Diversity in ambient noise in European freshwater habitats: noise levels, spectral profiles, and impact on fishes.

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.