In several recent studies, we examined the relationship between baseline hearing thresholds and noise level on temporary threshold shifts in fish hearing capabilities. It has been well documented in the mammalian literature that temporary threshold shifts reach an asymptote after a specific duration of noise exposure. These asymptotic threshold shifts (ATS) increase linearly with sound intensity. We examined whether this linear threshold shift relationship is valid for other hearing vertebrates (fish and birds).

Specifically, we tested the hypothesis that noise-induced threshold shifts in fish increase linearly with increasing sound pressure levels (SPL) above baseline thresholds (the linear threshold shift or LINTS hypothesis). To test this hypothesis we investigated the effect of intense continuous white noise exposure on the hearing loss of two species that vary considerably in hearing sensitivity- goldfish, Carassius auratus (a hearing specialist), and tilapia, Oreochromis niloticus, (a hearing generalist). The goal was to compare these hearing effects between species to elucidate a potential relationship between hearing sensitivity and susceptibility to acoustic stress.

Goldfish and tilapia were exposed to white noise from 0.1 to 4 kHz at 164-170 dB (re: 1 µPa) for either 0 (control), 7, 21 (for goldfish), or 28 d (for tilapia) in 600-L aquaria. Auditory thresholds were measured using the auditory brainstem response (ABR). This technique is a noninvasive method of measuring the whole brain response to auditory stimuli and is commonly used for measuring hearing in fishes and other vertebrates.

Our results show that noise differentially affects two teleost species that differ in hearing sensitivity and also confirms the hypothesis that hearing specialists are more greatly affected by noise exposure than are hearing generalists. While tilapia were minimally affected by 28 d of noise-exposure, goldfish exhibited significant TTS after 7 d of noise-exposure. The difference can be explained by a linear relationship between TTS and SPL above the fish’s baseline threshold. We suggest that the reason that tilapia did not exhibit threshold shifts in response to 170 db re: 1µPa white noise and goldfish did, is that TTS (and perhaps hearing damage) only occurs when noise is a certain SPL above the fish’s baseline. Because baseline thresholds for tilapia are 20-50 dB higher than those of goldfish, one might expect 20-50 dB greater SPLs (190-220 dB re: 1µPa) would be required to produce the same threshold shifts as found in goldfish exposed to 170 db re: 1µPa. This linear threshold shift (LINTS) hypothesis needs to be tested with more teleost species and a broader range of noise SPLs, but may become a useful tool for researchers examining how anthropogenic sounds might affect fishes. Such a linear relationship for teleosts is consistent with results for birds and mammals, but greater underwater SPLs are required to induce a comparable TTS as found in birds and mammals in air.

The LINTS relationship is robust and is predictive on many different levels. On the level of an individual animal, it predicts that, when stimulated with white noise, the threshold shift will be greatest at frequencies where the animal’s baseline hearing threshold is the lowest. Both hearing specialists (goldfish and fathead minnows) had significant LINTS regressions when plotted alone, suggesting that this relationship was true, at least for hearing specialists. Tilapia and bluegill did not exhibit a significant LINTS regression when plotted alone, but this relationship was not testable in these hearing generalists since no TTS occurred at any frequency (except 10 dB shift at 800 Hz for tilapia). On the next higher level of prediction, the LINTS hypothesis predicts that, for a given intensity of sound, more sensitive species will be more prone to TTS than less sensitive species. This was the case in comparing our specialist and generalist teleost species, and in comparing fish with mammals and birds.