- How do repeated airgun shots affect the hearing of the marine mammals that are most often exposed to seismic surveys (i.e. dolphins)?
Temporary Threshold Shift (TTS) has been understood to be a conservative benchmark for hearing injury in marine mammals. Often times, however, TTS curves are obtained after exposure to tones or broadband sound. There are no direct data for TTS in marine mammals exposed to multiple impulses from airguns, as wild animals would receive from seismic surveys.
This study aimed to provide such data for bottlenose dolphins, relating the sound characteristics of airgun impulses to the measured physiological effects. Hearing thresholds were measured at multiple frequencies using both psychophysical and electrophysiological methods and were compared to pre-exposure values to determine the amount of TTS induced. No TTS was observed in any of the behavioural tests in which three dolphins were exposed to 10 consecutive shots at 196 dB re 1 μPa2s cumulative sound exposure level. Electrophysiololcal measurements suggest that the loudest exposure levels may have been near the levels that reliably induce a threshold shift.
Overall, the data suggest that the potential for airguns to cause hearing loss in dolphins is lower than previously predicted, possibly as a result of the low-frequency output of airgun impulses compared to the high-frequency hearing ability of dolphins. Dolphins are exposed to airgun arrays more often than any other group of marine mammal, giving results from this model species broad applicability.
Objectives and methods
Using both behavioural and electrophysiological methods, measure the amount of TTS induced in trained dolphins after exposure to multiple impulses produced by a seismic airgun in a bay.
Relate features of airgun impulses (e.g., rms sound pressure level, peak pressure, sound exposure level, number of impulses) to observed TTS.
This study shows the low risk of TTS in dolphins exposed to seismic airgun arrays. Dolphins are the group of marine mammals that has the closest and most frequent proximity to airgun arrays. Results from this work will fill a key information gap on the level of TTS associated with multiple shots from an airgun. Results will be critical in informing industry risk assessments and regulators, guiding mitigation strategies and, potentially, affecting damage risk criteria.
Links to other research
Directly related to on going Programme work on TTS in Pinnipeds.
US Navy SPAWAR Lab (James Finneran)