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Federal Register / Vol. 86, No. 195 / Wednesday, October 13, 2021 / Proposed Rules
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Take section, and the Proposed Mitigation section, to draw conclusions regarding the likely impacts of these activities on the reproductive success or survivorship of individuals and how those impacts on individuals are likely to impact marine mammal species or stocks.
Acoustic effects on marine mammals during the specified activity can occur from vibratory and impact pile driving.
The effects of underwater noise from the Navys proposed activities have the potential to result in Level A and Level B harassment of marine mammals in the action area.
Description of Sound Sources The marine soundscape is comprised of both ambient and anthropogenic sounds. Ambient sound is defined as the all-encompassing background sound in a given place and is usually a composite of sound from many sources both near and far. The sound level of an area is defined by the total acoustical energy being generated by known and unknown sources. These sources may include physical e.g., waves, wind, precipitation, earthquakes, ice, atmospheric sound, biological e.g., sounds produced by marine mammals, fish, and invertebrates, and anthropogenic sound e.g., vessels, dredging, aircraft, construction.
The sum of the various natural and anthropogenic sound sources at any given location and timewhich comprise ambient sounddepends not only on the source levels as determined by current weather conditions and levels of biological and shipping activity but also on the ability of sound to propagate through the environment.
In turn, sound propagation is dependent on the spatially and temporally varying properties of the water column and sea floor, and is frequency-dependent. As a result of the dependence on a large number of varying factors, ambient sound levels can be expected to vary widely over both coarse and fine spatial and temporal scales. Sound levels at a given frequency and location can vary by 1020 dB from day to day Richardson et al. 1995. The result is that, depending on the source type and its intensity, sound from the specified activity may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals.
In-water construction activities associated with the project would include impact pile driving and vibratory pile driving. The sounds produced by these activities fall into one of two general sound types:
Impulsive and non-impulsive.

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Impulsive sounds e.g., explosions, gunshots, sonic booms, impact pile driving are typically transient, brief less than 1 second, broadband, and consist of high peak sound pressure with rapid rise time and rapid decay ANSI 1986; NIOSH 1998; ANSI 2005;
NMFS 2018a. Non-impulsive sounds e.g. aircraft, machinery operations such as drilling or dredging, vibratory pile driving, and active sonar systems can be broadband, narrowband or tonal, brief or prolonged continuous or intermittent, and typically do not have the high peak sound pressure with raid rise/decay time that impulsive sounds do ANSI 1995; NIOSH 1998; NMFS
2018a. The distinction between these two sound types is important because they have differing potential to cause physical effects, particularly with regard to hearing e.g., Ward 1997 in Southall et al. 2007.
Two types of pile hammers would be used on this project: Impact and vibratory. Impact hammers operate by repeatedly dropping a heavy piston onto a pile to drive the pile into the substrate.
Sound generated by impact hammers is characterized by rapid rise times and high peak levels, a potentially injurious combination Hastings and Popper 2005. Vibratory hammers install piles by vibrating them and allowing the weight of the hammer to push them into the sediment. Vibratory hammers produce significantly less sound than impact hammers. Peak sound pressure levels SPLs may be 180 dB or greater, but are generally 10 to 20 dB lower than SPLs generated during impact pile driving of the same-sized pile Oestman et al. 2009. Rise time is slower, reducing the probability and severity of injury, and sound energy is distributed over a greater amount of time Nedwell and Edwards 2002; Carlson et al. 2005.
The likely or possible impacts of the Navys proposed activity on marine mammals could involve both nonacoustic and acoustic stressors.
Potential non-acoustic stressors could result from the physical presence of the equipment and personnel. However, any impacts to marine mammals are expected to primarily be acoustic in nature. Acoustic stressors include effects of heavy equipment operation during pile driving.
Acoustic Impacts The introduction of anthropogenic noise into the aquatic environment from pile driving is the primary means by which marine mammals may be harassed from the Navys specified activity. In general, animals exposed to natural or anthropogenic sound may experience physical and psychological
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effects, ranging in magnitude from none to severe Southall et al. 2007. In general, exposure to pile driving noise has the potential to result in auditory threshold shifts and behavioral reactions e.g., avoidance, temporary cessation of foraging and vocalizing, changes in dive behavior. Exposure to anthropogenic noise can also lead to non-observable physiological responses such an increase in stress hormones.
Additional noise in a marine mammals habitat can mask acoustic cues used by marine mammals to carry out daily functions such as communication and predator and prey detection. The effects of pile driving noise on marine mammals are dependent on several factors, including, but not limited to, sound type e.g., impulsive vs. nonimpulsive, the species, age and sex class e.g., adult male vs. mom with calf, duration of exposure, the distance between the pile and the animal, received levels, behavior at time of exposure, and previous history with exposure Wartzok et al. 2004; Southall et al. 2007. Here we discuss physical auditory effects threshold shifts, followed by behavioral effects and potential impacts on habitat.
NMFS defines a noise-induced threshold shift TS 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. The amount of threshold shift is customarily expressed in dB. A TS can be permanent or temporary. As described in NMFS
2018, there are numerous factors to consider when examining the consequence of TS, including, but not limited to, the signal temporal pattern e.g., impulsive or non-impulsive, the likelihood an individual would be exposed for a long enough duration or to a high enough level to induce a TS, the magnitude of the TS, the time to recovery seconds to minutes or hours to days, the frequency range of the exposure i.e., spectral content, the hearing and vocalization frequency range of the exposed species relative to the signals frequency spectrum i.e., how an animal uses sound within the frequency band of the signal; e.g., Kastelein et al. 2014, and the overlap between the animal and the source e.g., spatial, temporal, and spectral.
Permanent Threshold Shift PTS
NMFS defines PTS as a permanent, irreversible 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. Available data from humans and other terrestrial mammals
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