`[Federal Register Volume 89, Number 206 (Thursday, October 24, 2024)]
[Notices]
[Pages 84872-84880]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-24748]


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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

[RTID 0648-XD212]


2024 Updated Guidance for Assessing the Effects of Anthropogenic 
Sound on Marine Mammal Hearing--Underwater and In-Air Criteria for 
Onset of Auditory Injury and Temporary Threshold Shifts (Version 3.0)

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

ACTION: Notice.

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SUMMARY: The National Marine Fisheries Service (NMFS) announces the 
availability of our final 2024 Update to: Technical Guidance for 
Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing 
(Version 3.0): Underwater and In-Air Criteria for Onset of Auditory 
Injury and Temporary Threshold Shifts (2024 Updated Technical 
Guidance). The 2024 Updated Technical Guidance provides updated 
information, or acoustic criteria, to predict when individual marine 
mammals, both in-air and underwater, will experience changes in their 
hearing sensitivity (auditory injury or temporary threshold shift) from 
exposure to anthropogenic sound sources. The 2024 Updated Technical 
Guidance replaces NMFS's current 2018 Revisions to: Technical Guidance 
for Assessing the Effects of Anthropogenic Sound on Marine Mammal 
Hearing (Version 2.0): Underwater Thresholds for Onset of Permanent and 
Temporary Threshold Shifts (2018 Revised Technical Guidance).

ADDRESSES: The final 2024 Updated Technical Guidance is available in 
electronic form via the internet https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance.

FOR FURTHER INFORMATION CONTACT: Amy R. Scholik-Schlomer, Office of 
Protected Resources, 301-427-8449, [email protected].

SUPPLEMENTARY INFORMATION: NMFS has updated its guidance for assessing 
the effects of anthropogenic sound on the hearing of marine mammal 
species under NMFS's jurisdiction. Specifically, the 2024 Updated 
Technical Guidance identifies the received levels and auditory 
weighting functions, or ``acoustic criteria,'' that describe the 
received levels (decibels (dB)) and frequencies (kilohertz (kHz)) where 
individual marine mammals are predicted to experience changes in their 
hearing sensitivity (auditory injury (AUD INJ) or temporary threshold 
shift (TTS)) from exposure to anthropogenic sound sources both in-air 
and underwater. This document is intended for use by NMFS analysts and 
managers and other relevant user groups and interested parties, 
including other Federal agencies, when seeking to determine whether and 
how their activities are expected to result in auditory impacts to 
marine mammals via acoustic exposure in-air and underwater. The 2024 
Updated Technical Guidance outlines NMFS's updated acoustic criteria 
and describes in detail how they were developed and how they will be 
updated in the future. For information on NMFS' 2018 Revised and the 
original 2016 Technical Guidance, refer to our 2016 Federal Register 
notification (81 FR 51694, August 4, 2016).
    For the 2024 Updated Technical Guidance, NMFS again worked with the 
U.S. Navy (Navy), which recently updated its marine mammal AUD INJ and 
TTS criteria (Finneran 2024), to incorporate the best available 
science. NMFS conducted an independent peer review in October/November 
2022. Details of the peer review, peer reviewer comments, and our 
response to these comments are available at the following website: 
https://www.noaa.gov/information-technology/update-to-20162018-technical-guidance-for-assessing-effects-of-anthropogenic-sound-on-marine-mammal. In May/June of 2023, NMFS solicited input from other 
relevant Federal agencies on the 2024 Updated Technical Guidance. 
Federal agency comments and NMFS responses to those comments are 
available at the following website: https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance. To complete the review process, NMFS solicited additional 
feedback from user groups and interested parties via a 45-day public 
comment period in May/June 2024. Comments received via the public 
comment period can be found at the following website: https://www.regulations.gov/document/NOAA-NMFS-2024-0026-0001.
    It is important to note that the use of the acoustic criteria 
within the 2024 Updated Technical Guidance should not be considered to 
represent the entirety of an impact assessment, but rather serve as one 
tool to help evaluate the effects of a proposed action. Furthermore, 
the 2024 Updated Technical Guidance does not create or confer any 
rights for or on any person, or operate to bind the public. For the 
purposes of assessing auditory impacts to marine mammals in support of 
regulatory processes under NMFS' authority, an alternative approach 
that has undergone independent peer review may be proposed (by Federal 
agencies or prospective action proponents or applicants) and used if 
case-specific information/data indicate that the alternative approach 
is likely to produce a more accurate portrayal of auditory impacts (AUD 
INJ or TTS) for the project being evaluated, if NMFS determines the 
approach satisfies the requirements of the applicable statutes and 
regulations.
    The 2024 Updated Technical Guidance reflects the current state of 
scientific knowledge regarding the characteristics of sound that have 
the potential to impact marine mammal hearing sensitivity. NMFS 
recognizes that the implementation of marine mammal weighting functions 
and the weighted SEL24h criteria may extend beyond the 
capabilities of some action proponents. Thus, NMFS has developed an 
optional, alternative tool for those

[[Page 84873]]

who cannot fully incorporate these factors into their own analyses (See 
2024 Updated Technical Guidance's companion optional User Spreadsheet 
tool; https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance).

Transitioning to 2024 Updated Technical Guidance

    NMFS has determined the updated thresholds and associated weighting 
functions in the 2024 Updated Technical Guidance represent the best 
available information for assessing whether exposure to specific 
activities is likely to result in changes in marine mammal hearing 
sensitivity (AUD INJ or TTS). Prospective applicants for incidental 
take authorizations under the Marine Mammal Protection Act (MMPA) and 
Federal agencies seeking Endangered Species Act (ESA) section 7 
consultations that have not yet started their acoustic analyses should 
begin using the 2024 Updated Technical Guidance immediately. At the 
same time, we recognize that for some proposed actions, analyses may 
have already substantially progressed using the existing criteria in 
the 2018 Revised Technical Guidance or other methods for assessing 
hearing effects, and it may be impractical to begin those analyses 
anew, taking into account timing constraints, expense, and other 
considerations. In such ``pipeline'' cases, the applicant or action 
agency should contact NMFS as soon as possible to discuss how to best 
include consideration of the 2024 Updated Technical Guidance to satisfy 
the applicable requirements. A non-exhaustive list of factors that 
could affect the extent to which the 2024 Updated Technical Guidance 
will be quantitatively incorporated for an action include: The relative 
degree to which the 2024 Updated Technical Guidance is expected to 
affect the results of the acoustic impact analyses; how far in the 
process the application or prospective application has progressed; when 
the activity is scheduled to begin or other timing constraints; the 
complexity of the analyses and the cost and practicality of redoing 
them; and the temporal and spatial scope of anticipated effects. We 
anticipate that after the initial transition period, all applications 
for MMPA incidental take authorization and all requests for ESA section 
7 consultations involving noise that may affect marine mammal hearing 
will include full consideration of the 2024 Updated Technical Guidance.

Regulatory Context

    NMFS uses acoustic criteria to help quantify ``take'' and as part 
of more comprehensive effects analyses under several statutes. The 2024 
Updated Technical Guidance's acoustic criteria do not represent the 
entirety of the comprehensive effects analysis, but rather serve as one 
tool among others (e.g., behavioral impact criteria, auditory masking 
assessments, evaluations to help understand the ultimate effects of any 
particular type of impact on an individual's fitness, population 
assessments, etc.) to help evaluate the effects of a proposed action 
and make findings required by NMFS' various statutes.
    Under current agency practice, NMFS considers the onset of auditory 
injury as an example of ``Level A Harassment'' as defined in the MMPA 
and as ``harm'' as defined in ESA regulations, such that exposing an 
animal to weighted received sound levels at or above the indicated 
permanent threshold shift (PTS) threshold is predicted to result in 
these two types of ``take'' (i.e., Level A Harassment under the MMPA 
and harm under ESA).
    As explained below, NMFS does not consider a TTS to be an auditory 
injury under the MMPA or ESA, and thus it does not qualify as Level A 
harassment or harm. Nevertheless, TTS is an adverse effect that 
historically has been treated as ``take'' by ``Level B Harassment'' 
under the MMPA and ``harassment'' under the ESA.

Marine Mammal Protection Act

    The MMPA prohibits the take of marine mammals, with certain 
exceptions, one of which is the issuance of incidental take 
authorizations (ITAs). Sections 101(a)(5)(A) & (D) of the MMPA (16 
U.S.C. 1361 et seq.) direct the Secretary of Commerce to allow, upon 
request, the incidental, but not intentional, taking of small numbers 
of marine mammals by U.S. citizens who engage in a specified activity 
(other than commercial fishing) within a specified geographical region 
if certain findings are made. Through delegation by the Secretary of 
Commerce, NMFS is required to authorize the incidental taking of marine 
mammals if the agency finds that the total 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 certain subsistence uses. NMFS must also set forth the 
permissible methods of taking and requirements pertaining to the 
mitigation, monitoring, and reporting of such takings. (The ``small 
numbers'' and ``specified geographical region'' provisions do not apply 
to military readiness activities.)
    The term ``take'' means to harass, hunt, capture, or kill, or 
attempt to harass, hunt, capture or kill any marine mammal. 16 U.S.C. 
1362(13).
    Except with respect to certain activities described below, 
``harassment'' means any act of pursuit, torment, or annoyance which:
     Has the potential to injure a marine mammal or marine 
mammal stock in the wild (Level A Harassment); or
     Has the potential to disturb a marine mammal or marine 
mammal stock in the wild by causing disruption of behavioral patterns, 
including, but not limited to, migration, breathing, nursing, breeding, 
feeding or sheltering (Level B Harassment).
    See id. at 1362(18)(A)(i) & (ii) (emphasis added).
    Congress amended the definition of ``harassment'' as it applies to 
a ``military readiness activity'' or research conducted by or on behalf 
of the Federal government consistent with MMPA section 104(c)(3) as 
follows (section 3(18)(B) of the MMPA):
     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
     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).
    See id. at 1362(18)(B)(i) & (ii) (emphasis added).
    The term ``negligible impact'' is defined as an impact resulting 
from the specified activity that cannot be reasonably expected to, and 
is not reasonably likely to, adversely affect the species or stock 
through effects on annual rates of recruitment or survival. 50 CFR 
216.103.
    In support of the analysis that is necessary to make the required 
statutory determinations, MMPA implementing regulations require ITA 
action proponents to provide NMFS with specific information. Although 
they may also be used to inform the development of mitigation measures, 
the updated acoustic criteria are particularly relevant to the 
following 2 of the 14 required pieces of information:
     The type of incidental taking authorization that is being 
requested (i.e., takes by Level B Harassment only; Level A Harassment; 
or serious injury/

[[Page 84874]]

mortality) and the method of incidental taking; and
     By age, sex, and reproductive condition (if possible), the 
number of marine mammals (by species) that may be taken by each type of 
taking identified in paragraph (a)(5) of this section, and the number 
of times such takings by each type of taking are likely to occur. 50 
CFR 216.104 (emphasis added).

Endangered Species Act

    Section 9 of the ESA prohibits the take of ESA-listed species, with 
limited exceptions. Section 7 of the ESA requires that each Federal 
agency, in consultation with NMFS and/or the U.S. Fish and Wildlife 
Service (USFWS), ensure that any action authorized, funded, or carried 
out by the agency is not likely to jeopardize the continued existence 
of any endangered or threatened species or result in the destruction or 
adverse modification of designated critical habitat. See 16 U.S.C. 
1536(a)(2). Provided that NMFS or the USFWS reaches these conclusions 
through a ``formal consultation'' process, incidental take of ESA-
listed species may be exempted from the section 9 take prohibition 
through an ``incidental take statement'' that must specify the impact, 
i.e., the amount or extent, of the taking on the species. See id. at 
section 1536(b)(4). Incidental take statements must also include 
reasonable and prudent measures necessary or appropriate to minimize 
the impact, and the terms and conditions required to implement those 
measures.
    Under ESA, ``take'' means to harass, harm, pursue, hunt, shoot, 
wound, kill, trap, capture, or collect, or to attempt to engage in any 
such conduct. See id. at section 1532(19). ``Harm'' is defined in NMFS 
regulations as ``an act which actually kills or injures fish or 
wildlife'' (and can include significant habitat modification or 
degradation). See 50 CFR 222.102.
    Under NMFS and the USFWS implementing regulations for section 7 of 
the ESA, ``jeopardize the continued existence of'' means to engage in 
an action that reasonably would be expected, directly or indirectly, to 
reduce appreciably the likelihood of both the survival and recovery of 
a listed species in the wild by reducing the reproduction, numbers, or 
distribution of that species. See id. at Sec.  402.02.
    In support of the analysis necessary to conduct the consultation, 
the ESA implementing regulations state that in order to initiate formal 
consultation, the Federal action agency must submit a written request 
for formal consultation to the Director (of NMFS or the USFWS) that 
includes, among other things, a description of the manner in which the 
action may affect any listed species. See id. at Sec.  402.14(c).

Application of Acoustic Criteria for Auditory Injury

    The acoustic criterias for AUD INJ will be used in conjunction with 
sound source characteristics, environmental factors that influence 
sound propagation, anticipated marine mammal occurrence and behavior in 
the vicinity of the activity, as well as other available activity-
specific factors, to quantitatively estimate (acknowledging the gaps in 
scientific knowledge and the inherent uncertainties in a marine 
environment) the takes of marine mammals (by Level A harassment and 
harm under the MMPA and ESA, respectively) and facilitate compliance 
with the MMPA and ESA, as described above.
    NMFS will use the same AUD INJ criteria in the identification and 
quantification of MMPA Level A harassment for both military readiness 
and non-military readiness activities. Because the acoustic criteria 
for AUD INJ predict the onset of AUD INJ, they are inclusive of the 
``potential'' and ``significant potential'' language in the two 
definitions of Level A harassment. The limited data now available do 
not support the parsing out of a meaningful quantitative difference 
between the ``potential'' and ``significant potential'' for injury and, 
therefore, the designated AUD INJ criteria will be treated as onset of 
Level A harassment for both types of activities.
    Estimating the numbers of take by Level A harassment and harm is 
one component of the fuller analyses that inform NMFS' ``negligible 
impact'' and ``jeopardy'' determinations under the MMPA and ESA, 
respectively. Last, the AUD INJ criteria may be used to inform the 
development of mitigation and monitoring measures (such as shut-down 
zones) pursuant to the MMPA or ESA.
    When initiating any of the MMPA or ESA processes described above, 
agencies and other action proponents should utilize the AUD INJ 
criteria, in combination with activity-specific information, to predict 
whether, and if so how many, instances of AUD INJ are expected to 
occur.

Application of Acoustic Criteria for Temporary Threshold Shift

    As previously stated, NMFS has not considered TTS an auditory 
injury for purposes of the MMPA and ESA, based on the work of a number 
of investigators that have measured TTS before and after exposure to 
intense sound. For example, Ward (1997) suggested that a TTS is within 
the normal bounds of physiological variability and tolerance and does 
not represent physical injury. In addition, Southall et al. (2007, 
2019) indicate that although AUD INJ is a tissue injury, TTS is not 
because the reduced hearing sensitivity following exposure to intense 
sound results primarily from fatigue, not loss, of cochlear hair cells 
and supporting structures, and is reversible. TTS is not considered 
Level A harassment under the MMPA. However, given the associated 
disruptions of behavioral patterns anticipated to co-occur with TTS in 
some cases, it has been considered take by Level B harassment under the 
MMPA and harassment under the ESA, which will be the subject of future 
guidance.
    MMPA Level B harassment and ESA harassment are broad categories 
that encompass not only TTS but also other behaviorally related impacts 
that almost always involve a lower onset threshold than that for onset 
of TTS. In quantifying take by Level B harassment or harassment, NMFS 
considers all effects that fall into those categories of take, not just 
TTS. NMFS is in the process of developing updated acoustic criteria for 
the onset of behavioral effects and will further consider the best 
approach for considering TTS at that time. When that process is 
completed, NMFS will provide further guidance regarding how to best 
consider and/or quantify TTS for non-pulse and impulse sources that do 
not involve instantaneous explosives (see exception below for 
underwater explosives). In the meantime, except in the case of 
instantaneous underwater explosives (discussed next), action proponents 
do not need to quantify estimates of TTS separately from their overall 
behavioral harassment take calculations. NMFS will consider the TTS 
acoustic criteria in the 2024 Updated Technical Guidance as part of the 
larger comprehensive effects analyses under the MMPA and the ESA.
    With respect to instantaneous underwater explosives (as 
distinguished from repeated explosives such as gunnery exercises), NMFS 
already requires quantification of TTS estimates because instantaneous 
explosives do not have a separate behavioral component from a lower 
exposure threshold and there is no time accumulation involved. The 
rationale for calculating TTS for instantaneous explosives continues to 
apply with the 2024 Updated Technical Guidance.

[[Page 84875]]

    The occurrence and estimated number of TTS takes is one component 
of the larger analysis that informs NMFS's ``negligible impact'' and 
``jeopardy'' determinations under the MMPA and ESA, respectively. As 
with AUD INJ, TTS acoustic criteria also may be used to inform the 
development of mitigation and monitoring measures pursuant to the MMPA 
or ESA.

Comments and Responses

    On May 3, 2024, NMFS published the draft 2024 Updated Technical 
Guidance for a 45-day public comment period (89 FR 36762). During the 
public comment period, NMFS received 7 comments from individual members 
of the public, Fugro, EnerGeo Alliance and American Petroleum 
Institute, National Resources Defense Council, and Ocean Conservation 
Research. Six commenters (i.e., one commenter provided two separate 
comments) provided substantive comments addressing technical aspects or 
issues relating to the implementation of criteria.
    Some comments were similar to those received during the public 
comment periods for the 2016 Technical Guidance. For example, these 
similar comments addressed topics such as why NMFS does not consider 
TTS as injury, use of mean/medians, pseudoreplication, uncertainty, 
development of criteria for low-frequency (LF) cetaceans, Tougaard et 
al. 2015, and Wright 2015. NMFS will not repeat our responses from 2016 
here but instead refers readers to our previous Federal Register 
notification (81 FR 51694, August 4, 2016) that previously addressed 
these comment topics.

2024 Updated Technical Guidance Scope

    Comment 1: One commenter stated that Sirenians should be included 
in the 2024 Updated Technical Guidance.
    Response: The 2024 Updated Technical Guidance does not pertain to 
marine mammal species under the U.S. Fish and Wildlife Service's 
jurisdiction (e.g., walrus, polar bears, manatees, dugongs, sea 
otters). Thus, Sirenians are not included in the main document. 
However, they are included in the Navy's Technical Report, attached to 
the 2024 Updated Technical Guidance as Appendix A.

Peer Review Process

    Comment 2: One set of commenters stated that while the main 
sections of the 2024 Updated Technical Guidance was peer reviewed by 
experts, the Navy's Technical Report does not seem to be.
    Response: The 2024 Updated Technical Guidance document, including 
the Navy's Technical Report (Appendix A), completed all stages of the 
review process (i.e., the entire document underwent NMFS internal 
review, peer review, Federal agency preview, and public comment). 
Furthermore, during the peer review, federal agency preview, and public 
comment period, NMFS worked directly with the Navy to address some of 
the comments received.

Marine Mammal Hearing

    Comment 3: To define a marine mammal hearing group's generalized 
hearing range, one commenter asked whether the 65 dB level above the 
threshold of maximum hearing sensitivity was chosen arbitrarily (i.e., 
they asked for a reference that supports choosing a threshold 
specifically 5 dB wider than that in humans).
    Response: NMFS did not choose this level arbitrarily. We 
specifically chose the 65 dB threshold to be slightly wider than how 
the hearing range for humans is defined in order to encompass the 
general uncertainty of marine mammal hearing ranges, since we do not 
have hearing data for all species.
    Comment 4: One commenter's letter stated that sperm whales are 
categorized as high-frequency (HF) cetaceans but produce lower 
frequency sounds and are expected to hear lower frequencies compared to 
smaller delphinids. The commenter pointed out that Southall et al. 2019 
indicated that sperm whales, killer whales, and beaked whales may be 
separated from other HF cetaceans in the future (i.e., compose a mid-
frequency (MF) cetacean hearing group), but Appendix A (Finneran 
Technical Report) does not address this potential future hearing group. 
The comment also notes there is potential variation of phocid hearing 
that may necessitate splitting this hearing group in the future. In a 
related comment, another commenter questioned why killer whale hearing 
data were not used to define audiograms for a separate MF cetacean 
group.
    Response: While the potential to separate some marine mammal 
species into a separate MF cetacean hearing group is not directly 
mentioned in Appendix A (Finneran Technical Report), NMFS does address 
this in our main document. Specifically, table 1, footnote 1, says 
Southall et al. 2019 indicates that as more data become available there 
may be separate hearing group designations for very low-frequency 
cetaceans (blue, fin, right, and bowhead whales) and MF cetaceans 
(sperm, killer, and beaked whales). However, at this point, all baleen 
whales are part of the low-frequency (LF) cetacean hearing group, and 
sperm, killer, and beaked whales are part of the HF cetacean hearing 
group. Additionally, recent data indicate that as more data become 
available for Monachinae seals, separate hearing group designations may 
be appropriate for the two phocid subfamilies (Ruscher et al. 2021; 
Sills et al. 2021). NMFS concurs with the aforementioned studies that 
there are currently not enough data to further separate out MF 
cetaceans from HF cetaceans or further divide the phocid hearing group.
    Comment 5: One comment letter questioned why the LF cetacean 
generalized hearing range expanded by 1 kHz, if there are no new data.
    Response: While there are no new data available for LF cetaceans 
specifically, the weighting function parameters slightly changed for 
all hearing groups as a result of maintaining consistency with other 
hearing groups. As a result, NMFS re-examined the generalized hearing 
range for all hearing groups.
    Comment 6: Numerous commenters inquired why auditory evoked 
potential (AEP) data are not being considered in deriving composite 
audiograms for various hearing groups and encouraged that all data be 
considered. Some commenters advocated for further research to focus on 
better integrating AEP data.
    Response: NMFS agrees that incorporating AEP data into composite 
audiograms would be preferred and would expand the number of species 
where hearing data are available. Nevertheless, there are some well-
defined reasons this is not done. Behavioral techniques measure 
perception of sound by a receiver, while AEP methods measure only 
neural activity (Jewett and Williston 1971) (i.e., the two 
methodologies are not necessarily equivalent). Behavioral techniques, 
which are considered most representative (i.e., ``gold standard'' for 
measuring auditory sensitivity, consistently produce lower thresholds, 
which are indicative of greater sensitivity, than those obtained by 
AEPs (e.g., Szymanski et al. 1999; Yuen et al. 2005; Houser and 
Finneran 2006). AEP data are considered if they are the only data 
available for a hearing group (e.g., LF cetaceans; See Response to next 
Comment). Currently, there are no established means for ``correcting'' 
AEP data so they are more comparable to behavioral data (Heffner and 
Heffner 2003; Finneran 2015; Sisneros et al. 2016; Erbe et al. 2016). 
NMFS is aware that the Navy's Living Marine Resources

[[Page 84876]]

(LMR) Program is supporting a project entitled Standardizing Auditory 
Evoked Potential Hearing Thresholds with Behavioral Hearing Thresholds 
by Dorian Houser, National Marine Mammal Foundation, which may allow 
for the addition of AEP data to future versions of our Technical 
Guidance: https://exwc.navfac.navy.mil/Portals/88/Documents/EXWC/Environmental_Security/Living%20Marine%20Resources/LMRFactSheet_Project47.pdf. NMFS also included this topic as a specific 
data gap in Appendix B to the 2024 Updated Technical Guidance 
(``Research Recommendations for Improved Criteria'').
    Comment 7: Many commenters were interested in learning about recent 
AEP hearing measurements collected on minke whales by the National 
Marine Mammal Foundation. One commenter cautioned that the Balaenoptera 
(blue, fin, Rice's, Bryde's, minke, and sei whales) are just one genus 
of whale that use sounds in ways differently than Megaptera (humpback 
whales), Eschrichtius (gray whales), Balaena (bowhead whales), or 
Eubalaena (right whales) do. Similarly, another comment stated that 
NMFS should acknowledge that, given substantial differences among these 
LF cetacean species in their anatomy, sound production, and acoustic 
ecology, minke whales may not appropriately represent the hearing 
sensitivity of blue, fin, humpback, gray, and certain other baleen 
whales in the LF cetacean group; and that a more conservative approach 
to weighting in the very low frequencies may still be necessary. The 
commenter also remarked that NMFS should commit to a re-evaluation of 
LF cetacean group thresholds as soon as those data are published.
    Response: NMFS is aware that the National Marine Mammal Foundation 
successfully collected preliminary hearing data on two minke whales 
during their third field season (2023) in Norway. These data have 
implications for not only the generalized hearing range for LF 
cetaceans but also on their weighting function. However, at this time, 
no official results have been published. Furthermore, a fourth field 
season (2024) was recently completed, where more data were collected. 
Thus, it is premature for NMFS to propose any changes at this time. 
However, mysticete hearing data is identified as a special circumstance 
that could merit re-evaluating the acoustic criteria in the 2024 
Updated Technical Guidance, once the data from both field seasons are 
published.
    NMFS anticipates the publication of these AEP data from minke 
whales will help better inform the composite audiogram and associated 
weighting function for LF cetaceans. We agree that while having direct 
measurements of minke whale hearing would represent a significant 
milestone in better understanding mysticete hearing, these data will 
need to be considered carefully in the context of how they may or may 
not be appropriate to fill data gaps for other LF cetacean species.
    Comment 8: One commenter asserted that table A.3, which provides 
composite audiogram parameters, contains a number of errors. One error 
they purportedly identified is the use of hertz (Hz) instead of kHz 
(e.g., the F1 parameter for HF cetaceans is 9910 kHz and for Sirenians 
is 1680 kHz). Another example is the upper roll-off for the very-high 
frequency (VHF) cetaceans that begins below 100 kHz in Figure A.4, yet 
the value for the F2 parameter (132 kHz) in table A.3 fails to match 
this result. As a consequence, they assert, they were unable to assess 
NMFS' use of [Delta]T values in extrapolating certain parameters across 
hearing groups.
    Response: Table A.3 is correct and does not contain any errors. 
This can be verified by calculating Eq. (3) (i.e., median threshold 
equation for composite audiogram) with the parameters from table A.3 
and comparing the results to the composite audiograms. The F1 and F2 
parameters in table A.3 are not roll-off frequencies, but instead are 
fitting parameters for use with Eq. (3) to best match the composite 
audiogram data. Because of the large number of fitting parameters, the 
values may not always make physical sense, especially for audiograms 
without a plateau region. This point is specifically made in the 
Finneran Technical Report (Appendix A, including the example cited by 
the reviewer) that the large number and possible high dependency of 
fitting parameters, in some cases the specific fitting parameter values 
may not make physical sense (e.g., HF group F1 = 9910 kHz), and the 
important point is how well the resulting curve fits the median 
threshold data.
    Comment 9: One commenter advocated that the F1 audiogram fitting 
parameter for LF cetaceans be changed from 412 Hz to 137 Hz.
    Response: NMFS disagrees that the F1 audiogram fitting parameter 
for LF cetaceans should be adjusted, since no new data have become 
available supporting this change. This parameter is the same as 
published in Southall et al. 2019, as well as what was in the 2018 
Revised Technical Guidance, and is appropriate based on our current 
understanding of LF cetacean hearing. Furthermore, when the new minke 
whale hearing data become available, it is likely that NMFS will begin 
the process of updating the acoustic criteria for LF cetaceans based on 
these data. Thus, the composite audiogram, thresholds, and weighting 
functions for this hearing group will all be re-examined and 
appropriate adjustments can be made.

Weighting Functions

    Comment 10: A group of commenters requested that Appendix A 
(Finneran Technical Report) provide a detailed description of the 
methodology used to derive equations and criteria. On a few occasions, 
it is noted that mean values are calculated from data sets for deriving 
the weighting function, but the number of samples used to derive means 
or medians are not readily available. When available, this should be 
provided along with standard deviations.
    Response: Figures A14 through A17 in Appendix A show the individual 
TTS onset values and the mean values used to fit the exposure 
functions. The number of samples and a sense of the variability can be 
determined from these plots.
    Comment 11: Several commenters had questions about the 2024 Updated 
Technical Guidance modifying the weighting function high-frequency 
exponent (b) from 2 to 5, which was done to fit better underwater 
otariid pinniped data. Many asked why this parameter was modified for 
all hearing groups and not just otariids.
    Response: This question was also asked during our Peer Review. In 
the 2018 Technical Guidance (and Southall et al. 2019), the b parameter 
was the same for all hearing groups. Thus, the changes made to the 2024 
Updated Technical Guidance are consistent keeping this parameter the 
same for all hearing groups. Increasing the b parameter from two to 
five was done to fit better the underwater otariid (OW) pinniped 
function without substantially affecting the other marine mammal 
hearing group fits. The decision to keep the same b parameter for all 
groups was made to try to reduce complexity where possible, and there 
are no data to currently suggest this parameter varies by marine mammal 
hearing group.
    Comment 12: A group of commenters asked for clarification on the R2 
value for underwater phocid (PW) pinnipeds (i.e., -4.69) in table A.7 
displaying weighting function parameters.
    Response: A similar question was also asked during our Peer Review. 
The negative R2 indicates the curve-fit does not follow the general 
trend in the data (i.e., the data would have fit better with

[[Page 84877]]

a flat line). This is a result of the assumption that the weighting 
function should be broader than the audiogram, thus F1 was decreased 
after fitting for the PW group to match the audiogram 10-dB bandwidth. 
This prevents the weighting function from adjusting to best-fit the 
data points and causes the very low R2 value.

Temporary Threshold Shifts

    Comment 13: A group of commenters stated that NMFS should clarify 
which TTS data in table 7 were collected using electrophysiological 
(AEP) methods vs. behavioral methods. They indicate that table 7 is 
misleading as it lists all the TTS studies available for integration, 
while only a small portion of these studies was used in the analysis.
    Response: NMFS agrees that this is important information to include 
and has updated table 7 to distinguish between which studies collected 
hearing measurements via behavioral methodology vs. AEPs.
    Comment 14: A group of commenters disagreed that impulsive sounds 
are more injurious than non-impulsive sounds. Specifically, their 
comment indicated that there are no direct or consistent data for 
marine mammals demonstrating that impulsive sounds are more injurious. 
They also indicated that marine mammals are able to self-mitigate, 
which might protect them from noise exposure.
    Response: While there are limited marine mammal TTS data associated 
with exposure to impulsive sound, there is a rich library of literature 
available for terrestrial mammals that indicates exposure to impulsive 
sounds more often leads to mechanical damage of the inner ear, as well 
as more complex patterns of hearing recovery (e.g., Henderson and 
Hamernik 1986; Hamernik and Hsueh 1991). Additionally, inner ear 
anatomy is conserved among all mammals, including marine marine mammals 
(Grunstra et al. 2024). Thus, lessons learned from terrestrial mammals 
likely translate to marine mammals. Furthermore, the marine mammal TTS 
data currently available do support that TTS and AUD INJ onset 
thresholds are lower for impulsive sounds compared to non-impulsive 
sounds.
    Appendix B (Research Recommendations for Improved Criteria) in the 
2024 Updated Technical Guidance acknowledges that odontocetes may have 
multiple means of reducing or ameliorating the effects of noise 
exposure. However, at this point, directly incorporating these 
mechanisms into our AUD INJ and TTS criteria and anticipating the 
likelihood of exposure ahead of an activity is difficult. More 
information on these mechanisms, especially associated with real-world 
exposure scenarios, would be useful.
    Comment 15: One commenter remarked that Southall et al. 2019 found 
an approximately +/-6 dB difference in measured versus predicted TTS 
onset data. The commenter advocated there should be a similar measure 
of accuracy in Appendix A (Finneran Technical Report), but that the 
document provides no measure of natural variability or uncertainty, or 
any indication of predictability. Thus, they recommend that the agency 
implement a 6-dB reduction to its TTS and AUD INJ thresholds in line 
with the suggestions by Tougaard et al. 2015.
    Response: There are numerous figures showing TTS onset data and the 
relationship between the exposure functions and the TTS data. These 
graphs indicate the variability in the TTS data and the differences 
between the measurements of TTS onset and the predicted exposure 
functions. Furthermore, NMFS disagrees that a 6-dB reduction is 
necessary or justifiable (and despite Southall et al. 2019 indicating 
variability in TTS data for VHF cetaceans, they did not alter their 
proposed TTS thresholds).
    Comment 16: A group of commenters noted that there are significant 
changes for HF cetaceans criteria compared to the 2018 Technical 
Guidance, based on data from Finneran et al. 2023, and indicated that 
these changes were not highlighted in the draft 2024 Updated Technical 
Guidance. The comment expressed concern that since numerous HF species 
travel in large groups, there is a potential that take will be 
overestimated, as the 2024 Updated Technical Guidance does not account 
for behavioral responses of the animals. They also noted that in Figure 
A.14 for 2 kHz, it seems like the mean used for fitting value is much 
lower than the actual mean of the TTS onset values obtained by three 
studies (filled icons).
    Response: This change was highlighted in Section 1.3 of the draft 
2024 Updated Technical Guidance (Changes Associated with 2024 Updated 
Technical Guidance), specifically the fifth bullet: Lower TTS and AUD 
INJ thresholds ([cumulative sound exposure level metric] SEL24h metric) 
for HF cetaceans, below 10 kHz, based on new data (Finneran et al. 
2023a). Furthermore, the behavioral response of marine mammals to 
anthropogenic sounds is outside the scope of the 2024 Updated Technical 
Guidance. Finally, there are two TTS onset data points at 2 kHz (brown 
triangles in Figure A.14). The mean value (large circle in Figure A.14, 
visually between the two data points) was used during curve-fitting.
    Comment 17: A group of commenters noted Appendix A (Navy's 
Technical Report) indicates ``For VHF, new data suggest substantially 
higher onset TTS SELs at frequencies above ~10 kHz compared to the 
Phase 3 predictions, with high variability in the TTS onset data for 
harbor porpoises at 63 kHz (~40 dB difference in TTS onset for the two 
porpoises). Furthermore, the harbor porpoise behavioral TTS onset SELs 
are significantly higher than SELs resulting in large amounts (e.g., 
23-45 dB) of AEP TTS in Yangtze finless porpoise (see Fig. A.8). 
Although some differences in AEP/behavioral TTS data are expected, 
these large differences indicate that caution is warranted in adopting 
the high-frequency behavioral TTS data at the present time. For this 
reason, the VHF behavioral TTS onset data at frequencies >10 kHz were 
not used during the exposure function fitting process.'' They commented 
this is problematic as it deviates from the other species group and 
processes used throughout the document as behavioral methodology is 
preferred.
    Response: Numeric TTS onsets for the VHF cetaceans were derived 
using only behavioral data. For conditions where both behavioral and 
AEP data were available, the behavioral TTS data were used, because the 
relationship between AEP and behavioral TTS onsets is not clear. 
However, the AEP data cannot be completely ignored, and large 
differences between AEP and behavioral TTS onsets raise concerns. 
Because of the large difference in VHF cetacean behavioral TTS onset 
across individuals from the same study, and large differences between 
some of the behavioral and AEP data, the highest frequency VHF cetacean 
data were not used during the fitting process; only the lower frequency 
behavioral TTS data were used. This is consistent with the approach 
taken with other species groups, such as PW pinnipeds.
    Comment 18: A commenter noted the 2024 Updated Technical Guidance 
generates notably higher TTS onset weighted exposure levels for harbor 
porpoise than the species-specific Tougaard et al. 2022 publication, 
even when accounting for different weightings.
    Response: Tougaard et al. 2022 advocates examining harbor porpoise 
TTS data available since 2015 and calls for revisiting the synthesis 
culminating in Southall et al. 2019. NMFS' 2024 Updated Technical 
Guidance synthesizes all the same studies mentioned in Tougaard et al. 
2022 (table

[[Page 84878]]

IV from that publication), including those since Southall et al. 2019.
    For impulsive sounds, compared to Southall et al. 2019 (which 
Tougaard et al. 2022 supports), the TTS SEL24h thresholds in 
the 2024 Updated Technical Guidance are 4 dB higher. For non-impulsive 
sounds, compared to Southall et al. 2019 (which Tougaard et al. 2022 
indicates updated data below 10 kHz correspond well, with more 
differences above 10 kHz), the TTS SEL24h thresholds in the 
2024 Updated Technical Guidance are 8 dB higher. However, it should be 
noted that the weighting function for VHF cetaceans has also shifted, 
making them more susceptible to noise-induced hearing loss below 10 
kHz, where the majority of anthropogenic sound sources have their 
energy. Thus, both the thresholds and the weighting factors need to be 
considered in concert when evaluating the acoustic criteria for any 
marine mammal hearing group.
    Comment 19: A group of commenters noted that Appendix A (Finneran 
Technical Report) indicates that harbor seal TTS onset data below 2.5 
kHz were excluded from the Phase 4 fitting process and asked how this 
data exclusion would affect potential applicants more concerned about 
Moanachinae vs Phocinae pinnipeds.
    Response: All phocids exposed in water are within the PW pinniped 
group and use the same criteria, therefore the exclusion affects 
Monachinae and Phocinae in the same way.
    Comment 20: A commenter questioned NMFS' inclusion of a 132 dB data 
point in calculating the mean difference between TTS onset and the 
auditory threshold at f0. The commenter states that if 132 dB is not an 
outlier among the other cited values (116 dB, 116 dB, and 118 dB), then 
they do not know what would count as one. More generally, the commenter 
states that a visual review of Figures A.14 to A.17 indicates the 
presence of multiple apparent outliers, which collectively are likely 
to influence the fit of the exposure function.
    Response: In combining values or deriving central tendencies, the 
default approach was to use the mean. Identifying true outliers in the 
various datasets was difficult because the sample sizes were typically 
small, and in many cases, the underlying distributions were unknown. 
Even if individual species group distributions were Gaussian, it is 
probable that sampling from the different groups would result in a 
unique, non-Gaussian distribution, and thus a Gaussian distribution for 
purposes of statistically testing for outliers could not be assumed. 
Datasets were therefore considered on a case-by-case basis, taking into 
account not only the range of numeric values, but the specific 
circumstances under which the data were obtained. There were only four 
instances where changing from the mean to the median was warranted:
    (1) Estimating hearing thresholds for each group to create the 
composite audiograms. Here, the number of samples was relatively large, 
and there could be large fluctuations in thresholds across studies at 
any given frequency. Using the median value was the simplest way to 
estimate the composite audiogram, while reducing the influence of any 
outliers;
    (2) Calculating the parameter DT1. DT1 and DT2 (i.e., the amounts 
that the composite audiogram exceeded the minimum hearing threshold at 
the weighting function frequency parameters f1 and f2, respectively). 
Values of DT1 and DT2 were calculated for HF cetaceans, VHF cetaceans, 
OW pinnipeds, and PW pinnipeds. For the other species groups, the mean 
or median of these values was used to estimate f1 and f2 from their 
composite audiogram. DT1 values for groups HF cetaceans, VHF cetaceans, 
OW pinnipeds, and PW pinnipeds were 36.8, 11.5, 3.9, 6.5 dB, with mean 
= 14.7 and median = 9. The difference between 36.8 and its nearest 
neighbor was 25.3, which was almost 10x larger than the smallest value. 
For this reason, the median was used rather than the mean. In contrast, 
for DT2, values were 38.6, 22.7, 38.9, 39.4, with mean = 34.9 and 
median = 38.8. Here the mean was used rather than the median, despite a 
difference of 15.9 between 22.7 and its nearest neighbor;
    (3) Cumulative weighted impulse SEL-based TTS onset values for HF 
cetaceans. There were impulsive TTS onset data from four individuals in 
the HF cetacean group: 177, 178, 175, 188 dB SEL (mean = 180, median = 
178). The associated differences between steady-state and impulsive TTS 
onsets (Cs-Ci) were 4, 3, 6, and -7 dB (mean = 1.5, median = 3.5). 
Here, the median was used over concerns that the dolphin subject with 
the 188 dB onset may not be representative, based on his higher TTS 
onset and inverted relationship between steady-state and impulsive TTS 
onset compared to the other HF cetacean individuals. The inverted 
relationship between the steady-state and impulsive TTS onset values 
meant that including this subject would have made the impulsive TTS 
onsets more similar to steady-state onsets, which does not match our 
current understanding of TTS (impulsive noise is typically more 
hazardous); and
    (4) Estimating the audiogram function parameter B for mysticetes. 
To estimate B for LF cetaceans, the median of the B values from the 
composite audiograms for the other in-water species groups was used. 
The individual values were: 1.66, 24.5, 2.5, 0.786, and 1.79 (mean = 
6.25, median = 1.79). The range of values here is extreme, with the 
largest value >31x the smallest and almost 10x larger than its nearest 
neighbor. For this reason, the median was used.
    In terms of the specific comments, the differences between hearing 
threshold and TTS onset at f0 (the frequency of best sensitivity) were 
132, 118, 116, and 118 dB for HF cetaceans, VHF cetaceans, OW 
pinnipeds, and PW pinnipeds (mean = 121, median = 118). The range of 
values and differences between neighbors is more similar to those for 
DT2, where the mean was used, compared to DT1, where the median was 
used. For this reason, the mean was used. Figures A.14 through A.17 
show all available TTS data, regardless of the amount of TTS. These 
graphs therefore cannot be used to assess whether a data point is an 
``outlier'' or not.
    Comment 21: With the inclusion of in-air pinniped acoustic criteria 
in the 2024 Updated Technical Guidance, a group of commenters asked how 
NMFS plans on managing pinnipeds entering and leaving the water (e.g., 
Does NMFS anticipate having animals 100 percent in air and/or 100 
percent underwater during exposure calculations?).
    Response: The inclusion of the in-air pinniped criteria in our 2024 
Updated Technical Guidance was to promote consistency with other 
documents that previously have included in-air criteria, such as 
Southall et al. 2007 and Southall et al. 2019. Their inclusion does not 
necessitate anything changing in terms of how previous analyses have 
been completed for pinnipeds.

Recovery and Effective Quiet

    Comment 22: A group of commenters requested that recovery should be 
considered within the 2024 Updated Technical Guidance. They recommended 
that NMFS consider reviewing terrestrial data and comparing it to the 
cited two references (Finneran et al. 2010a and Finneran and Schlundt 
2013) that presented a model to approximate recovery in bottlenose 
dolphins. They stated that expansion to other odontocetes seems very 
reasonable and more supportable compared to some of the other decisions 
made in the 2024 Updated Technical Guidance.

[[Page 84879]]

    Response: NMFS acknowledges that after sound exposure ceases or 
between successive sound exposures, the potential for recovery from 
hearing loss exists, with AUD INJ resulting in incomplete recovery and 
TTS resulting in complete recovery. Nevertheless, predicting recovery 
from sound exposure can be complicated. Currently, recovery in wild 
marine mammals cannot be accurately quantified. As mentioned in the 
Comment, Finneran et al. 2010a and Finneran and Schlundt 2013 proposed 
a model that approximates recovery in bottlenose dolphins exposed to 
tones. However, the applicability of this model to other species, other 
sound sources, and other exposure conditions has yet to be determined. 
As more data become available for a broader array of species and sound 
sources, the incorporation of recovery can be considered for future 
iterations of the Technical Guidance.
    Comment 23: A group of commenters indicated that it was unclear why 
effective quiet (i.e., the maximum sound pressure level that will fail 
to produce any significant threshold shift in hearing despite duration 
of exposure and amount of accumulation) was not integrated in the 2024 
Updated Technical Guidance.
    Response: While NMFS agrees that effective quiet is an important 
consideration, there are limited data available to define effective 
quiet for marine mammals. As more data become available (identified as 
a data gap in Appendix B, Research Recommendations for Improved 
Criteria), they will be useful for a better understanding of 
appropriate accumulation periods for the weighted SEL24h 
metric and noise-induced hearing loss, as well as whether there is 
potential for low-level (e.g., Copping et al. 2014; Schuster et al. 
2015; Copping and Hemery 2020; Tougaard et al. 2020; St[ouml]ber and 
Thomsen 2021; Kulkarni and Edwards 2022), continuously operating 
sources (e.g., alternative energy tidal, wave, or wind turbines) to 
result in noise-induced hearing loss or not (i.e., below effective 
quiet).

Auditory Injury

    Comment 24: One commenter recommended that NMFS move away from 
establishing AUD INJ/TTS criteria and instead consider an ``Auditory 
Damage Index,'' which could include considerations of long-term hearing 
degeneration because of acute or chronic noise exposure and better 
allow for the assessment of a continuum of effects.
    Response: NMFS agrees that noise-induced hearing loss follows a 
continuum (Houser 2021). However, to best quantify this continuum in a 
regulatory context, NMFS has established an onset criteria for both TTS 
and AUD INJ.
    Comment 25: A commenter noted that the definition of AUD INJ 
includes but is not limited to PTS. They commented that AUD INJ 
threshold levels in many cases are higher than previous PTS levels in 
the 2018 Technical Guidance and asked if it is possible to determine 
levels that will result in the ``loss of cochlear neuron synapses or 
auditory neuropathy,'' and if NMFS is expanding the range of auditory 
damage to be considered, whether threshold levels in all cases should 
be reduced.
    Another commenter had a similar comment, where they were concerned 
that this criterion is not conservative for purposes of representing 
directly-induced AUD INJ. They indicated that 40 dB TTS cannot continue 
to represent AUD INJ once indirect or accumulated injury is added. 
Conceptually, if directly-induced PTS alone equates to 40 dB TTS, then 
the two processes together must equate to less than 40 dB TTS. Thus, 
levels of noise exposure that can result in auditory neuropathy are 
capable of inducing PTS or AUD INJ.
    Response: NMFS disagrees that based on our inclusion of AUD INJ, 
our thresholds should be adjusted or reduced. As stated in the 2024 
Updated Technical Guidance, in situations where destruction of auditory 
tissue has occurred in terrestrial mammals, threshold shifts were 30 to 
50 dB measured 24 hours after the exposure. There is no evidence that 
an exposure resulting in <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 is used as an upper limit of 
a threshold shift to prevent AUD INJ (i.e., it is assumed that only 
exposures beyond those capable of causing 40 dB of TTS have the 
potential to result in AUD INJ, which may or may not result in PTS).
    Comment 26: A group of commenters requested clarification regarding 
use of the phrase a ``few minutes'' in Appendix A (Finneran Technical 
Report) where it talks about 40 dB of TTS, measured a few minutes after 
exposure, being used as a conservative estimate for the onset of PTS. 
The commenters indicated that this phrase was vague and should be 
clarified.
    Response: In this context, a ``few minutes'' means the range of 
time over which marine mammal TTS initial post-exposure thresholds are 
obtained, typically 2 to 4 minutes.

Metrics

    Comment 27: One commenter noted that 2024 Updated Technical 
Guidance indicates that SEL24h metric is not intended to 
estimate impact of noise exposure ``over various spatial and temporal 
scales.'' The comment asked if this means the SEL24h metric 
is not intended for accumulating exposures that occur at considerably 
different locations and times; and the reason for such a limitation if 
that is the case. The commenter asked if auditory recovery plays a role 
in the explanation.
    Response: As the 2024 Updated Technical Guidance indicates, current 
data available for deriving criteria using the SEL24h metric 
are based on exposure to only a single source and therefore may not be 
appropriate for situations where exposure to multiple sources is 
occurring. As more data become available, the use of this metric can be 
re-evaluated for application of exposure from multiple activities 
occurring in space and time.
    While auditory recovery is an important consideration, predicting 
recovery from sound exposure can be complicated. Currently, recovery in 
wild marine mammals cannot be accurately quantified. For the 2024 
Updated Technical Guidance criteria, for intermittent, repeated 
exposures within a 24-hour period, NMFS assumes there is no recovery 
between subsequent exposures, although auditory recovery has been 
demonstrated in terrestrial mammals (Clark et al. 1987; Ward 1991) and 
more recently in a marine mammal studies (Finneran et al. 2010b; 
Kastelein et al. 2014a; Kastelein et al. 2015b). As more data become 
available, this topic can be further evaluated and potentially 
considered in future versions of this guidance.
    Comment 28: A commenter recommended the incorporation and use of 
the kurtosis metric in the 2024 Updated Technical Guidance.
    Response: NMFS agrees that kurtosis (i.e., a statistical quantity 
that represents the impulsiveness or ``peakedness'' of the event), can 
be a useful consideration for distinguishing between impulsive and non-
impulsive sounds. However, there are questions of how to apply this 
metric to marine mammal acoustic criteria (Von Benda-Beckmann et al. 
2022). NMFS has identified kurtosis as a topic for further research in 
the 2024 Updated Technical Guidance (Appendix B: Research 
Recommendations for Improved Criteria). While kurtosis may be useful in 
helping determine when impulsive vs. non-impulsive criteria might be 
applicable for a particular sound source in a specific situation, it

[[Page 84880]]

does not necessitate any changes to the criteria in the 2024 Updated 
Technical Guidance (i.e., kurtosis affects implementation of the 
acoustic criteria, not the criteria themselves).
    Comment 29: A group of commenters noted that for VHF cetaceans, 
there is one study (Kastelein et al. 2017c) where a higher peak sound 
pressure level (PK SPL) (199 dB) did not trigger TTS (maximum threshold 
shift of 3 to 5 dB), but Lucke et al. 2009 obtained significant TTS 
with a lower level PK SPL (195 dB) using AEP measurements. The 
commenters state these contradicting results highlight that PK SPL is 
currently not a robust and good predictor of TTS, and suggest that NMFS 
provide stronger reasoning on the choice of Lucke et al. 2009, while 
Kastelein et al. 2017c used behavioral methods (preferred method in 
Acoustic Guidance and Appendix A, Navy's Technical Report).
    Response: NMFS acknowledges there are limited marine mammal data 
available for impulsive sounds reporting the PK SPL metric. However, we 
disagree that PK SPL is not a good predictor of TTS and believe it has 
inherent value in establishing marine mammal AUD INJ and TTS criteria. 
As stated in the 2024 Updated Technical Guidance, sound exposure 
containing transient components (e.g., short duration and high 
amplitude; impulsive sounds) can create a greater risk of causing 
direct mechanical fatigue to the inner ear (as opposed to strictly 
metabolic) compared to sounds that are strictly non-impulsive 
(Henderson and Hamernik 1986; Levine et al. 1998; Henderson et al. 
2008). Often the risk of damage from these transient components does 
not depend on the duration of exposure. Thus, weighted 
SEL24h is not an appropriate metric to capture all the 
effects of impulsive sounds, which is why instantaneous PK SPL has also 
been chosen as part of NMFS's dual metric criteria for impulsive 
sounds. Of note, human noise standards recognize and provide separate 
criteria for impulsive sound sources using the PK SPL metric 
(Occupational Safety and Health Administration 29 CFR 1910.95; Starck 
et al. 2003).
    As indicated in the document (Appendix A, Navy's Technical Report), 
PK SPL thresholds for TTS were based on TTS data from single impulsive 
sound exposures that produced 6 dB or more TTS for the HF and VHF 
cetaceans (the only groups for which data are available). The PK SPL 
thresholds from these data were 224 and 196 dB, for HF and VHF 
cetaceans, respectively (table A.5, Finneran et al. 2002; Lucke et al. 
2009). The choice of relying on Lucke et al. 2009, even though it 
relies on AEP data, is due to the limited nature of the impulse TTS 
data for marine mammals and the likelihood that the VHF cetaceans are 
more susceptible than the HF cetaceans (i.e., use of the HF cetacean 
value is not appropriate). Based on the limited data, it is reasonable 
to assume that the exposures described by Lucke et al. 2009, which 
produced AEP-measured TTS of up to 20 dB, would have resulted in a 
behavioral TTS of at least 6 dB. Finally, Kastelein et al. 2017c is not 
used because it did not meet our definition of TTS as requiring a 
threshold shift of at least 6 dB.

Future Updates to Technical Guidance

    Comment 30: A group of commenters stated it is unclear how/when 
NMFS will decide the appropriate timeline to next update the Technical 
Guidance. They also questioned how NMFS will integrate future data in 
future iterations and whether an update would require another Navy 
Technical Report or Southall et al. publication.
    Response: The 2024 Updated Technical Guidance provides a procedure 
and timeline for future updates in Section 3.1., where it indicates 
that NMFS will continue to monitor and evaluate new data as they become 
available and periodically convene staff from our various offices, 
regions, and science centers to revise the Updated Technical Guidance 
as appropriate (anticipating updates to occur on a three to five year 
cycle). A new Navy Technical Report and/or Southall et al. publication 
would be considered if either becomes available.
    Finally, as mentioned in an earlier response, NMFS is aware that 
the National Marine Mammal Foundation successfully collected 
preliminary hearing data on 2 minke whales during their third field 
season (2023) in Norway. However, at this time, no official results 
have been published. Furthermore, a fourth field season (2024) was 
recently completed, where more data were collected. Thus, it is 
premature for NMFS to propose any changes at this time. However, 
mysticete hearing data is identified as a special circumstance that 
could merit re-evaluating the acoustic criteria in the 2024 Updated 
Technical Guidance, once the data from both field seasons are 
published.

Miscellaneous Issues

    Comment 31: A group of commenters stated that the absence of 
consideration of ambient noise in measurements targeted at measuring a 
single source can be problematic, as certain environments have already 
elevated ambient noise levels even without the introduction of any 
specific source or activity. NMFS listed ``Multiple sources'' as a 
research priority in Appendix B (Research Recommendations for Improved 
Criteria), and while the commenters agree that information on multiple 
sources might be important to better characterize how the acoustic 
environment of animals might change, it is also critical to recognize 
the importance of ambient noise, particularly if NMFS recommends 
applicants consider recording broadband measurements.
    Response: NMFS agrees that characterizing the existing soundscape 
is an important consideration and has added it to the Appendix B 
(Research Recommendations for Improved Criteria) as suggested.
    Comment 32: Several commenters inquired about the status of NMFS 
updating behavioral disturbance criteria for marine mammals.
    Response: Behavioral disturbance criteria are outside the scope of 
the 2024 Updated Technical guidance. However, NMFS is currently in the 
process of developing draft marine mammal behavioral disturbance 
criteria. To date, we have completed a NMFS internal review and have 
started the peer review via the Center for Independent Experts (July 
2024). For more information on how this review is progressing, please 
see: https://www.noaa.gov/information-technology/national-marine-fisheries-services-development-of-recommended-behavioral-disturbance-criteria-for.

    Dated: October 21, 2024.
Kimberly Damon-Randall,
Director, Office of Protected Resources, National Marine Fisheries 
Service.
[FR Doc. 2024-24748 Filed 10-23-24; 8:45 am]
BILLING CODE 3510-22-P`