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Federal Register / Vol. 86, No. 167 / Wednesday, September 1, 2021 / Notices basic factors can contribute to a basic calculation to provide an initial prediction of takes, additional information that can qualitatively inform take estimates is also sometimes available e.g., previous monitoring results or average group size. Below, we describe the factors considered here in more detail and present the take estimate.
Acoustic Thresholds NMFS recommends the use of acoustic thresholds that identify the received level of underwater sound above which exposed marine mammals will be reasonably expected to be behaviorally harassed equated to Level B harassment or to incur PTS of some degree equated to Level A harassment.
Level B Harassment for non-explosive sourcesThough significantly driven by received level, the onset of behavioral disturbance from anthropogenic noise exposure is also informed to varying degrees by other factors related to the source e.g., frequency, predictability, duty cycle, the environment e.g., bathymetry, and the receiving animals hearing, motivation, experience, demography, behavioral context and can be difficult to predict Southall et
al., 2007, Ellison et al., 2012. Based on what the available science indicates and the practical need to use a threshold based on a factor that is both predictable and measurable for most activities, NMFS uses a generalized acoustic threshold based on received level to estimate the onset of behavioral harassment. NMFS predicts that marine mammals are likely to be behaviorally harassed in a manner we consider Level B harassment when exposed to underwater anthropogenic noise above received levels of 120 dB re 1 mPa root mean square rms for continuous e.g., vibratory hammer and above 160 dB re 1 mPa rms for non-explosive impulsive e.g., impact hammers pile-driving or intermittent e.g., scientific sonar sources.
The Navys pile driving/removal activities includes the use of stationary, non-impulsive, and continuous noise sources vibratory hammer, diamond wire saw, underwater chainsaw, single use or concurrent use of pile clippers, and therefore the 120 dB re 1 mPa rms is applicable. However, as discussed above, the Navy measurements support an ambient noise estimate of 129.6 dB
re 1 mPa rms in the project area.
Accordingly, we have adjusted the
standard Level B harassment threshold of 120 dB to 129.6 dB, as it likely provides a more realistic and accurate basis for predicting Level B harassment in the San Diego Bay area.
Level A harassment for non-explosive sourcesNMFS Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing Version 2.0 NMFS, 2018a identifies dual criteria to assess auditory injury Level A harassment to five different marine mammal groups based on hearing sensitivity as a result of exposure to noise from two different types of sources impulsive or nonimpulsive. The Navys pile driving/
removal activities includes the use of non-impulsive vibratory pile removal and other cutting and removal methods sources.
These thresholds are provided in Table 2 below. The references, analysis, and methodology used in the development of the thresholds are described in NMFS 2018a Technical Guidance, which may be accessed at https www.fisheries.noaa.gov/
national/marine-mammal-protection/
marine-mammal-acoustic-technicalguidance.

TABLE 2THRESHOLDS IDENTIFYING THE ONSET OF PERMANENT THRESHOLD SHIFT PTS
PTS onset acoustic thresholds 1
received level
Hearing group
Impulsive Low-Frequency LF Cetaceans
Mid-Frequency MF Cetaceans
High-Frequency HF Cetaceans
Phocid Pinnipeds PW Underwater
Otariid Pinnipeds OW Underwater

Cell Cell Cell Cell Cell
1:
3:
5:
7:
9:

Lpk,flat:
Lpk,flat:
Lpk,flat:
Lpk,flat:
Lpk,flat:

219
230
202
218
232

dB;
dB;
dB;
dB;
dB;

Non-impulsive
LE,LF,24h: 183 dB
LE,MF,24h: 185 dB
LE,HF,24h: 155 dB
LE,PW,24h: 185 dB
LE,OW,24h: 203 dB

Cell Cell Cell Cell Cell
2: LE,LF,24h: 199 dB.
4: LE,MF,24h: 198 dB.
6: LE,HF,24h: 173 dB.
8: LE,PW,24h: 201 dB.
10: LE,OW,24h: 219 dB.

1 Dual metric acoustic thresholds for impulsive sounds: Use whichever results in the largest isopleth for calculating PTS onset. If a non-impulsive sound has the potential of exceeding the peak sound pressure level thresholds associated with impulsive sounds, these thresholds should also be considered.
Note: Peak sound pressure Lpk has a reference value of 1 Pa, and cumulative sound exposure level LE has a reference value of 1Pa2s.
In this Table, thresholds are abbreviated to reflect American National Standards Institute standards ANSI 2013. However, peak sound pressure is defined by ANSI as incorporating frequency weighting, which is not the intent for this Technical Guidance. Hence, the subscript flat is being included to indicate peak sound pressure should be flat weighted or unweighted within the generalized hearing range. The subscript associated with cumulative sound exposure level thresholds indicates the designated marine mammal auditory weighting function LF, MF, and HF
cetaceans, and PW and OW pinnipeds and that the recommended accumulation period is 24 hours. The cumulative sound exposure level thresholds could be exceeded in a multitude of ways i.e., varying exposure levels and durations, duty cycle. When possible, it is valuable for action proponents to indicate the conditions under which these acoustic thresholds would be exceeded.

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Ensonified Area Here, we describe operational and environmental parameters of the activity that will feed into identifying the area ensonified above the acoustic thresholds, which include source levels, durations, and transmission loss coefficient.
The sound field in the project area is the existing background noise plus additional construction noise from this project. Marine mammals are expected to be affected via sound generated by
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the primary components of the project i.e., vibratory pile removal, diamond wire saw, single use or concurrent use of pile clippers, and underwater chainsaws.
Vibratory hammers produce constant sound when operating, and produce vibrations that liquefy the sediment surrounding the pile, allowing it to penetrate to the required seating depth or be withdrawn more easily. The actual durations of each method vary depending on the type and size of the pile.

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In order to calculate the distance to the Level B harassment sound threshold for piles of various sizes being used in this project, the Navy used acoustic monitoring data from other locations and projects to develop source levels for the various pile types, sizes, and methods of removal. Data for the removal methods i.e., a diamond wire saw, individual use or concurrent use of pile clippers, and an underwater chainsaw comes from data gathered at other nearby or related Navy projects as reported in their San Diego Noise
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