Determining sound exposure levels for marine fauna during a past seismic survey to inform effective mitigation measures in South African waters

Abstract

Concerns have been raised about the impact of seismic surveys on marine fauna in South Africa due to inadequate environmental management plans (EMPs). This study assessed sound exposure levels (SELs) expected to be experienced by marine fauna during a 2013 seismic survey in Algoa Bay, home to critically endangered African penguins (Spheniscus demersus), Cape fur seals (Arctocephalus pusillus), and several cetacean species. Using acoustic propagation modelling, SELs were estimated for cetaceans, seals and penguins seen from the vessel throughout the survey. At 500 m, single-pulse exposures were below temporary and permanent threshold shifts in assessed species. 48 % of baleen whales experienced cumulative SELs >168 dBre1μPa²s, capable of causing temporary threshold shifts between 800 and 3000 m from the source, while 17 % were exposed to >183 dBre1μPa²s, potentially leading to permanent threshold shifts between 800 and 2400 m from the source. No odontocetes were predicted to be exposed to cumulative sound levels exceeding the established limits for temporary or permanent threshold shifts during the survey period. African penguins, sensitive to SELs above 120 dBre1μPa²s, were exposed to levels between 152 and 183 dBre1μPa²s over the six-week survey, raising concerns about long-term effects. These findings highlight the need to mitigate against the effects of cumulative exposure of marine fauna to noise pollution. We recommend establishing an independent regulatory board, comprising government officials, industry experts, acousticians, and marine scientists, responsible for developing science-based mitigation measures, regulating, and overseeing the EMP process and content, and advising the government on policies to protect marine fauna from noise pollution.