Temporary Threshold Shift in Porpoise Hearing: Effect of Experimental Protocol

Two recent studies of auditory temporary threshold shift (TTS) in a harbor porpoise at 25 and 28 kHz are compared to previously published results revealing a very large difference in sensitivity to TTS correlating with experimental paradigm. For fatiguing noise above 10 kHz, the TTS-onset thresholds measured with behavioral (psychophysical) methods following noise exposure to free-swimming porpoises are 40–60 dB higher than TTS-onset thresholds measured with auditory brain stem recordings on porpoises exposed while stationary. The uncertainty about sensitivity of porpoises to noise exposure above 10 kHz have very large implications for regulation of underwater noise and call for immediate further experiments. Such experiments should disentangle and quantify the confounding effects of exposure (stationary vs. free-swimming) and threshold measurements (behavioral vs. brain stem recordings), and the results are required to establish the VHF weighting function above 10 kHz, applicable to porpoises in assessment of risk of impact from high-frequency noise sources.