Bottlenose dolphin (Tursiops truncatus) temporary threshold shift in response to frequency-modulated and pure-tone exposures centered at 28 kHz

Abstract

There is a scarcity of marine mammal temporary threshold shift (TTS) data for noise exposures representative of naval continuous active sonar (CAS). In this study, bottlenose dolphin (n = 2) TTS was measured following exposure to frequency-modulated (FM) tones with temporal characteristics representative of CAS, but frequencies shifted to the dolphins' region of best hearing sensitivity (20–40 kHz). Exposures were conducted at multiple sound pressure levels and durations ranging from 2 to 60 min. Additional exposures to 28-kHz pure tones at matched cumulative sound exposure levels examined the effect of exposure bandwidth on TTS. Hearing thresholds were measured using behavioral methods and supplemented by electrophysiological auditory brainstem response and auditory steady-state response methods. Larger TTS magnitudes were observed for the FM versus the pure-tone exposures in both dolphins. A consistent pattern of TTS was not observed using either electrophysiological method, contrary to what has been noted in some previous studies. Future testing will evaluate TTS following exposure to simulated CAS centered near 3 kHz, at the lower-frequencyend of the bottlenose dolphin’s hearing range but representative of the actual frequencies of naval CAS. [Work funded by US Navy Living Marine Resources.]