Effects of impulsive noise on marine mammals : investigating range-dependent risk

This work was funded as part of the Department of Energy and Climate Change’s Offshore Energy Strategic Environmental Assessment programme, with additional resources from the National Capability funding from the Natural Environment Research Council to the Sea Mammal Research Unit (grant no. SMRU1001).Concerns exist about the impacts of underwater noise on marine mammals. These include auditory damage, which is a significant risk for marine mammals exposed to impulsive sounds such as explosions, pile‐driving, and seismic air guns. Currently, impact assessments use different risk criteria for impulsive and non‐impulsive sounds (e.g., ships, drilling). However, as impulsive sounds dissipate through the environment, they potentially lose hazardous features (e.g., sudden onset) and become non‐impulsive at some distance from the source. Despite management implications, a lack of data on range‐dependent characteristics currently limits their inclusion in impact assessments. We address this using acoustic recordings of seismic air guns and pile‐driving to quantify range dependency in impulsive characteristics using four criteria: (1) rise time 5,000 Pa/s; (3) duration 15 dB. We demonstrate that some characteristics changed markedly within ranges of ~10 km, and that the mean probability of exceeding criteria 1 and 2 was 3.5 km. In contrast, the mean probability of exceeding criteria 3 remained >0.5 up to ~37.0 km, and the mean probability of exceeding criteria 4 remained