SUMMARY Several groups of fishes, among them two thirds of all freshwater fishes,have developed hearing specializations that enhance auditory sensitivity and broaden frequency ranges compared with hearing non-specialists (generalists),which lack such adaptations. It has been speculated that the enhanced sensitivities of these so-called hearing specialists have evolved in quiet habitats such as lakes, backwaters of rivers, slowly flowing streams or the deep sea. To test this hypothesis, noise levels and frequency spectra of four different freshwater habitats near Vienna, Austria (Danube River, Triesting stream, Lake Neusiedl, backwaters of the Danube River), were recorded and played back to native fish species while simultaneously measuring their auditory thresholds using the auditory evoked potential (AEP) recording technique. As a representative of hearing specialists, we chose the common carp (Cyprinus carpio, Cyprinidae) and for the hearing generalists the European perch (Perca fluviatilis, Percidae). Data show that the carp's hearing is only moderately masked by the quiet habitat noise level of standing waters (mean threshold shift 9 dB) but is heavily affected by stream and river noise by up to 49 dB in its best hearing range (0.5-1.0 kHz). In contrast, the perch's hearing thresholds were only slightly affected (mean up to 12 dB, at 0.1 kHz) by the highest noise levels presented. Our results indicate that hearing abilities of specialists such as carp are well adapted to the lowest noise levels encountered in freshwater habitats and that their hearing is considerably masked in some parts of their distribution range. Hearing in non-specialists such as perch, on the other hand, is only slightly or not at all impaired in all habitats.
The effects of intense white noise (158 dB re 1 microPa for 12 and 24 h) on the hearing abilities of two otophysine fish species--the nonvocal goldfish Carassius auramus and the vocalizing catfish Pimelodus pictus--were investigated in relation to noise exposure duration. Hearing sensitivity was determined utilizing the auditory brainstem response (ABR) recording technique. Measurements in the frequency range between 0.2 and 4.0 kHz were conducted prior and directly after noise exposure as well as after 3, 7, and 14 days of recovery. Both species showed a significant loss of sensitivity (up to 26 dB in C. auratus and 32 dB in P. pictus) immediately after noise exposure, with the greatest hearing loss in the range of their most sensitive frequencies. Hearing loss differed between both species, and was more pronounced in the catfish. Exposure duration had no influence on hearing loss. Hearing thresholds of C. auratus recovered within three days, whereas those of P. pictus only returned to their initial values within 14 days after exposure in all but one frequency. The results indicate that hearing specialists are affected differently by noise exposure and that acoustic communication might be restricted in noisy habitats.
In order to assess the effects of high-speed boating on fish communities, noise levels were measured during the first Class 1 powerboat race on the Austrian Lake Traunsee. The noise spectra were compared to natural ambient noise and hearing abilities of four native fish species. Sound pressure levels (SPLs) were significantly elevated during the training heats and the race compared with natural levels, reaching up to 128 dB re 1 microPa (instantaneous SPL) at a distance of 300 m to the powerboats. Continuous equivalent SPLs were significantly lower during training and the pole position race compared to the race itself because fewer boats were simultaneously on the lake. The hearing abilities of the native hearing specialists and generalists were investigated. While carp and roach (two cyprinids) showed enhanced auditory sensitivity typical for hearing specialists, perch and whitefish were much less sensitive to sounds. Comparisons between power boat noise spectra and audiograms showed that the cyprinids can detect the boats up to several hundred meters distance because the main noise energy is well within the most sensitive hearing range. The hearing generalists, however, probably only perceive the first harmonic of the boat noise at close distances.
The detectability of acoustic signals depends on the hearing abilities of receivers and the prevailing ambient noise in a given habitat. Ambient noise is inherent in all terrestrial and aquatic habitats and has the potential to severely mask relevant acoustic signals. In order to assess the detectability of sounds to fishes, the linear equivalent sound pressure levels (L(Leq)) of twelve European freshwater habitats were measured and spectra of the ambient noise recordings analyzed. Stagnant habitats such as lakes and backwaters are quiet, with noise levels below 100 dB re 1 microPa (L(Leq)) under no-wind conditions. Typically, most environmental noise is concentrated in the lower frequency range below 500 Hz. Noise levels in fast-flowing waters were typically above 110 dB and peaked at 135 dB (Danube River in a free-flowing area). Contrary to stagnant habitats, high amounts of sound energy were present in the high frequency range above 1 kHz, leaving a low-energy "noise window" below 1 kHz. Comparisons between the habitat noise types presented here and prior data on auditory masking indicate that fishes with enhanced hearing abilities are only moderately masked in stagnant, quiet habitats, whereas they would be considerably masked in fast-flowing habitats.
Changes in habitat acoustics over the year can potentially affect fish hearing and orientation to sound, especially in temperate climates. This is the first study where year-round changes in ambient noise in aquatic habitats were assessed. Seven different European fresh-water habitats were chosen for this study. Sound pressure level (SPL) and spectral composition of the ambient noise varied in both quiet stagnant habitats (lakes, backwaters) and in flowing habitats (streams, rivers). Linear equivalent SPL (L(Leq, 60s)) tended to be lower in stagnant habitats (means: 91.6-111.7 dB) than in flowing habitats (means: 111.2-133.4 dB). The changes in SPL were smallest in the river (means: 4.2-4.4 dB, maxima: 8.5-10.1 dB), whereas significantly higher values were measured in stagnant habitats and the stream (means: 9.9-14.9 dB, maxima: 25.1-30.9 dB). The spectral compositions of the ambient noise determined at different times of the year were highly correlated to each other at the river sites (mean cross-correlation coefficients: 0.85 and 0.94) and were weaker or not correlated at the other study sites (means: 0.24-0.76). The changes in ambient noise spectra were negatively correlated to changes in SPL, indicating that large changes in SPLs were accompanied by large changes in spectral composition and vice versa. Comparison of these ecoacoustical data with a preceding study (Amoser and Ladich in J Exp Biol 208:3533-3542, 2005) indicates that the auditory sensitivity in hearing specialists is affected by changes in ambient noise levels and spectra throughout a year and that this effect tends to be more pronounced in stagnant waters and the stream than at river sites. On the other hand, absolute noise levels result in a higher degree of masking in flowing waters.
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