Temporary shift in masked hearing thresholds in a harbor porpoise (Phocoena phocoena) after exposure to seismic airgun stimuli

An auditory study was conducted to derive data on temporary threshold shift TTS induced bysingle impulses. This information should serve as basis for the definition of noise exposurecriteria for harbor porpoises. The measurements of TTS were conducted on a harbor porpoise bymeasuring the auditory evoked potentials in response to amplitude-modulated sounds. Afterobtaining baseline hearing data the animal was exposed to single airgun stimuli at increasingreceived levels. Immediately after each exposure the animal’s hearing threshold was tested forsignificant changes. The received levels of the airgun impulses were increased until TTS wasreached. At 4 kHz the predefined TTS criterion was exceeded at a received sound pressure level of199.7 dBpk-pk re 1 Pa and a sound exposure level SEL of 164.3 dB re 1 Pa2 s. The animalconsistently showed aversive behavioral reactions at received sound pressure levels above174 dBpk-pk re 1 Pa or a SEL of 145 dB re 1 Pa2 s. Elevated levels of baseline hearingsensitivity indicate potentially masked acoustic thresholds. Therefore, the resulting TTS levelsshould be considered masked temporary threshold shift MTTS levels. The MTTS levels are lowerthan for any other cetacean species tested so far