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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices
avoidance was temporary Lindeboom et al., 2011.
Overall, the available literature suggests harbor seals and harbor porpoises have shown avoidance of pile driving at offshore wind projects during the construction phase in some instances, with the duration of avoidance varying greatly, and with repopulation of the area generally occurring post-construction. The literature suggests that marine mammal responses to pile driving in the offshore environment are not predictable and may be context-dependent. It should also be noted that the only studies available on marine mammal responses to offshore wind-related pile driving have focused on species which are known to be more behaviorally sensitive to auditory stimuli than the other species that occur in the project area.
Therefore, the documented behavioral responses of harbor porpoises and harbor seals to pile driving in Europe should be considered as a worst case scenario in terms of the potential responses among all marine mammals to offshore pile driving, and these responses cannot reliably predict the responses that will occur in other marine mammal species. Harwood et al.
2014 discuss a theoretical framework to predict the population level consequences of disturbance from offshore renewable energy development in the UK on bottlenose dolphins and minke whales among other species, providing illustrative examples of the extent to which each species might be exposed to behavioral disturbance or experience PTS on a given construction day, as well as probabilities of different levels of population decline at the end of the modeled construction period. For bottlenose dolphins, most of the simulated populations had declined in abundance by less than 5 percent by the time construction of the offshore wind project ended; of the simulated minke whale populations, the mean decline in abundance was approximately 3
percent. The results, which relied heavily on assumptions and expert opinion, highlight the need for empirical data to support more robust predictive capabilities for assessment of population level impacts of offshore wind development on affected species Harwood et al., 2014.
Noise generated from vibratory pile driving is mostly concentrated at lower frequencies. Rise time is slower, and sound energy is distributed over a great amount of time, reducing the probability and severity of injury Nedwell and Edwards, 2002; Carlson et al. 2005.
Vibratory hammers produce peak SPLs that may be 180 dB or greater, but are
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generally 10 to 20 dB lower than SPLs generated during impact pile driving of the same-sized pile Oestman et al., 2009. Measurements from vibratory pile driving of sheet piles during construction activities for bridges and piers indicate that root mean square sound pressure level SPLrms produced by this activity can range from 130 to 170 dB referenced to 1 micropascal squared seconds dB re 1 mPa2 s; re 1
mPa depending on the measured distance from the source and physical properties of the location Buehler et al., 2015; Illingworth and Rodkin, Inc., 2017.
Masking, which occurs when the receipt of a sound is interfered with by a coincident sound at similar frequencies and similar or higher levels, may occur during the short periods of vibratory pile driving; however, this is unlikely to become biologically significant. It is possible that vibratory pile driving resulting from construction and removal of the temporary cofferdam may mask acoustic signals important to low frequency marine mammals, but the short-term duration approximately 36
hours over 3 non-consecutive days, 18
hours each for installation and removal would result in limited impacts from masking. In this case, vibratory pile driving durations are relatively short and no significant seal rookeries or haulouts, or cetacean foraging habitats are located near the inshore proposed cofferdam locations.
While thresholds for auditory impairment consider exposure time, the metrics used for the behavioral harassment threshold do not consider the duration of the animals exposure to a sound level. Therefore, the traditional assessment for behavioral exposures is dependent solely on the presence or absence of a species within the area ensonified above the threshold. Also, animals are less likely to respond to sounds from more distance sources, even when equivalent sound levels elicit responses at closer ranges; both proximity and received levels are important factors in aversion responses Dunlop et al., 2017.
HRG surveys may temporarily impact marine mammals in the area due to elevated in-water sound levels. Animals exposed to active acoustic sources during the HRG survey are unlikely to incur TTS hearing impairment due to the characteristics of the sound sources, which include relatively narrow beamwidths e.g., shallow sub-bottom profilers and generally very short pulses and duration of the sound. Even for high-frequency cetacean species e.g., harbor porpoises, which may have increased sensitivity to TTS Lucke et
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al., 2009; Kastelein et al., 2012, individuals would have to make a very close approach and also remain very close to vessels operating these sources in order to receive the multiple exposures at relatively high levels that would be necessary to cause TTS.
Intermittent exposuresas would occur due to the brief, transient signals produced by these sourcesrequire a higher cumulative SEL to induce TTS
than would continuous exposures of the same duration i.e., intermittent exposure results in lower levels of TTS
Mooney et al., 2009; Finneran et al., 2010. Moreover, most marine mammals would more likely avoid a loud sound source rather than swim in such close proximity as to result in TTS. Kremser et al. 2005 noted that the probability of a cetacean swimming through the area of exposure when a sub-bottom profiler emits a pulse is smallbecause if the animal was in the area, it would have to pass the transducer at close range in order to be subjected to sound levels that could cause TTS and would likely exhibit avoidance behavior to the area near the transducer rather than swim through at such a close range.
Further, the restricted beam shape of the majority of the geophysical survey equipment planned for use Table 2
makes it unlikely that an animal would be exposed more than briefly during the passage of the vessel.
The onset of behavioral disturbance from anthropogenic sound depends on both external factors characteristics of sound sources and their paths and the specific characteristics of the receiving animals hearing, motivation, experience, demography and is difficult to predict Southall et al., 2007, Ellison et al., 2012. It is possible that pile driving could result in temporary, shortterm changes in an animals typical behavioral patterns and/or temporary avoidance of the affected area. These behavioral changes may include Richardson et al., 1995: Changing durations of surfacing and dives, number of blows per surfacing, or moving direction and/or speed;
reduced/increased vocal activities;
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;
and/or flight responses. The biological significance of many of these behavioral disturbances is difficult to predict, especially if the detected disturbances appear minor. However, the consequences of behavioral modification could be expected to be
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