Temporary threshold shift in bottlenose dolphins (Tursiops truncatus) exposed to mid-frequency tones

Abstract

A behavioral response paradigm was used to measure hearing thresholds in bottlenose dolphins before and after exposure to 3 kHz tones with sound exposure levels (SELs) from 100 to 203 dB re 1μPa2s. Experiments were conducted in a relatively quiet pool with ambient noise levels below 55 dB re 1μPa2∕Hz at frequencies above 1 kHz. Experiments 1 and 2 featured 1-s exposures with hearing tested at 4.5 and 3 kHz, respectively. Experiment 3 featured 2-, 4-, and 8-s exposures with hearing tested at 4.5 kHz. For experiment 2, there were no significant differences between control and exposure sessions. For experiments 1 and 3, exposures with SEL=197dB re 1μPa2s and SEL⩾195dB re 1μPa2s, respectively, resulted in significantly higher TTS4 than control sessions. For experiment 3 at SEL=195dB re 1μPa2s, the mean TTS4 was 2.8 dB. These data are consistent with prior studies of TTS in dolphins exposed to pure tones and octave band noise and suggest that a SEL of 195 dB re 1μPa2s is a reasonable threshold for the onset of TTS in dolphins and white whales exposed to midfrequency tones.