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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Proposed Rules available e.g., previous monitoring results or average group size. Below, we describe the factors considered here in more detail and present the proposed take estimate.
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 equated to Level B harassment or to incur PTS of some degree equated to Level A harassment.
As described in detail for NEFSC and other science centers in previously issued Federal Register notices e.g., 85
FR 53606, August 28, 2020; 88 FR
27028, May 6, 2020, the use of the sources used by NMFS Science Centers, including NEFSC, do not have the potential to cause Level A harassment;
therefore, our discussion is limited to behavioral harassment Level B
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 rms for continuous e.g., vibratory piledriving, drilling and above 160 dB re 1
mPa rms for intermittent e.g., scientific sonar sources. NEFSC surveys include the use of non-impulsive, intermittent sources and therefore the 160 dB re 1
mPa rms threshold is applicable.
The operating frequencies of active acoustic systems used by the NEFSC
range from 30333 kHz see Table 2.
Examination of these sources considers operational patterns of use relative to each other, and which sources would have the largest potential impact zone when used simultaneously. NEFSC
determined that the EK60, ME70, and DSM 300 sources comprise the total
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effective exposures relative to linekilometers surveyed see Section 6.5 of the Application. Acoustic disturbance takes are calculated for these three dominant sources. Of these dominant acoustic sources, only the EK60 can use a frequency within the hearing range of baleen whales 18k Hz. Therefore, for North Atlantic right whales and all other baleen whales, Level B harassment is only expected for exposure to the EK60. The other two dominant sources are outside of their hearing range. The ADCP Ocean Surveyor operates at 75
kHz, which is outside of baleen whale hearing capabilities. Therefore, we would not expect any exposures to these signals to result in behavioral harassment in baleen whales.
The assessment paradigm for active acoustic sources used in NEFSC
fisheries research is relatively straightforward and has a number of key simple and conservative assumptions.
NMFS current acoustic guidance requires in most cases that we assume Level B harassment occurs when a marine mammal receives an acoustic signal at or above a simple step-function threshold. Estimating the number of exposures at the specified received level requires several determinations, each of which is described sequentially below:
1 A detailed characterization of the acoustic characteristics of the effective sound source or sources in operation;
2 The operational areas exposed to levels at or above those associated with Level B harassment when these sources are in operation;
3 A method for quantifying the resulting sound fields around these sources; and 4 An estimate of the average density for marine mammal species in each area of operation.
Quantifying the spatial and temporal dimension of the sound exposure footprint or swath width of the active acoustic devices in operation on moving vessels and their relationship to the average density of marine mammals enables a quantitative estimate of the number of individuals for which sound levels exceed the relevant threshold for each area. The number of potential incidents of Level B harassment is ultimately estimated as the product of the volume of water ensonified at 160
dB rms or higher and the volumetric density of animals determined from simple assumptions about their vertical stratification in the water column.
Specifically, reasonable assumptions based on what is known about diving behavior across different marine mammal species were made to segregate those that predominately remain in the upper 200 m of the water column versus
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those that regularly dive deeper during foraging and transit. Methods for estimating each of these calculations are described in greater detail in the following sections, along with the simplifying assumptions made, and followed by the take estimates.
Sound source characteristicsAn initial characterization of the general source parameters for the primary active acoustic sources operated by the NEFSC
was conducted, enabling a full assessment of all sound sources used by the NEFSC. This auditing of the active acoustic sources also enabled a determination of the predominant sources that, when operated, would have sound footprints exceeding those from any other simultaneously used sources. These sources were effectively those used directly in acoustic propagation modeling to estimate the zones within which the 160 dB rms received level would occur.
Many of these sources can be operated in different modes and with different output parameters. In modeling their potential impact areas, those features among the sources identified in Table 2
e.g., lowest operating frequency that would lead to the most precautionary estimate of maximum received level ranges i.e., largest ensonified area were used. The effective beam patterns took into account the normal modes in which these sources are typically operated.
While these signals are brief and intermittent, a conservative assumption was taken in ignoring the temporal pattern of transmitted pulses in calculating Level B harassment events.
Operating characteristics of each of the predominant sound sources were used in the calculation of effective linekilometers and area of exposure for each source in each survey.
Calculating effective line-kilometers As described below, based on the operating parameters for each source type, an estimated volume of water ensonified at or above the 160 dB rms threshold was calculated. In all cases where multiple sources are operated simultaneously, the one with the largest estimated acoustic footprint was considered to be the effective source.
Two depth zones were defined for each of the four research areas: 0200 m and >200 m. Effective line distance and volume ensonified was calculated for each depth strata 0200 m and >200
m, where appropriate. In some cases, this resulted in different sources being predominant in each depth stratum for all line km i.e., the total linear distance traveled during acoustic survey operations when multiple sources were in operation. This was accounted for in estimating overall exposures for species
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