Effects of Increased Ambient Sound on Fish Hearing
Effects of Increased Ambient Sounds on Hearing in Fishes With and Without Hearing Adaptations
It is well known that increases in background noise can cause temporary hearing loss (often called Temporary Threshold Shift - TTS) in mammals and birds. A number of labs, including ours, have attempted to ascertain if TTS occurs in fishes. Former postdoc Michael Smith led a series of studies (which he continues even today as professor at Western Kentucky University) to examine the effects of long-term exposure to sounds in both a cichlid (tilapia) and goldfish. Results showed that long-term exposure to noise will produce TTS in goldfish but not very much effect in tilapia (Figure 1). The likely reason for this is that goldfish, which have specializations to enhance hearing bandwidth and sensitivity, hear much lower sounds than do tilapia, which have no special adaptations to enhance hearing. Since both species were exposed to the same sound levels, the sounds were actually much further above hearing thresholds in goldfish than in tilapia. We hypothesized that to get more TTS in tilapia, the sounds would have to be much louder so that they would be as far above threshold as in the goldfish.
Figure 1. Auditory thresholds of tilapia (left) and goldfish (right) after 7, 21 or 28 days of white noise exposure. ABRs were detectable from 100 to 800 Hz for tilapia. Tilapia exposed for 28 d exhibited an overall treatment effect, but this effect was only significant at 800 Hz (P=0.02). ABRs were detectable up to 4 kHz in goldfish. In contrast to tilapia, goldfish had significant threshold shifts at all frequencies after only 7 d of noise exposure. After 7 d, further noise exposure did not produce greater threshold shifts, suggesting an asymptote had been reached. Thresholds returned to baseline levels after 14 d of recovery from noise exposure.
Figure 2 shows the relationship between amount of hearing loss (TTS) in fishes that have adaptations to enhance hearing and in other species that do not.
Figure 2 (left). Relationship between TTS and noise SPL above baseline levels in four fish species (bluegill and tilapia are hearing generalists, minnows and goldfish are hearing specialists). Sunfish and minnow data are from Scholik & Yan 2001, 2002b. A significant linear relationship exists for all species, for hearing specialists alone, but not for hearing generalists alone. Thus, it is unclear if the LINTS hypothesis is valid for only hearing specialist fishes or whether the SPL was simply not great enough for TTS in generalists.
Our results show that noise differentially affects two teleost species that differ in hearing sensitivity and also confirms the hypothesis that fishes with hearing adaptations are more greatly affected by noise exposure than species that do not. 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.