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Federal Register / Vol. 86, No. 109 / Wednesday, June 9, 2021 / Notices
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which data are available. Sea otters and pinnipeds share a common mammalian aural physiology Echteler et al. 1994;
Solntseva 2007. Both are adapted to amphibious hearing, and both use sound in the same way primarily for inair communication rather than feeding.
Exposure Thresholds The National Marine Fisheries Service NMFS established noise exposure criteria for identifying underwater noise levels capable of causing Level A
harassment injury of otariid pinnipeds NMFS 2018. Sea otter-specific criteria have not been determined. However, because of their biological similarities, we assume that NMFS noise exposure criteria for otariid pinniped injury is a suitable surrogate for sea otter impacts.
Those criteria are based on estimated levels of sound exposure capable of causing a permanent shift in sensitivity of hearing e.g., a permanent threshold shift PTS NMFS 2018. A PTS occurs when noise exposure causes hairs within the inner ear system to die.
The NMFS 2018 criteria for sound exposure incorporate two metrics of exposure: The peak level of instantaneous exposure likely to cause PTS, and the cumulative sound exposure level during a 24-hour period SELcum. They also include weighting adjustments for the sensitivity of different species to varying frequencies.
The PTS-based injury criteria were developed from theoretical extrapolation of observations of temporary threshold shifts TTS
detected in lab settings during sound exposure trials. Studies were summarized by Finneran 2015. For otariid pinnipeds, PTS is predicted to occur at 232 dB peak or 203 dB SELcum for impulsive sound, or 219 dB SELcum for non-impulsive continuous sound.
The NMFS criteria for take by Level A harassment represents the best available information for predicting injury from exposure to underwater sound among pinnipeds, and in the absence of data specific to otters, we assume these criteria also represent appropriate exposure limits for Level A
take of sea otters.
The NMFS 2018 criteria do not identify thresholds for avoidance of Level B take. For pinnipeds, the NMFS
has adopted a 160-dB threshold for Level B take from exposure to impulse noise and a 120-dB threshold for continuous noise NMFS 1998; HESS
1999; NMFS undated. These thresholds were developed from observations of mysticete baleen whales responding to airgun operations e.g., Malme et al.
1983a, b; Richardson et al. 1986, 1995
and from equating Level B take with
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noise levels capable of causing TTS in lab settings.
We have evaluated these thresholds and determined that the Level B
threshold of 120-dB for non-impulsive noise is not applicable to sea otters. The 120-dB threshold is based on studies conducted by Malme et al. in the 1980s, during which gray whales Eschrichtius robustus were exposed to experimental playbacks of industrial noise. Gray whales are in the group of marine mammals believed to be most sensitive to low-frequency sounds, with an estimated audible frequency range of approximately 10 Hz to 30 kHz Finneran 2015. During the study, conducted at St. Lawrence Island, Alaska, Malme et al. 1988 observed the behavioral responses of gray whales to the playback of drillship noise and concluded that exposure to levels of 120 dB or more would probably cause avoidance of the area by more than onehalf of the gray whales. Sea otters do not usually occur at St. Lawrence Island, Alaska, but similar playback studies conducted off the coast of California Malme 1983a, 1984
included a southern sea otter monitoring component Riedman 1983, 1984. While the 1983 and 1984 studies detected probabilities of avoidance in gray whales comparable to those reported in Malme et al. 1988, there was no evidence of disturbance reactions or avoidance in southern sea otters. Thus, given the different range of frequencies to which sea otters and gray whales are sensitive, the NMFS 120-dB
threshold based on gray whale behavior is not appropriate for predicting sea otter behavioral responses, particularly for low-frequency sound.
Although no specific thresholds have been developed for sea otters, several alternative behavioral response thresholds have been developed for pinnipeds. Southall et al. 2007, 2019
assessed behavioral response studies and found considerable variability among pinnipeds. The authors determined that exposures between approximately 90 to 140 dB generally do not appear to induce strong behavioral responses in pinnipeds in water.
However, they found behavioral effects, including avoidance, become more likely in the range between 120 to 160
dB, and most marine mammals showed some, albeit variable, responses to sound between 140 to 180 dB. Wood et al. 2012 later adapted the approach identified in Southall et al. 2007 to develop a probabilistic scale for marine mammal taxa at which 10 percent, 50
percent, and 90 percent of individuals exposed are assumed to produce a behavioral response. For many marine
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mammals, including pinnipeds, these response rates were set at sound pressure levels of 140, 160, and 180 dB, respectively.
Based on the lack of sea otter disturbance response or any other reaction to the 1980s playback studies and the absence of a clear pattern of disturbance or avoidance behaviors attributable to underwater sound levels up to about 160 dB resulting from lowfrequency broadband noise, we assume 120 dB is not an appropriate behavioral response threshold for sea otters exposed to continuous underwater noise.
Thresholds based on TTS have been used as a proxy for Level B harassment i.e., 70 FR 1871, January 11, 2005; 71
FR 3260, January 20, 2006; and 73 FR
41318, July 18, 2008. Southall et al.
2007 derived TTS thresholds for pinnipeds based on 212 dB peak and 171 dB SELcum. Exposures resulting in TTS in pinnipeds were found to range from 152 to 174 dB 183 to 206 dB SEL
Kastak et al. 2005, with a persistent TTS, if not a PTS, after 60 seconds of 184 dB SEL Kastak et al. 2008.
Kastelein et al. 2012 found small but statistically significant TTSs at approximately 170 dB SEL 136 dB, 60
minutes min and 178 dB SEL 148 dB, 15 min. Finneran 2015 summarized these and other studies, and the NMFS
2018 has used the data to develop TTS
threshold for otariid pinnipeds of 188
dB SELcum for impulsive sounds and 199
dB SELcum for non-impulsive sounds.
Exposure to impulsive sound levels greater than 160 dB can elicit behavioral changes in marine mammals that may lead to detrimental disruption of normal behavioral routines. Thus, using information available for other marine mammals as a surrogate and taking into consideration the best available scientific information about sea otters, the Service has set 160 dB of received underwater sound as a threshold for Level B take by disturbance for sea otters for this proposed IHA based on the work of Ghoul and Reichmuth 2012a, b, McShane et al. 1995, NOAA 2005, Riedman 1983, Richardson et al. 1995, and others.
Exposure to unmitigated in-water noise levels between 125 Hz and 38 kHz that are greater than 160 dBfor both impulsive and non-impulsive sound sourceswill be considered by the Service as Level B take; thresholds for potentially injurious Level A take will be 232 dB peak or 203 dB SEL for impulsive sounds and 219 dB SEL for continuous sounds Table 1.
The area in which underwater noise in the frequency range of sea otter hearing will exceed thresholds is termed
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