Seasonal migration and habitat use of adult barbel ( Barbus barbus ) and nase ( Chondrostoma nasus ) along a river stretch of the Austrian Danube River

Migration patterns and habitat use of adult barbel ( Barbus barbus ) and nase ( Chondrostoma nasus ) were monitored by radio telemetry over a period of 13 months along a 58-km-long section of the Austrian part of the Danube River. The study site is confined upstream and downstream by two hydropower plants, and contains a larger tributary, the Pielach River. Telemetry transmitters were implanted into fish caught in this tributary after spawning in June (25 individuals per species). The results show that both species use the entire available width and depth spectrum of the Danube along the full migratable river length. Nase had an average home range of 22.4 km, while that of barbel was 34.4 km. The habitat use of the two species differs significantly. While the nase was primarily encountered in the free-flowing section, barbel mainly used deep areas of the impoundment during the year. Nase showed a distinct site fidelity to certain areas in the free-flowing reach which were periodically revisited. During the spawning season, distinct homing behavior was observed in both species. All seven nase that could still be detected during the spawning season returned to the tributary (homing rate 100%). Six homing nase migrated up to the first migration barrier in the tributary but did not pass the existing fish passage facility. In contrast, only nine barbel returned to spawn in the tributary (homing rate 50%), while nine barbel were most likely using a spawning location in the head of impoundment section. Homing fish entered the tributary during darkness.