Moderate levels of high-frequency noise mask harbor porpoise hearing, but do not cause temporary threshold shift

The potential for masking and temporary threshold shift (TTS) of a harbor porpoise exposed to high-frequency noise was investigated using levels and a duration that match likely vessel noise exposures at sea. An auditory evoked potential (AEP) technique allowed immediate assessment of hearing sensitivity during and after 20 s noise exposures centered on the 125 kHz 1/3 octave band. When the noise was delivered concomitantly with the stimuli, a 125 kHz 1/3 octave level of 85 dB re 1 μPa root mean square (rms) was enough to mask the hearing of click energy levels of 83 and 97 dB re 1 μPa2 s, and no AEPs could be measured when the noise reached a rms level of 120 dB re 1 μPa rms. These masking levels in the 100-150 kHz echolocation and communication band of porpoises are realized at ranges of several hundred meters from vessels with screws causing cavitations. After a period of more intense noise exposure level up to 147 dB re 1 μPa2 s at 125 kHz, responses to the click stimuli were not lower than at baseline levels. Since exposure levels this high are rarely encountered at high frequencies, it is therefore unlikely that high-frequency components of vessel noise can cause TTS, even in harbor porpoises within 10 s of meters of passing vessels. The AEP responses observed after exposures support the hypothesis that harbor porpoises can actively reduce their hearing sensitivity during noise exposure to maintain high hearing acuity immediately after exposure.