The U.S. National Marine Fisheries Service has issued an Incidental Harassment Authorization to Hilcorp Alaska, LLC to take, by harassment, small numbers of marine mammals incidental to a shallow geohazard survey in the Beaufort Sea, Alaska, during the 2015 Arctic open-water season. This IHA is effective July 1, 2015, through September 30, 2015.
NMFS’ Federal Register notice of this final IHA responds to comments filed by the U.S. Marine Mammal Commission on the proposed IHA. There follows NMFS’ rejection of the MMC’s argument that “the sub-bottom profiler, echosounder, and other sonars are nonimpulsive acoustic sources and that NMFS should use the behavioral harassment threshold of 120 dB re 1 mPa instead of 160 dB”:
“Comment 1: The Commission states that the sub-bottom profiler, echosounder, and other sonars are nonimpulsive acoustic sources and that NMFS should use the behavioral harassment threshold of 120 dB re 1 mPa instead of 160 dB, which is the threshold for impulse sound. Further, the Commission recommends that NMFS require Hilcorp to monitor the larger 120-dB re 1 mPa harassment zone of 450 m for the purpose of enumerating marine mammal takes associated with the use of the sub-bottom profiler.
Response: NMFS does not agree with the Commission’s statement that signals from a sub-bottom profiler, echosounder, and other sonar equipment proposed to be used by Hilcorp are non-impulsive. In classifying underwater noise types, NMFS recognizes two categories: continuous sounds and intermittent sounds. Continuous sounds are those whose sound pressure level remains above that of the ambient sound, with negligibly small fluctuations in level (NIOSH, 1998; ANSI, 2005), while intermittent sounds are defined as sounds with interrupted levels of low or no sound (NIOSH, 1998). Thus, signals from sub-bottom profiler, echosounder, and other sonar equipment to be used by Hilcorp are not continuous sounds but rather intermittent sounds. Intermittent sounds can further be defined as either impulsive or nonimpulsive. Impulsive sounds have been defined as sounds that are typically transient, brief (< 1 sec), broadband, and consist of a high peak pressure with rapid rise time and rapid decay (ANSI, 1986; NIOSH, 1998). Signals from these sources to be used by Hilcorp also have durations that are typically very brief (< 1 sec), with temporal characteristics that more closely resemble those of impulsive sounds than non-impulsive sounds, which typically have more gradual rise times and longer decays (ANSI, 1995; NIOSH, 1998). With regard to behavioral thresholds, we therefore consider the temporal and spectral characteristics of signals from the subbottom profiler, echosounder, and other sonar equipment to be used by Hilcorp to more closely resemble those of an impulse sound than a continuous sound.
Therefore, NMFS considers that using the 160 dB re 1 mPa threshold for Level B harassment for marine mammal noise exposure by Hilcorp’s sub-bottom profiler is more appropriate than the continuous threshold of 120 dB re 1 mPa. Subsequently, the Level B zone of influence (ZOI) is established as the isopleths where the received level is 160 dB re 1 mPa and higher, which will be monitored by the protected species observers (PSOs).”
Click here to read NMFS’ entire Federal Register notice of this IHA and NMFS’ entire response to comments.
