Temporary threshold shift from continuous 20–40 kHz hyperbolic upsweeps in bottlenose dolphins (Tursiops truncatus)

Naval continuous active sonar (CAS) can operate at high duty cycles and result in sound exposure levels (SELs, in dB re 1 μPa2s) that may induce temporary threshold shift (TTS) in marine mammals. Estimating the impacts of CAS on marine mammal hearing is difficult however, given the scarcity of TTS data for long-duration tonal exposures. For this ongoing study, bottlenose dolphin TTS was measured following exposure to continuous playback of 19-s hyperbolic upsweeps from 20–40 kHz, at multiple sound pressure levels and exposure durations from 2 to 60 min (100% duty cycle). The upsweep was based on characteristics of naval CAS, but frequency-shifted into the dolphin’s range of best hearing to predict how baleen whales might be affected by CAS exposures below 10 kHz. Hearing was measured using both psychophysical and auditory brainstem response measurements. To date, the greatest cumulative exposures of 180 dB SEL (normal-hearing dolphin) and 183 dB SEL (dolphin with elevated hearing thresholds) resulted in less than 6 dB of behavioral TTS. Auditory brainstem response amplitudes for these exposures did not indicate noise-induced fatigue. Future testing will evaluate TTS following exposure to 28-kHz pure tones with comparable SELs. [Work funded by US Navy Living Marine Resources.]