Temporary threshold shifts in the bottlenose dolphin (Tursiops truncatus), varying noise duration and intensity

Abstract

There is much concern regarding increasing noise levels in the ocean and how it may affect marine mammals. However, there is a little information regarding how sound affects marine mammals and no published data examining the relationship between broadband noise intensity and exposure duration. This study explored the effects of octave-band noise on the hearing of a bottlenose dolphin by inducing temporary hearing threshold shifts (TTS). Sound pressure level (SPL) and exposure duration were varied to measure the effects of noise duration and intensity. Hearing thresholds were measured using auditory evoked potentials before and after sound exposure to track and map TTS and recovery. Shifts were frequency dependent and recovery time depended on shift and frequency, but full recovery was relatively rapid, usually within 20 min and always within 40 min. As exposure time was halved, TTS generally occurred with an increase in noise SPL. However, with shorter, louder noise, threshold shifts were not linear but rather shorter sounds required greater sound exposure levels to induce TTS, a contrast to some published literature. From the data a novel algorithm was written that predicts the physiological effects of anthropogenic noise if the intensity and duration of exposure are known.