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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices sound exposure. Marine mammals exposed to high-intensity sound, or to lower-intensity sound for prolonged periods, can experience hearing threshold shift TS, which is the loss of hearing sensitivity at certain frequency ranges Finneran, 2015. 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.
Due to the characteristics of airgun arrays as a distributed sound source, maximum estimated Level A
harassment isopleths for species of certain hearing groups are assumed to fall within the near field of the array.
For these species, i.e., mid-frequency cetaceans and all pinnipeds, animals in the vicinity of LDEOs proposed seismic survey activity are unlikely to incur PTS. For low-frequency cetaceans and high-frequency cetaceans, potential exposures sufficient to cause low-level PTS may occur on the basis of cumulative exposure level and instantaneous exposure to peak pressure levels, respectively. However, when considered in conjunction with the potential for aversive behavior, relative motion of the exposed animal and the sound source, and the anticipated efficacy of the proposed mitigation requirements, a reasonable conclusion may be drawn that PTS is not a likely outcome for any species. However, we propose to authorize take by Level A
harassment, where indicated by the quantitative exposure analysis, for species from the lowand highfrequency cetacean hearing groups.
Please see Estimated Take and Proposed Mitigation for further discussion.
Behavioral 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.
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.

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In addition, sound can disrupt behavior through masking, or interfering with, an animals ability to detect, recognize, or discriminate between acoustic signals of interest e.g., those used for intraspecific communication and social interactions, prey detection, predator avoidance, navigation.
Masking occurs when the receipt of a sound is interfered with by another coincident sound at similar frequencies and at similar or higher intensity, and may occur whether the sound is natural e.g., snapping shrimp, wind, waves, precipitation or anthropogenic e.g., shipping, sonar, seismic exploration in origin.
Sound may affect marine mammals through impacts on the abundance, behavior, or distribution of prey species e.g., crustaceans, cephalopods, fish, zooplankton i.e., effects to marine mammal habitat. Prey species exposed to sound might move away from the sound source, experience TTS, experience masking of biologically relevant sounds, or show no obvious direct effects. The most likely impacts if any for most prey species in a given area would be temporary avoidance of the area. Surveys using active acoustic sound sources move through an area relatively quickly, limiting exposure to multiple pulses. In all cases, sound levels would return to ambient once a survey ends and the noise source is shut down and, when exposure to sound ends, behavioral and/or physiological responses are expected to end relatively quickly. Finally, the survey equipment will not have significant impacts to the seafloor and does not represent a source of pollution.
Vessel Strike Vessel collisions with marine mammals, or ship strikes, can result in death or serious injury of the animal.
These interactions are typically associated with large whales, which are less maneuverable than are smaller cetaceans or pinnipeds in relation to large vessels. The severity of injuries typically depends on the size and speed of the vessel, with the probability of death or serious injury increasing as vessel speed increases Knowlton and Kraus, 2001; Laist et al., 2001;
Vanderlaan and Taggart, 2007; Conn and Silber, 2013. Impact forces increase with speed, as does the probability of a strike at a given distance Silber et al., 2010; Gende et al., 2011. The chances of a lethal injury decline from approximately 80 percent at 15 kn to approximately 20 percent at 8.6 kn. At speeds below 11.8 kn, the chances of lethal injury drop below 50 percent Vanderlaan and Taggart, 2007.

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Ship strikes generally involve commercial shipping, which is much more common in both space and time than is geophysical survey activity and which typically involves larger vessels moving at faster speeds. Jensen and Silber 2004 summarized ship strikes of large whales worldwide from 1975
2003 and found that most collisions occurred in the open ocean and involved large vessels e.g., commercial shipping. Commercial fishing vessels were responsible for 3 percent of recorded collisions, while no such incidents were reported for geophysical survey vessels during that time period.
For vessels used in geophysical survey activities, vessel speed while towing gear is typically only 45 kn. At these speeds, both the possibility of striking a marine mammal and the possibility of a strike resulting in serious injury or mortality are so low as to be discountable. At average transit speed for geophysical survey vessels approximately 10 kn, the probability of serious injury or mortality resulting from a strike if it occurred is less than 50 percent Vanderlaan and Taggart, 2007; Conn and Silber, 2013. However, the likelihood of a strike actually happening is again low given the smaller size of these vessels and generally slower speeds. We anticipate that vessel collisions involving seismic data acquisition vessels towing gear, while not impossible, represent unlikely, unpredictable events for which there are no preventive measures.
Given the required mitigation measures, the relatively slow speeds of vessels towing gear, the presence of bridge crew watching for obstacles at all times including marine mammals, the presence of marine mammal observers, and the small number of seismic survey cruises relative to commercial ship traffic, we believe that the possibility of ship strike is discountable and, further, that were a strike of a large whale to occur, it would be unlikely to result in serious injury or mortality. No incidental take resulting from ship strike is anticipated or proposed for authorization, and this potential effect of the specified activity will not be discussed further in the following analysis.
The potential effects of LDEOs specified survey activity are expected to be limited to Level B harassment consisting of behavioral harassment and/or temporary auditory effects and, for certain species of lowand highfrequency cetaceans only, low-level permanent auditory effects. No permanent auditory effects for any species belonging to other hearing groups, or significant impacts to marine
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