Seasonal and spatial variability of abiotic habitat conditions and fish community composition at tributary confluences in an alpine river network

Habitat quality, diversity and connectivity are key variables governing biodiversity patterns, but existing conservation and restoration strategies hardly consider their important interlinkages in river networks. In order to develop best practice river network restoration strategies, fish diversity and the spatio-temporal occurrence of target species of conservation in relation to abiotic habitat conditions were assessed in the alpine River Inn network comprising main stem, tributaries and confluences. Fish sampling was carried out by standardized electrofishing during spring, summer and autumn. In addition, important physicochemical variables were collected to interlink abiotic habitat characteristics with the fish community patterns. Differences in the physicochemical habitat conditions between the main stem and the tributaries were particularly evident in turbidity, electrical conductivity and temperature. This was also reflected in seasonal distribution patterns of fishes which differed between tributaries and main stem, even at small spatial scales. Cyprinids such as common nase and barbel were generally found more frequently in the tributaries where they successfully reproduced. In contrast, cold-water adapted species such as grayling, brown trout and European bullhead were detected much more frequently in the River Inn main stem, mostly as juveniles. Across species, distribution patterns were strongly driven by spawning migration into tributaries in spring and back-drift of larvae and juveniles to the main stem between summer and autumn. Consequently, the diverse habitats of interconnected river networks are key in sustaining the life cycles of diverse and healthy fish populations, and they can provide refuge areas increasing the resilience of such systems. The findings are directly relevant for restoration projects in large alpine rivers to address two primary aspects: Firstly, to create heterogeneous habitat structures that reflect the diverse life cycle requirements of target species, and secondly, to improve multidimensional river network connectivity.