Page 40 - Summer 2015

Underwater Noise and Marine Protected Species
other federal agencies (hereafter referred to as “applicants”) seeking take authorization to determine under what circum- stances take is likely to occur. Generic acoustic thresholds were developed for auditory injury based on expert opinion (NOAA, 1998; High Energy Seismic Survey [HESS], 1999) because no direct noise-induced threshold measurements were available for marine mammals at the time. Addition- ally, existing observations of avoidance from free-ranging individuals, associated with noise exposure, were used to develop behavioral thresholds (e.g., Richardson et al., 1985, 1986, 1990) (Table 1).
Acoustic thresholds are used to estimate when animals are likely to be harassed, which results in an estimate of the number of takes on a species-by-species basis, and to in- form the development of appropriate mitigation measures (e.g., shut-down procedures) targeted at reducing impacts. Acoustic thresholds are therefore an important tool used in the larger analysis of an activity.
The NMFS current generic acoustic thresholds are extreme- ly simple, which has made them easy to implement for appli- cants as well as for NMFS analysts and managers. However, as new science becomes available and the understanding of potential impacts of man-made noise on marine species increases, the characteristics of what constitutes meaning- ful acoustic thresholds become more complicated, and our analyses regarding acoustic impacts should necessarily re- flect this added complexity.
Accordingly, the NMFS has been working to reevaluate and update its acoustic thresholds, but various parts of these ef- forts have evolved at different paces. For example, underwa- ter explosive thresholds for nonacoustic (lung and gastroin- testinal tract) injury and mortality have been updated more recently, as have acoustic impact thresholds for tactical sonar (Southall et al., 2007; Finneran and Jenkins, 2012), whereas thresholds for behavioral harassment and airborne sources for pinnipeds are still under different stages of development. The goal is to establish consistent national guidance for all our acoustic thresholds, but we are first focusing on updat- ing those associated with the effects of noise on hearing.
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Developing Updated Acoustic
Threshold Levels for Impacts on Hearing The NMFS, on behalf of NOAA, has proposed updated Acoustic Guidance (hereafter referred to as “draft Guid- ance”) for assessing the effects of anthropogenic sound on marine mammal species under our jurisdiction (NOAA, 2013). Specifically, the draft Guidance focuses on the im- pacts of underwater noise on marine mammal hearing (i.e., updated acoustic threshold levels for the onset of permanent threshold shift [PTS] and temporary threshold shifts [TTS] for all sound sources). The draft Guidance is intended to be used by NOAA managers and other applicants to better predict exposures that have the potential to trigger certain requirements under one or more of NOAA statutes (e.g., MMPA, ESA, and the National Marine Sanctuaries Act, un- der a sister agency of the NMFS, the National Ocean Ser- vice). Although the acoustic thresholds often get a lot of attention because of the overall take numbers they help gen- erate, they are but one tool utilized within a larger impact assessment to help evaluate the effects of a proposed activ- ity on marine mammals and make findings required by our various statutes.
To develop updated threshold levels for auditory impacts, NOAA compiled, interpreted, and synthesized the best available information on the effects of man-made sound on marine mammal hearing. Additionally, because the draft Guidance is classified as a “highly influential scientific as- sessment” by the President’s Office of Management and Bud- get (OMB), it was also required to undergo an independent peer review (OMB, 2005). The draft Guidance is significant for NOAA because it is the first time the agency has present- ed this information in a single comprehensive document to establish national guidance. In addition, the draft Guidance is created as a “living” document with a mechanism for up- dating it as new data become available.
The draft Guidance builds on previous scientific recommen- dations for acoustic threshold levels by Southall et al. (2007) because more is known about sound sources as well as about marine mammal acoustics and hearing than when our ge- neric thresholds were first derived. However, as with any science field, there are data gaps and associated uncertainty (e.g., TTS data are only available for seven marine mammal species, from few individuals; PTS data are not available for marine mammals and so data for terrestrial mammals are extrapolated and used as surrogates). Nevertheless, NOAA used the best scientific information available in the develop- ment of updated Guidance.
Table 1: Generic NMFS marine mammal underwater acoustic thresholds.
Criterion
Threshold Root-Mean-Square Sound Pressure Level
Permanent threshold shift (auditory injury)
180 dB re 1 μPa (cetaceans) 190 dB re 1 μPa (pinnipeds)
Behavioral harassment associated with impulse sounds (e.g., seismic and impact pile driving)
160 dB re 1 μPa
Behavioral harassment associated with continuous sounds (e.g., drilling and vibratory pile driving)
120 dB re 1 μPa