Summary
Airgun ‘shots’ from seismic surveys are powerful enough to be heard across ocean basins. At close range, though, there is extraordinarily little understanding of the potential for these intense sounds to cause temporary or permanent hearing threshold shifts (TTS/PTS) in marine mammals. While low frequency specializing baleen whales are likely to be most susceptible, there is just a single empirical measurement of sound levels from an airgun-like sound causing TTS in a beluga whale. Considering this extreme paucity of data, a series of simulations were developed to account for inter-individual variation and uncertainty in assessing the distances at which baleen whales may be susceptible to TTS/PTS from seismic surveys. For each of 100 simulation runs: 10,000 ‘baleen whales’ are assigned a TTS onset level based on a) normal variation expected within a population; b) uncertainty over where the one experimentally measured level falls in that population’s distribution; and c) a distribution of potential weighting values to transfer TTS onset levels from beluga to baleen whales. Each whale is randomly distributed in a seismic ‘arena’ and exposed to 1 pass of a seismic vessel with cumulative exposure level calculated. Initial results indicate TTS (and even PTS) may be a risk at greater ranges than may have been suspected. For example, while using empirically supported received levels of sound throughout the arena, in one scenario, 35% of whales(5th/95th percentiles of 17/66%) at 1km were exposed to levels sufficient for TTS onset, and up to 11% (5th/median/95th percentiles of 0/2/11%) of whales were exposed to levels sufficient for PTS onset. Until more data can be collected to reduce uncertainty and refine models like these, these preliminary results suggest there is a reasonable likelihood that whales at distances of a kilometer or more from seismic surveys could experience temporary or even in some scenarios, permanent threshold shifts.