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Soundings Archive

NMFS Defends 160 dB Behavioral Take Standard
NMFS has issued an Incidental Harassment Authorization to the University of Alaska Geophysics Institute to take marine mammals, by harassment, incidental to conducting a marine geophysical seismic survey in the Arctic Ocean during September-October 2011. NMFS issued this IHA is issued in accordance with the Marine Mammal Protection Act.

NMFS' Federal Register notice of the IHA responds to comments by the North Slope Borough on the proposed IHA. One of NSB's comments was:

    "that NMFS should require applicants to assess impacts of surveys to bowhead whales to the 120 dB level, especially in this case because the survey will overlap in time with migrating bowheads and the hunts in Barrow and Wainwright."
NMFS's lengthy response to this comment is of great regulatory significance. It is set forth in part as follows:
    "[W]hile NMFS considers impacts to bowhead whales from sound levels below 160 dB, NMFS believes that it cannot scientifically support adopting any single SPL value below 160 dB and apply it across the board for all species and in all circumstances. As stated in the past, NMFS does not believe that minor course corrections during a migration rise to a level of being a significant behavioral response. To show the contextual nature of this minor behavioral modification, recent monitoring studies of Canadian seismic operations indicate that when, not migrating, but involved in feeding, bowhead whales do not move away from a noise source at an SPL of 160 dB. Therefore, while bowheads may avoid an area of 20 km (12.4 mi) around a noise source, when that determination requires a post-survey computer analysis to find that bowheads have made a 1 or 2 degree course change, NMFS believes that does not rise to a level of a ``take.'' NMFS therefore continues to estimate ``takings'' under the MMPA from impulse noises, such as seismic, as being at a distance of 160 dB (re 1 [mu]Pa).

    Although it is possible that marine mammals could react to any sound levels detectable above the ambient noise level within the animals' respective frequency response range, this does not mean that such animals would react in a biologically significant way. According to experts on marine mammal behavior, the degree of reaction which constitutes a 'take,'' i.e., a reaction deemed to be biologically significant that could potentially disrupt the migration, breathing, nursing, breeding, feeding, or sheltering, etc., of a marine mammal is complex and context specific, and it depends on several variables in addition to the received level of the sound by the animals. These additional variables include, but are not limited to, other source characteristics (such as frequency range, duty cycle, continuous vs. impulse vs. intermittent sounds, duration, moving vs. stationary sources, etc.); specific species, populations, and/or stocks; prior experience of the animals (na[iuml]ve vs. previously exposed); habituation or sensitization of the sound by the animals; and behavior context (whether the animal perceives the sound as predatory or simply annoyance), etc. (Southall et al., 2007).

    Therefore, unless and until an improved approach is developed and peer-reviewed, NMFS will continue to use the 160-dB threshold for determining the level of take of marine mammals by Level B harassment for impulse noise (such as from airguns). While NMFS does not consider exposures to sounds below 160-dB (rms) as likely to result in take of marine mammals by Level B harassment, NMFS acknowledges that some behaviors that might result from exposures at these lower levels do have the potential to impact a subsistence hunt."
  • Click here to read NMFS' Federal Register notice of this IHA
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