Ökoakustische Einflüsse auf das Hören der Fische

Fishes have evolved a wide diversity of hearing sensitivities whose functional significance is unknown. While hearing generalists (e.g., perches, salmonids) are able to detect low frequency sounds up to a few hundred hertz, hearing specialists (e.g. minnows, catfishes) possess accessory hearing structures (based on coupling their inner ears to air-filled cavities) which broadens the frequency range up to several kilohertz and increases hearing sensitivity. Was intraspecific acoustic communication a driving force in improving hearing? This is unlikely because sound production is widespread among both hearing generalists and specialists. Interestingly, hearing specialists such as carps, catfishes and elephantnose fishes are mostly inhabitants of freshwaters, which points to ecoacoustical constraints for the enhancement of hearing. We therefore postulate that low environmental (ambient) noise was the main reason for the selection of enhanced hearing sensitivities in fishes. We plan to support this hypothesis by three series of experiments. In order to analyze the correlation between hearing and ambient noise we plan to measure the sound pressure levels and record the noise spectra in different aquatic habitats in Austria (streamlets, rivers, lakes, backwaters); this will then be related to the species inhabiting these freshwaters. Austrian freshwaters are characterized by a large number of hearing specialists (cyprinids) and a few generalists (perches and salmonids). Sound levels are therefore expected to be low where specialists occur. To support our expectation we will measure hearing thresholds of representative native fish species utilizing, the auditory evoked potential (AEP) recording technique. The AEP technique has been established in our lab in the preceding project. The hearing curves (audiograms) will be determined under quiet laboratory conditions and during the simultaneous presentation of various habitat noises. This will clarify if and to what extent hearing thresholds are shifted (or are masked) due to the noise. In order to determine whether ambient noise affects acoustic communication, sounds of hearing generalists and specialists will be recorded, played back during the simultaneous presentation of ambient noise, and their detection measured using the AEP technique. This will show whether sound communication is adapted to the environmental noise or whether the detection of conspecific sounds is impaired. This will be the first comprehensive investigation showing the influence of environmental noise on the evolution of hearing and sound communication in fishes. It will in particular demonstrate the importance of ecoacoustical constraints on the selection of accessory hearing structures in fishes.