Research Document - 2008/087

Updated Review of Scientific Information on Impacts of Seismic Survey Sound on Marine Mammals, 2004-present

By P. Abgrall, V.D. Moulton and W.J. Richardson

Abstract

This document provides an updated review of recent scientific information on the effects of seismic survey sound from airgun(s) on marine mammals. It reviews conclusions in the Canadian Science Advisory Secretariat (CSAS) Habitat Status Report 2004/002 and describes published information that has become available since 2004 (through February 2008). The types of impacts reviewed closely follow those outlined in the original CSAS Habitat Status Report and include mortality and physical effects, direct behavioural effects, functional consequences of physical and behavioural effects, chronic effects, and indirect effects.

The review of mortality and physical effects considers strandings/mortality, hearing impairment, and non-auditory effects. There is no conclusive evidence that airgun pulses cause stranding or death in marine mammals, or non-auditory effects (e.g., gas bubble formation). However, incidents involving beaked whale strandings near naval exercises which used mid-frequency sonar suggest a need for caution in conducting seismic surveys in areas occu­pied by beaked whales. A key paper (Southall et al. 2007) provides new recommendations for injury criterion in marine mammals exposed to sound. Permanent threshold shift (PTS) in hearing is recommended as an injury criterion. On a SEL (sound exposure level) basis, there is a risk of PTS when received sound levels exceed 15 dB above the SEL which may elicit temporary threshold shift (TTS) in hearing. On a peak pressure basis, sounds might cause immediate PTS if received sound levels exceed 6 dB above TTSpeak threshold. For exposure to multiple pulses like those from airgun(s), PTS is assumed to occur if either of the following SEL or peak pressure criterion are exceeded:

• Cetaceans: ~198 dB re 1 μPa² ∙ s or 230 dB re 1 μPa (peak)
• Pinnipeds:  ~186 dB re 1 μPa² ∙ s or 218 dB re 1 μPa (peak)

The direct behavioural effects considered include displacement and migratory diversion, changes in dive and respiratory patterns, changes in social behaviour, and changes in vocalization patterns. Baleen whales generally tend to avoid at least the immediate area around operating airguns, but avoidance radii are quite variable. Bowhead and gray whales in summer feeding habitat often exhibit only localized avoidance of airgun sources. Dolphins show variable responses to seismic surveys but seem more responsive than suspected earlier. Beluga whales exhibited 10-20 km avoidance of a seismic vessel; received sound levels were estimated at 150-130 dB re 1 μPa (rms). In contrast, there is little evidence that sperm whales avoid seismic surveys and a study of ringed seals concluded that they showed very limited avoidance of an operating airgun array. Dive and respiratory patterns vary in response to seismic exposure. Some preliminary analyses suggest that foraging attempts by sperm whales in the Gulf of Mexico decrease during seismic exposure. There are also variable results reported for changes in vocalization patterns of marine mammals in response to seismic sounds. Some baleen and toothed whales continued calling in the presence of seismic operations while others stopped calling and resumed after the survey.

Very little information addresses the potential for functional consequences of physical and behavioural effects, chronic effects, and indirect effects of seismic survey sound on marine mammals. Masking effects of pulsed sounds on marine mammal calls another natural sounds are expected to be limited, although there are very few specific data on this. 

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