When Is Temporary Threshold Shift Injurious to Marine Mammals?

Evidence for synaptopathy, the acute loss of afferent auditory nerve terminals, and degeneration spiral ganglion cells associated with temporary threshold shift (TTS) in traditional laboratory animal models (e.g., mice, guinea pigs) has brought into question whether TTS should be considered a non-injurious form noise impact marine mammals. Laboratory studies also demonstrate that both neuropathic non-neuropathic forms exist, synaptopathy neural beginning over narrow range exposures differing by ~6–9 dB, all which result significant TTS. Most mammals characterize within minutes exposure cessation, generally does not achieve levels measured animals, have had initial measurements made 6–24 h post-exposure. Given recovery ear following cessation exposure, it seems unlikely magnitude nearly shifts studied to date would sufficient induce neuropathy. Although no empirical evidence exists support this proposition, regulatory application thresholds based on onset (6 dB) is certain capture recoverable fatigue without tissue destruction.