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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices
in such effects see below for further discussion. Potential effects from impulsive sound sources can range in severity from effects such as behavioral disturbance or tactile perception to physical discomfort, slight injury of the internal organs and the auditory system, or mortality Yelverton et al., 1973.
Non-auditory physiological effects or injuries that theoretically might occur in marine mammals exposed to high level underwater sound or as a secondary effect of extreme behavioral reactions e.g., change in dive profile as a result of an avoidance reaction caused by exposure to sound include neurological effects, bubble formation, resonance effects, and other types of organ or tissue damage Cox et al., 2006; Southall et al., 2007; Zimmer and Tyack, 2007;
Tal et al., 2015. The construction activities considered here do not involve the use of devices such as explosives or mid-frequency tactical sonar that are associated with these types of effects.
Threshold ShiftMarine mammals exposed to high-intensity sound, or to lower-intensity sound for prolonged periods, can experience hearing threshold shift TS, which NMFS
defines as a change, usually an increase, in the threshold of audibility at a specified frequency or portion of an individuals hearing range above a previously established reference level NMFS, 2018. TS can be permanent PTS, in which case the loss of hearing sensitivity is not fully recoverable, or temporary TTS, in which case the animals hearing threshold would recover over time Southall et al., 2007.
Repeated sound exposure that leads to TTS could cause PTS. In severe cases of PTS, there can be total or partial deafness, while in most cases the animal has an impaired ability to hear sounds in specific frequency ranges Kryter, 1985.
When PTS occurs, there is physical damage to the sound receptors in the ear i.e., tissue damage, whereas TTS
represents primarily tissue fatigue and is reversible Southall et al., 2007. In addition, other investigators have suggested that TTS is within the normal bounds of physiological variability and tolerance and does not represent physical injury e.g., Ward, 1997.
Therefore, NMFS does not consider TTS
to constitute auditory injury.
Relationships between TTS and PTS
thresholds have not been studied in marine mammals, and there is no PTS
data for cetaceans, but such relationships are assumed to be similar to those in humans and other terrestrial mammals. PTS typically occurs at exposure levels at least several decibels
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above a 40-dB threshold shift approximates PTS onset; e.g., Kryter et al., 1966; Miller, 1974 that inducing mild TTS a 6-dB threshold shift approximates TTS onset; e.g., Southall et al., 2007. Based on data from terrestrial mammals, a precautionary assumption is that the PTS thresholds for impulse sounds such as impact pile driving pulses as received close to the source are at least 6 dB higher than the TTS threshold on a peak-pressure basis and PTS cumulative sound exposure level thresholds are 15 to 20 dB higher than TTS cumulative sound exposure level thresholds Southall et al., 2007.
Given the higher level of sound or longer exposure duration necessary to cause PTS as compared with TTS, it is considerably less likely that PTS could occur.
TTS is the mildest form of hearing impairment that can occur during exposure to sound Kryter, 1985. While experiencing TTS, the hearing threshold rises, and a sound must be at a higher level in order to be heard. In terrestrial and marine mammals, TTS can last from minutes or hours to days in cases of strong TTS. In many cases, hearing sensitivity recovers rapidly after exposure to the sound ends. Few data on sound levels and durations necessary to elicit mild TTS have been obtained for marine mammals.
Marine mammal hearing plays a critical role in communication with conspecifics, and interpretation of environmental cues for purposes such as predator avoidance and prey capture.
Depending on the degree elevation of threshold in dB, duration i.e., recovery time, and frequency range of TTS, and the context in which it is experienced, TTS can have effects on marine mammals ranging from discountable to serious. For example, a marine mammal may be able to readily compensate for a brief, relatively small amount of TTS
in a non-critical frequency range that occurs during a time where ambient noise is lower and there are not as many competing sounds present.
Alternatively, a larger amount and longer duration of TTS sustained during time when communication is critical for successful mother/calf interactions could have more serious impacts.
Currently, TTS data only exist for four species of cetaceans bottlenose dolphin, beluga whale Delphinapterus leucas, harbor porpoise, and Yangtze finless porpoise Neophocoena asiaeorientalis and three species of pinnipeds northern elephant seal Mirounga angustirostris, harbor seal, and California sea lion Zalophus californianus exposed to a limited number of sound sources i.e., mostly
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tones and octave-band noise in laboratory settings Finneran, 2015.
TTS was not observed in trained spotted Phoca largha and ringed Pusa hispida seals exposed to impulsive noise at levels matching previous predictions of TTS onset Reichmuth et al., 2016. In general, harbor seals and harbor porpoises have a lower TTS
onset than other measured pinniped or cetacean species Finneran, 2015.
Additionally, the existing marine mammal TTS data come from a limited number of individuals within these species. There are no data available on noise-induced hearing loss for mysticetes. For summaries of data on TTS or PTS in marine mammals or for further discussion of TTS or PTS onset thresholds, please see Southall et al.
2007, Finneran and Jenkins 2012, Finneran 2015, and NMFS 2018.
Behavioral EffectsBehavioral disturbance may include a variety of effects, including subtle changes in behavior e.g., minor or brief avoidance of an area or changes in vocalizations, more conspicuous changes in similar behavioral activities, and more sustained and/or potentially severe reactions, such as displacement from or abandonment of high-quality habitat.
Behavioral responses to sound are highly variable and context-specific and any reactions depend on numerous intrinsic and extrinsic factors e.g., species, state of maturity, experience, current activity, reproductive state, auditory sensitivity, time of day, as well as the interplay between factors e.g., Richardson et al., 1995; Wartzok et al., 2003; Southall et al., 2007; Weilgart, 2007; Archer et al., 2010. Behavioral reactions can vary not only among individuals but also within an individual, depending on previous experience with a sound source, context, and numerous other factors Ellison et al., 2012, and can vary depending on characteristics associated with the sound source e.g., whether it is moving or stationary, number of sources, distance from the source.
Please see Appendices BC of Southall et al. 2007 for a review of studies involving marine mammal behavioral responses to sound.
Habituation can occur when an animals response to a stimulus wanes with repeated exposure, usually in the absence of unpleasant associated events Wartzok et al., 2003. Animals are most likely to habituate to sounds that are predictable and unvarying. It is important to note that habituation is appropriately considered as a progressive reduction in response to stimuli that are perceived as neither aversive nor beneficial, rather than as,
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