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Federal Register / Vol. 86, No. 195 / Wednesday, October 13, 2021 / Proposed Rules indicate that a 40 dB threshold shift approximates PTS onset see Ward et al.
1958, 1959; Ward 1960; Kryter et al.
1966; Miller 1974; Ahroon et al. 1996;
Henderson et al. 2008. PTS levels for marine mammals are estimates, and, with the exception of a single study unintentionally inducing PTS in a harbor seal Kastak et al. 2008, there are no empirical data measuring PTS in marine mammals, largely due to the fact that, for various ethical reasons, experiments involving anthropogenic noise exposure at levels inducing PTS
are not typically pursued or authorized NMFS 2018.
Temporary Threshold Shift TTS
TTS is a temporary, reversible 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. Based on data from cetacean TTS measurements see Southall et al. 2007, a TTS of 6 dB is considered the minimum threshold shift clearly larger than any day-to-day or session-to-session variation in a subjects normal hearing ability Schlundt et al. 2000; Finneran et al.
2000, 2002. As described in Finneran 2015, marine mammal studies have shown the amount of TTS increases with cumulative sound exposure level SELcum in an accelerating fashion: At low exposures with lower SELcum, the amount of TTS is typically small and the growth curves have shallow slopes.
At exposures with higher SELcum, the growth curves become steeper and approach linear relationships with the noise SEL.
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 similar to those discussed in auditory masking, below. 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 takes place during a time when the animal is traveling through the open ocean, 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 a time when communication is critical for successful mother/calf interactions could have more serious impacts. We note that reduced hearing sensitivity as a simple function of aging has been observed in marine mammals, as well as humans and other taxa Southall et al.
2007, so we can infer that strategies exist for coping with this condition to
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some degree, though likely not without cost.
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 five species of pinnipeds exposed to a limited number of sound sources i.e., mostly 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. No data are available on noiseinduced hearing loss for mysticetes. For summaries of data on TTS in marine mammals or for further discussion of TTS onset thresholds, please see Southall et al. 2007, Finneran and Jenkins 2012, Finneran 2015, and Table 5 in NMFS 2018. Installing piles requires a combination of impact pile driving and vibratory pile driving. For this project, these activities would not occur at the same time and there would be pauses in activities producing the sound during each day. Given these pauses and that many marine mammals are likely moving through the ensonified area and not remaining for extended periods of time, the potential for TS declines.
Behavioral HarassmentExposure to noise from pile driving and removal also has the potential to behaviorally disturb marine mammals. Available studies show wide variation in response to underwater sound; therefore, it is difficult to predict specifically how any given sound in a particular instance might affect marine mammals perceiving the signal. If a marine mammal does react briefly to an underwater sound by changing its behavior or moving a small distance, the impacts of the change are unlikely to be significant to the individual, let alone the stock or population. However, if a sound source displaces marine mammals from an important feeding or breeding area for a prolonged period, impacts on individuals and populations could be significant e.g., Lusseau and Bejder 2007; Weilgart 2007; NRC 2005.
Disturbance may result in changing durations of surfacing and dives, number of blows per surfacing, or moving direction and/or speed;
reduced/increased vocal activities;

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changing/cessation of certain behavioral activities such as socializing or feeding; visible startle response or aggressive behavior such as tail/fluke slapping or jaw clapping; avoidance of areas where sound sources are located.
Pinnipeds may increase their haulout time, possibly to avoid in-water disturbance Thorson and Reyff 2006.
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. In general, pinnipeds seem more tolerant of, or at least habituate more quickly to, potentially disturbing underwater sound than do cetaceans, and generally seem to be less responsive to exposure to industrial sound than most cetaceans.
Please see Appendices BC of Southall et al. 2007 for a review of studies involving marine mammal behavioral responses to sound.
Disruption of feeding behavior can be difficult to correlate with anthropogenic sound exposure, so it is usually inferred by observed displacement from known foraging areas, the appearance of secondary indicators e.g., bubble nets or sediment plumes, or changes in dive behavior. As for other types of behavioral response, the frequency, duration, and temporal pattern of signal presentation, as well as differences in species sensitivity, are likely contributing factors to differences in response in any given circumstance e.g., Croll et al. 2001; Nowacek et al.
2004; Madsen et al. 2006; Yazvenko et al. 2007. A determination of whether foraging disruptions incur fitness consequences would require information on or estimates of the energetic requirements of the affected individuals and the relationship between prey availability, foraging effort and success, and the life history stage of the animal.
Stress responsesAn animals perception of a threat may be sufficient to trigger stress responses consisting of some combination of behavioral
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