Water level induced changes of habitat quality determine fish community composition in restored and modified riverbanks of a large alpine river

Bank habitats provide important functions for riverine fish. Yet, they have been heavily modified by land use, technical flood protection measures, and hydropower installations. Fish species requiring specific habitats to complete their life cycle have strongly declined and therefore become target species of river restoration measures. This study compared abiotic conditions and fish community composition of three bank habitat types in a large alpine river, comprising different degrees of alteration compared to the natural state (concrete profile, bank riprap, and naturally restored riverbank). Significant differences in abiotic habitat characteristics such as bed material, water depth, turbidity, submerged vegetation, and temperature were detected between the three bank habitat types and sampling seasons. These water level-dependent structural changes had the strongest effect on fish community composition as detected by distance-based linear modeling. Small specimens between 3 and 13 cm TL and juveniles were most abundant in the restored areas, except for Lota lota, which was most abundant in the man-made bank riprap. Target species of conservation were mostly detected in restored areas, particularly the critical young life stages of Chondrostoma nasus, Barbus barbus, and Thymallus thymallus. Water level strongly determined accessibility and suitability of bank habitats, with shallow, gravel-dominated habitats comprising flat bank angles being most beneficial for these species. The findings of this study provide evidence for the success of bank habitat restoration in structurally impacted alpine rivers on target species of conservation. Fluctuating water levels and discharges typical for alpine rivers should be better considered in restoration planning, particularly in light of climate change, affecting the timing and amplitude of discharge in these systems.