Key Factors for the Findability of Fish Passes in Large Epipotamal Rivers: The Case of the River Drava

Abstract

Restoring the longitudinal connectivity of rivers through fish passes is of great importance for achieving good ecological status of surface waters. However, the key stimuli determining the findability of the entrance of these structures is still subject to debate. In this article, the influence of water temperature, light and acoustic stimuli in addition to flow parameters on fish movement is assessed. Analyses are based on a comprehensive dataset of 40,000 fish migrations covering time accurate recording of ascending individuals during the observation periods from 2015 to 2020 in various fish passes on the river Drava in Carinthia, Austria. The data are assessed by technical, fish ecological and statistical methods. Results indicate that the effect of the water temperature gradient between the fish pass and the main river has most impact on the ascent rates, whereas the two factors of light and acoustic, as well as most of the flow parameters, have no or negligible effect on the findability. A favourable thermal environment can be important to ensure efficient upstream migration and thus facilitate the findability of fish passes.