Disrupted currents and shifting riverbanks: unveiling the structure of fish assemblages in the fragmented Austrian Danube

Abstract This study aims to characterize shoreline fish assemblages along different bank habitat types in the fragmented Austrian Danube and to investigate the impacts of fragmentation and habitat alterations caused by the construction of hydropower plants (HPPs). Nocturnal electrofishing surveys were conducted over 2 years in three river reaches separated by HPPs forming distinct sub-ecosystems within a larger Danube meta-ecosystem. The upper two reaches include both flowing and impounded sections, while the lowermost reach is free flowing. Of 46 species caught, 15 typical Danube species were selected for detailed analysis. Fish assemblages and abundance were compared across five river sections with varying longitudinal connectivity, four bank habitat types, three seasons, and two water levels. Results show that the combination of bank habitat type and section determines the habitat quality thus also determines the assemblage of fish species. Comparison with historical data and the pre-dam reference condition (Leitbild) reveals changes in species diversity/abundance over the past 40 years. The findings underscore the critical need for large-scale conservation and restoration measures to preserve fish biodiversity in regulated large river systems.