.
(2023) and Finneran
et al.
(2024) found that odontocetes that had participated in TTS experiments in the past could have learned from that experience and subsequently protected their hearing during new sound exposure experiments.

Behavioral Responses

Behavioral responses to sound are highly variable and context-specific (Nowacek
et al.,
2007; Southall
et al.,
2007; Southall
et al.,
2019). 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, self-mitigation) 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; Southall
et al.,
2016; Finneran, 2018; Finneran
et al.,
2024; Nachtigall and Supin, 2013, 2014, 2015; Nachtigall
et al.,
2015, 2016a, 2016b, 2018). Related to the sound itself, the perceived proximity 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), familiarity of the sound, and navigational constraints may affect the way an animal responds to the sound (Ellison
et al.,
2012; Southall
et al.,
2007; DeRuiter
et al.,
2013a; Southall
et al.,
2021; Wartzok
et al.,
2003). Individuals (of different age, sex, 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. Southall
et al.,
(2007) and Southall
et al.
(2021) have developed and subsequently refined methods developed to categorize and assess the severity of acute behavioral responses, considering impacts to individuals that may consequently impact populations. 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.

Studies by DeRuiter
et al.,
(2013a) 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.,
(2013b) examined behavioral responses of goose-beaked whales to MF sonar and found that whales responded strongly at low received levels (89-127 dB re 1 µPa) 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 responses.

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.,
whether 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 animals may choose to stay, even when experiencing stress or hearing loss. Forney
et al.,
(2017) recommend considering both the costs of remaining in an area of noise exposure such as TTS, PTS, or masking, which could lead to an increased risk of predation or other threats or a decreased capability to forage, and the costs of displacement, including potential increased risk of vessel strike, increased risks of predation or competition for resources, or decreased habitat suitable for foraging, resting, or socializing. This sort of contextual information is challenging to predict with accuracy for ongoing activities that occur over large spatial and temporal expanses. However, distance is one contextual factor for which data exist to quantitatively inform a take estimate, and the method for predicting Level B harassment in this proposed rule does consider distance to the source. Other factors are often considered qualitatively in the analysis of the likely consequences of sound exposure, where supporting information is available.

Friedlaender
et al.,
(2016) provided the first integration of direct measures of prey distribution and density variables incorporated into across-individual analyses of behavior responses of blue whales to sonar and demonstrated a five-fold increase in the ability to quantify variability in blue whale diving behavior. These results illustrate that responses evaluated without such measurements for foraging animals may be misleading, which again illustrates the context-dependent nature of the probability of response.

Exposure of marine mammals to sound sources can result in, but is not

limited to, no response or any of the following observable responses: (1) increased alertness; (2) orientation or attraction to a sound source; (3) vocal modifications; (4) cessation of feeding; (5) cessation of social interaction; (6) alteration of movement or diving behavior; habitat abandonment (temporary or permanent); and (in severe cases) (7) panic, flight, stampede, or stranding, potentially resulting in death (Southall
et al.,
2007). A review of marine mammal responses to anthropogenic sound was first conducted by Richardson (1995). More recent reviews (Nowacek
et al.,
2007; DeRuiter
et al.,
2013a and 2013b; Ellison
et al.,
2012; Gomez
et al.,
2016; Erbe
et al.,
2025) address studies conducted since 1995 and focused on observations where the received sound level of the exposed marine mammal(s) was known or could be estimated. Gomez
et al.,
(2016) conducted a review of the literature considering the contextual information of exposure in addition to received level and found that higher received levels were not always associated with more severe behavioral responses and vice versa. Southall
et al.,
(2016) states that results demonstrate that some individuals of different species display clear yet varied responses, some of which have negative implications, while others appear to tolerate high levels, and that responses may not be fully predictable with simple acoustic exposure metrics (
e.g.,
received sound level). Rather, the authors state that differences among species and individuals along with contextual aspects of exposure (
e.g.,
behavioral state) appear to affect response probability (Southall
et al.,
2019). The following parts provide examples of behavioral responses to stressors that provide an idea of the variability in responses that would be expected given the differential sensitivities of marine mammal species to sound and the wide range of potential acoustic sources to which a marine mammal may be exposed. Behavioral responses that could occur for a given sound exposure should be determined from the literature that is available for each species (see appendix B of the 2025 SURTASS Draft SEIS/OEIS for a comprehensive list of behavioral studies and species-specific findings) or extrapolated from closely related species when no information exists, along with contextual factors.

Responses Due to Sonar and Other Transducers

Baleen whales are hypothesized to react more strongly to LF sounds that overlap with their vocalization range. One series of behavioral response studies (BRSs) was undertaken in 1997-1998 pursuant to the Navy's Low-Frequency Sound Scientific Research Program (LFS SRP). The frequency bands of LF sonars used were between 100 and 500 Hz, with received levels between 115 and 150 dB re 1 μPa. Exposures occurred on fin whale and blue whale foraging grounds, on humpback whale breeding grounds, and along gray whale (
Eschrichtius robustus
) migratory routes. These studies found short-term responses to LF sound by some individual fin and humpback whales, including changes in vocal activity and avoidance of the source vessel, while other fin, humpback, and blue whale individuals did not respond at all. When the source was in the path of migrating gray whales, they changed course up to 2 km to avoid the sound, but when the source was outside their path, little response was observed (Clark and Fristrup, 2001; Croll
et al.,
2001; Fristrup
et al.,
2003; Miller
et al.,
2000; Nowacek
et al.,
2007). These responses were short-lived across all individuals, and animals returned to their normal activities within tens of minutes after initial exposure (Clark and Fristrup, 2001). The context of an exposure scenario is important for determining the probability, magnitude, and duration of a response (Ellison
et al.,
2012; Southall
et al.,
2021).

At this time, no other BRSs have used an LFA (less than 1 kHz) sound source, so the applicability of all BRSs discussed herein to determining potential behavioral responses to the specified activities is limited. Specifically, while there are several studies illustrating the responses of LF species to MF sources (which are still in or near the most sensitive part of their predicted hearing range), many of the studies discussed below relate to the responses of HF hearing specialists to MF sources, which are essentially sources with frequencies in or near the most sensitive area of the species hearing, whereas (as noted above), all but LF species have significantly reduced sensitivity (9-65 dB and above) in the range of SURTASS LFA sonar. However, these data can generally inform the analysis of marine mammal response to sonar.

Mysticetes responses to sonar and other duty-cycled tonal sounds are dependent upon the characteristics of the signal, behavioral state of the animal, sensitivity and previous experience of an individual, and other contextual factors including distance of the source, movement of the source, physical presence of vessels, time of year, and geographic location (Goldbogen
et al.,
2013; Harris
et al.,
2019a; Harris
et al.,
2015; Martin
et al.,
2015; Sivle
et al.,
2015b). For example, a BRS in Southern California demonstrated that individual behavioral state was critically important in determining response of blue whales to Navy sonar. In this BRS, some blue whales engaged in deep (greater than 50 m) feeding behavior had greater dive responses than those in shallow feeding or non-feeding conditions, while some blue whales that were engaged in shallow feeding behavior demonstrated no clear changes in diving or movement even when received levels were high (approximately 160 dB re 1 µPa) from exposures to 3-4 kHz sonar signals, while others showed a clear response at exposures at lower received level of sonar and pseudorandom noise (Goldbogen
et al.,
2013). Generally, behavioral responses were brief and of low to moderate severity, and the whales returned to baseline behavior shortly after the end of the acoustic exposure (DeRuiter
et al.,
2017; Goldbogen
et al.,
2013; Southall
et al.,
2019c). To better understand the context of these behavioral responses, Friedlaender
et al.,
(2016) mapped the prey field of the deep-diving blue whales and found that the response to sound was more apparent for individuals engaged in feeding than those that were not. The probability of a moderate behavioral response increased when the source was closer for these foraging blue whales, although there was a high degree of uncertainty in that relationship (Southall
et al.,
2019b). In the same BRS, none of the tagged fin whales demonstrated more than a brief or minor response regardless of their behavioral state (Harris
et al.,
2019a). The fin whales were exposed to both mid-frequency simulated sonar and pseudorandom noise of similar frequency, duration, and source level. They were less sensitive to disturbance than blue whales, with no significant differences in response between behavioral states or signal types. The authors rated responses as low-to-moderate severity with no negative impact to foraging success (Southall
et al.,
2023).

Similarly, while the rates of foraging lunges decrease in humpback whales due to sonar exposure, there was variability in the response across individuals, with one animal ceasing to forage completely and another animal starting to forage during the exposure (Sivle
et al.,
2016). In addition, almost half of the animals that exhibited avoidance behavior were foraging before the exposure, but the others were not;

the animals that exhibited avoidance behavior while not feeding responded at a slightly lower received level and greater distance than those that were feeding (Wensveen
et al.,
2017). These findings indicate that the behavioral state of the animal plays a role in the type and severity of a behavioral response. Henderson
et al.
(2019) examined tagged humpback whale dive and movement behavior, including individuals incidentally exposed to Navy sonar during training activities, at the Pacific Missile Range Facility (PMRF) off Kaua'i, Hawaii. Tracking data showed that, regardless of exposure to sonar, individual humpbacks spent limited time, no more than a few days, in the vicinity of Kaua'i. Potential behavioral responses due to sonar exposure were limited and may have been influenced by breeding and social behaviors. Martin
et al.,
(2015) found that the density of calling minke whales was reduced during periods of Navy training involving sonar relative to the periods before training began and increased again in the days following the completion of training activities. The responses of individual whales could not be assessed, so in this case it is unknown whether the decrease in calling animals indicated that the animals left the range or simply ceased calling. Harris
et al.,
(2019b) utilized acoustically generated minke whale tracks to statistically demonstrate changes in the spatial distribution of minke whale acoustic presence before, during, and after surface ship MFAS training. The spatial distribution of probability of acoustic presence was different in the “during” phase compared to the “before” phase, and the probability of presence at the center of ship activity during MFAS training was close to zero for both years. The “after” phases for both years retained lower probabilities of presence suggesting the return to baseline conditions may take more than 5 days. The results show a clear spatial redistribution of calling minke whales during surface ship MFAS training; however, a limitation of passive acoustic monitoring is that one cannot conclude if the whales moved away, went silent, or a combination of the two.

Building on this work, Durbach
et al.,
(2021) used the same data and determined that individual minke whales tended to be in either a fast or slow movement behavioral state while on the missile range, whereas the whales tended to be in the slow state in baseline or before periods but transitioned into the fast state with more directed movement during sonar exposures. They also moved away from the area of sonar activity on the range, either to the north or east depending on where the activity was located; this explains the spatial redistribution found by Harris
et al.,
(2019b). Minke whales were also more likely to stop calling when in the fast movement behavioral state regardless of whether there was sonar activity and stop calling when in the slow movement behavioral state during sonar activity (Durbach
et al.,
2021). Similarly, minke whale detections were reduced or ceased altogether during periods of sonar use off Jacksonville, Florida, (Norris
et al.,
2012; Simeone
et al.,
2015; U.S. Department of the Navy, 2013), especially with an increased ping rate (Charif
et al.,
2015).

Odontocetes have varied, context-dependent behavioral responses to sonar and other transducers. Much of the research on odontocetes has been focused on understanding the impacts of sonar and other transducers on beaked whales because they were hypothesized to be more susceptible to behavioral disturbance after several strandings of beaked whales in which military MFAS was identified as a contributing factor (see
Stranding and Mortality
section). Subsequent BRSs have shown that beaked whales are likely more sensitive to disturbance than most other cetaceans. Many species of odontocetes have been studied during BRSs (though not for low frequency sources), including Blainville's beaked whale, goose-beaked whale, Baird's beaked whale, northern bottlenose whale, harbor porpoise, pilot whale, killer whale, sperm whale, false killer whale, melon-headed whale, bottlenose dolphin, rough-toothed dolphin, Risso's dolphin, Pacific white-sided dolphin, and Commerson's dolphin. Observed responses by Blainville's beaked whales, goose-beaked whales, Baird's beaked whales, and northern bottlenose whales (the largest of the beaked whales), to mid-frequency sonar sounds include cessation of clicking, decline in group vocal periods, termination of foraging dives, changes in direction to avoid the sound source, slower ascent rates to the surface, longer deep and shallow dive durations, and other unusual dive behaviors (DeRuiter
et al.,
2013b; Hewitt
et al.,
2022; Jacobson
et al.,
2022; McCarthy
et al.,
2011; Miller
et al.,
2015; Moretti
et al.,
2014; Southall
et al.,
2011; Stimpert
et al.,
2014; Tyack
et al.,
2011).

During a BRS in Southern California, a tagged Baird's beaked whale exposed to simulated MFA sonar within 3 km increased swim speed and modified its dive behavior (Stimpert
et al.,
2014). One goose-beaked whale was also incidentally exposed to real Navy sonar located over 100 km away in addition to the source used in the controlled exposure study, and the authors did not detect similar responses at comparable received levels. Received levels from the MFA sonar signals from the controlled (3.4 to 9.5 km) exposures were calculated as 84-144 dB re 1 μPa, and incidental (118 km) exposures were calculated as 78-106 dB re 1 μPa, indicating that context of the exposures (
e.g.,
source proximity, controlled source ramp-up) may have been a significant factor in the responses to the simulated sonars (DeRuiter
et al.,
2013b).

Long-term tagging work during the same BRS demonstrated that the longer duration dives considered a behavioral response by DeRuiter
et al.
(2013b) fell within the normal range of dive durations found for eight tagged goose-beaked whales on the Southern California Offshore Range (Schorr
et al.,
2014). However, the longer inter-deep dive intervals found by DeRuiter
et al.,
(2013b), which were among the longest found by Schorr
et al.,
(2014) and Falcone
et al.,
(2017), may indicate a response to sonar. Williams
et al.,
(2017) note that during normal deep dives or during fast swim speeds, beaked whales and other marine mammals use strategies to reduce their stroke rates (
e.g.,
leaping, wave surfing when swimming, interspersing glides between bouts of stroking when diving). The authors determined that in the post-exposure dives by the tagged goose-beaked whales described in DeRuiter
et al.,
(2013b), the whales ceased gliding and swam with almost continuous strokes. This change in swim behavior was calculated to increase metabolic costs by about 30.5 percent and increase the amount of energy expending on fast swim speeds from 27 to 59 percent of their overall energy budget. This repartitioning of energy was detected in the model up to 1.7 hours after the single sonar exposure. Therefore, while the overall post-exposure dive durations were similar, the metabolic energy calculated by Williams
et al.,
(2017) was higher. However, Southall
et al.,
(2019a) found that prey availability was higher in the western area of the Southern California Offshore Range where goose-beaked whales preferentially occurred, while prey resources were lower in the eastern area and moderate in the area just north of the Range. This high prey availability may indicate that goose-beaked whales need fewer foraging dives to meet energy requirements than

would be needed in another area with fewer resources.

During a BRS in Norway, northern bottlenose whales avoided a sonar sound source over a wide range of distances (0.8 to 28 km) and estimated avoidance thresholds ranging from received SPLs of 117 to 126 dB re 1 μPa. The behavioral response characteristics and avoidance thresholds were comparable to those previously observed in beaked whale studies; however, researchers did not observe an effect of distance on behavioral response and found that onset and intensity of behavioral response were better predicted by received SPL. There was one instance where an individual northern bottlenose whale approached the vessel, circled the sound source (source level was only 122 dB re 1 μPa), and resumed foraging after the exposure. Conversely, one northern bottlenose whale exposed to a sonar source was documented performing the longest and deepest dive on record for the species, and continued swimming away from the source for more than 7 hours (Miller
et al.,
2015; Siegal
et al.,
2022; Wensveen
et al.,
2019).

Research on Blainville's beaked whales at the Atlantic Undersea Test and Evaluation Center (AUTEC) range has shown that individuals move off-range during sonar use, only returning after the cessation of sonar transmission (Boyd
et al.,
2009; Henderson
et al.,
2015; Jones‐Todd
et al.,
2021; Manzano-Roth
et al.,
2022; Manzano-Roth
et al.,
2016; McCarthy
et al.,
2011; Tyack
et al.,
2011). Five Blainville's beaked whales estimated to be within 2 to 29 km of the AUTEC range at the onset of active sonar were displaced a maximum of 28 to 68 km after moving away from the range, although one individual did approach the range during active sonar use. Researchers found a decline in deep dives at the onset of the training and an increase in time spent on foraging dives as whales moved away from the range. Predicted received levels at which presumed responses were observed were comparable to those previously observed in beaked whale studies. Acoustic data indicated that vocal periods were detected on the range within 72 hours after training ended (Joyce
et al.,
2019). However, Blainville's beaked whales have been documented to remain on-range to forage throughout the year (Henderson
et al.,
2016), indicating the AUTEC range may be a preferred foraging habitat regardless of the effects of active sonar noise, or it could be that there are no long-term consequences of the sonar activity. In the SOCAL Range Complex, researchers conducting photo-identification studies have identified approximately 100 individual goose-beaked whales, with 40 percent having been seen in one or more prior years, with re-sightings up to 7 years apart, indicating a possible on-range resident population (Falcone and Schorr, 2014; Falcone
et al.,
2009).

The probability of Blainville's beaked whale group vocal periods on the PMRF were modeled during periods of: (1) no naval activity; (2) naval activity without hull-mounted MFA sonar; and (3) naval activity with hull-mounted MFA sonar (Jacobson
et al.,
2022). At a received level of 150 dB re 1 μPa RMS SPL, the probability of detecting a group vocal period during MFA sonar use decreased by 77 percent compared to periods when general training activity was ongoing, and by 87 percent compared to baseline (no naval activity) conditions. Jacobsen
et al.,
(2022) found a greater reduction in probability of a group vocal period with MFA sonar than observed in a prior study of the same species at the AUTEC range (Moretti
et al.,
2014), which may be due to the baseline period in the AUTEC study including naval activity without MFA sonar, potentially lowering the baseline group vocal period activity in that study, or due to differences in the residency of the populations at each range.

Stanistreet
et al.
(2022) used passive acoustic recordings during a multinational naval activity to assess marine mammal acoustic presence and behavioral response to especially long bouts of sonar lasting up to 13 consecutive hours, occurring repeatedly over 8 days (median and maximum SPL = 120 dB and 164 dB). Goose-beaked whales and sperm whales substantially reduced how often they produced clicks during sonar activity, indicating a decrease or cessation in foraging behavior. Few previous studies have shown sustained changes in foraging or displacement of sperm whales, but there was an absence of sperm whale clicks for 6 consecutive days of sonar activity. Sperm whales returned to baseline levels of clicks within days after the activity, but beaked whale click detection rates remained low even 7 days after the exercise. In addition, there were no click detections from a Mesoplodon beaked whale species within the area during, and at least 7 days after, the sonar activity. Clicks from northern bottlenose whales and Sowerby's beaked whales were also detected but were not frequent enough at the recording site used to compare clicks between baseline and sonar conditions.

Goose-beaked whale behavioral responses (
i.e.,
deep and shallow dive durations, surface interval durations, inter-deep dive intervals) on the Southern California Anti-Submarine Warfare Range were modeled against predictor values that included helicopter dipping sonar, mid-power MFA sonar and hull-mounted, high-power MFA sonar along with other non-MFA sonar predictors (Falcone
et al.,
2017). Falcone
et al.
(2017) found both shallow and deep dive durations increased as the proximity to both mid- and high-powered sources decreased and found that surface intervals and inter-deep dive intervals increased in the presence of both types of sonars (helicopter dipping and hull-mounted), although surface intervals shortened during periods without MFA sonar. Proximity of source and receiver were important considerations, as the responses to the mid-power MFA sonar at closer ranges were comparable to the responses to the higher source level vessel sonar, as was the context of the exposure. Helicopter dipping sonars are shorter duration and randomly located, therefore more difficult to predict or track by beaked whales and potentially more likely to elicit a response, especially at closer distances (6 to 25 km) (Falcone
et al.,
2017).

Sea floor depths and quantity of light (
i.e.,
lunar cycle) are also important variables to consider in BRSs, as goose-beaked whale foraging dive depth increased with sea floor depth (maximum 2,000 m) and the amount of time spent at foraging depths (and likely foraging) was greater at night (likely avoiding predation by staying deeper during periods of bright lunar illumination), although they spent more time near the surface during the night, as well, particularly on dark nights with little moonlight (Barlow
et al.,
2020). Sonar occurred during 10 percent of the dives studied and had little effect on the resulting dive metrics. Watwood
et al.,
(2017) found that the longer the duration of a sonar event, the greater reduction in detected goose-beaked whale group dives and, as helicopter dipping events occurred more frequently but with shorter durations than periods of hull-mounted sonar, when looking at the number of detected group dives there was a greater reduction during periods of hull-mounted sonar than during helicopter dipping sonar. DiMarzio
et al.
(2019) also found that group vocal periods (
i.e.,
clusters of foraging pulses), on average, decreased during sonar events on the Southern California Anti-Submarine Warfare Range, though the decline from before the event to during the event was significantly less for helicopter dipping

events than hull-mounted events, and there was no difference in the magnitude of the decline between vessel-only events and events with both vessels and helicopters. Manzano-Roth
et al.
(2022) analyzed long-term passive acoustic monitoring data from the PMRF in Kaua'i, Hawaii, and found beaked whales reduced group vocal periods during submarine command course events and remained low for a minimum of 3 days after the MFA sonar activity.

Harbor porpoise behavioral responses have been researched extensively using acoustic deterrent and acoustic harassment devices; however, BRSs using sonar are limited. Kastelein
et al.
(2018b) found harbor porpoises did not respond to low-duty cycle mid-frequency sonar tones (3.5-4.1 kHz at 2.7 percent duty cycle;
e.g.,
one tone per minute) at any received level, but one individual did respond (
i.e.,
increased jumping, increased respiration rates) to high-duty cycle sonar tones (3.5-4.1 kHz at 96 percent duty cycle;
e.g.,
continuous tone for almost a minute).

Behavioral responses by odontocetes (other than beaked whales and harbor porpoises) to sonar and other transducers include horizontal avoidance, reduced breathing rates, changes in behavioral state, changes in dive behavior (Antunes
et al.,
2014; Isojunno
et al.,
2018; Isojunno
et al.,
2017; Isojunno
et al.,
2020; Miller, 2012; Miller
et al.,
2011; Miller
et al.,
2014; Southall
et al.,
2024), and, in one study, separation of a killer whale calf from its group (Miller
et al.,
2011). Some species of dolphin (
e.g.,
bottlenose, spotted, spinner, Clymene, Pacific white-sided, rough-toothed) are frequently documented bowriding with vessels and the drive to engage in bowriding, whether for pleasure or energetic savings (Fiori
et al.,
2024) may supersede the impact of associated sonar noise (Würsig
et al.,
1998).

In controlled exposure experiments on captive odontocetes, Houser
et al.
(2013a) recorded behavioral responses from bottlenose d

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2026-04668. Public record. Not legal advice.