The importance of structural features for spawning habitat of nase Chondrostoma nasus (L.) and barbel Barbus barbus (L.) in a pre-Alpine river

This study develops univariate utilisation- and preference indices and analyses multivariate microhabitat use of spawning nase Chondrostoma nasus (L.) and barbel Barbus barbus (L.) in the Pielach River, a pre-Alpine tributary to the Danube, Austria. During the spawning season we daily surveyed species presence, number of individuals and habitat size. Habitat features, i.e. flow velocity, water depth, shading, cover, flow protection, type of structure, substrate and embeddedness were recorded at ten spawning grounds used by 1900 spawners within one spawning season. Spawning habitat features were compared with available habitat. Nase spawns in fast-flowing water (~1 m/s) that is significantly faster than available habitat. In contrast, barbel constructs redds that differ in water temperature, depth, velocity and cover structure from those of nase. Multivariate analyses (PCA) underlined the importance of shading and, as a consequence, the occurrence of vegetation along river banks for both fish species. This study demonstrates that efficient river restoration requires re-establishing riparian vegetation besides hydromorphological habitat improvements in order to provide adequate spawning grounds for nase and barbel.