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Federal Register / Vol. 86, No. 108 / Tuesday, June 8, 2021 / Notices m are small enough that the mitigation and monitoring measures are expected to avoid the potential for such taking.
As mentioned previously, distances to thresholds for slight lung injury and mortality are not modeled here, but are expected to have a similar small magnitude and range of values tens of meters as those reported for the Statter Harbor project 84 FR 11066; March 25, 2019. In conjunction with small zone sizes for onset of and 50 percent probability of gastrointestinal tract injury and NMFS expectation that zone sizes for slight lung injury will be small, blasting will occur in a confined area that allows for effective observation with only one entrance to the basin that will be blocked by a bubble curtain during blasting events, within a large shutdown zone equivalent to the Level A harassment zone for harbor porpoises, all of which make it unlikely that these types of non-auditory injuries will occur. Therefore, the potential for nonauditory physical injury is considered discountable, and any takes by Level A
harassment are expected to occur due to PTS.
As described previously, no mortality is anticipated or authorized for these activities. The method by which take is estimated is described below.
Generally speaking, NMFS estimates take by considering: 1 Acoustic thresholds above which NMFS believes marine mammals will be behaviorally harassed or incur some degree of permanent hearing impairment; 2 the area or volume of water that will be ensonified above these levels in a day;
3 the density or occurrence of marine mammals within these ensonified areas;
and, 4 and the number of days of
activities. NMFS notes that while these 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, NMFS describes the factors considered here in more detail and presents the authorized take.
Acoustic Thresholds NMFS recommends the use of acoustic thresholds that identify the received level of underwater sound above which exposed marine mammals would be reasonably expected to be behaviorally harassed or experience TTS equated to Level B harassment, or to incur PTS of some degree equated to Level A harassment. Thresholds have also been developed to identify the pressure levels above which animals may incur different types of tissue damage from exposure to pressure waves from explosive detonations.
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 NMFS considers Level B behavioral harassment when exposed to underwater anthropogenic noise above received levels of 120 dB re 1 mPa rms for continuous e.g., vibratory pile-driving, drilling and above 160 dB re 1 mPa rms for impulsive and/or intermittent e.g., impact pile driving sources.
The Navys Portsmouth Naval Shipyard modification and expansion project includes the use of continuous, or non-impulsive, i.e., vibratory pile driving and drilling and impulsive i.e., impact pile driving sources; therefore, the 120 and 160 dB re 1 mPa rms thresholds are appropriate.
Level A harassment for non-explosive sourcesNMFS Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing Version 2.0 NMFS, 2018 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. As noted above, the Navys planned activity includes the use of impulsive and non-impulsive sources.
These thresholds are provided in Table 4. The references, analysis, and methodology used in the development of the thresholds are described in NMFS 2018 Technical Guidance, which may be accessed at: https
www.fisheries.noaa.gov/national/
marine-mammal-protection/marinemammal-acoustic-technical-guidance.

TABLE 4THRESHOLDS IDENTIFYING THE ONSET OF PERMANENT THRESHOLD SHIFT
PTS onset acoustic thresholds
received level
Hearing group
Impulsive
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High-Frequency HF Cetaceans
Phocid Pinnipeds PW Underwater

Non-impulsive
Cell 5: Lpk,flat: 202 dB; LE,HF,24h: 155 dB
Cell 7: Lpk,flat: 218 dB; LE,PW,24h: 185 dB

Cell 6: LE,HF,24h: 173 dB.
Cell 8: LE,PW,24h: 201 dB.

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 HF cetaceans and PW
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 will be exceeded.

Explosive sourcesBased on the best available science, NMFS uses the acoustic and pressure thresholds
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indicated in Table 5 to predict the onset of behavioral harassment, PTS, and nonauditory impacts. Because of the
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instantaneous nature of blasting, there is no established Level B behavioral harassment threshold associated with
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