Acoustical stress and hearing sensitivity in fishes: does the linear threshold shift hypothesis hold water?

Mammals exposed to loud aerial sounds exhibit temporary threshold shifts (TTS) that are linearly related to increases of sound pressure above baseline hearing levels. It was unknown if this relationship held true for aquatic ectotherms such as fishes. To test this linear threshold shift hypothesis (LINTS) in fishes, we examined the effects of increased ambient sound on hearing of two species differing in hearing capabilities: goldfish (Carassius auratus; a hearing specialist) and tilapia (Oreochromis niloticus; a hearing generalist). Fish were exposed to 1-28 days of either quiet (110 dB re 1 muPa) or continuous white noise. First, we examined the effect of noise sound pressure level (SPL; 130, 140, 160 or 170 dB re 1 muPa) on goldfish hearing thresholds after 24 h of noise exposure. Second, in a long-term experiment using 170 dB re 1 muPa white noise, we continuously exposed goldfish and tilapia for either 7 or 21-28 days. In both experiments, we measured alterations in hearing capabilities (using auditory brainstem responses) of noise-exposed fish. While tilapia exposed to noise for 28 days showed little or no hearing loss, goldfish exhibited considerable threshold shifts that reached an asymptote of up to 25 dB after only 24 h of exposure. There was a positive linear relationship between noise-induced TTS and the sound pressure difference between the noise and the baseline hearing thresholds in goldfish but not in tilapia. A similar relationship was found for published noise-induced threshold shifts in birds and mammals, but the slope of the linear relationship was greater in these groups than for fish. The linear threshold shift relationship provides insights into differential susceptibility of hearing specialist and generalist fishes to noise-induced hearing loss for a given SPL and provides a framework for future research on noise-induced threshold shifts in fishes and other animals.