Underwater noise can affect the hearing and behavior of marine mammals, as shown in previous research studies (Finneran, 2015). Very loud sounds, such as pile driving, air guns, and military sonars have been shown to inflict hearing loss in marine mammals. As a result, there are regulations in place in many countries limiting noise exposure that may cause a temporary threshold shift (TTS) in marine life, specifically, porpoises and seals. Unfortunately, the most recent guidelines are from 2015 and need updating.
Researchers in Denmark compiled data on TTS onset thresholds (the sound-exposure level required to induce 6 dB of TTS) from the literature. They note that a substantial number of studies of TTS in porpoises and seals have been conducted in recent years and, thus, a large body of empirical data has become available since 2015. The authors also note that there is “substantial interest” from regulatory bodies and other stakeholders regarding the estimated thresholds for TTS onset, as these form the basis of legislation and regulation of noise-generating activities in many countries.
The review of new experimental data for porpoises and seals showed significant discrepancies between newer data and the existing TTS onset thresholds for both mammals. However, the authors reported that the TTS onset thresholds that are currently being used are still conservative enough to protect the hearing of our marine friends from the noise of pile drivers, seismic air guns, and military low- and mid- frequency sonars. Finally, the authors report substantial uncertainties remain regarding the impact of low-frequency noise exposure for seals and high-frequency noise exposure for porpoises and strongly encourage new experiments.
Finneran JJ. (2015) Noise-induced hearing loss in marine mammals: a review of temporary threshold shift studies from 1996 to 2015. J Acoust Soc Am 138(3):1702–1726.
Tougaard J, Beedholm K, Madsen PT. (2022) Thresholds for noise-induced hearing loss in harbor porpoises and phocid seals. J Acoust Soc Am 151(6):4252–4263.
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