SEASONAL MIGRATION PATTERNS OF NASE AND BARBEL IN THE DANUBE AND ITS TRIBUTARIES Unfer, G., C. Frangez & S. Schmutz Department of Hydrobiology, Fisheries and Aquaculture; BOKU - University of Natural Resources and Applied Life Sciences, Vienna; Mail: [email protected]; Tel : +43-1-47654-5213 (Fax 5217) Nase (Chondrostoma nasus) and barbel (Barbus barbus) are classified as mediumdistance migrants and their migration patterns are well known for small rivers. However, the seasonal migration patterns in large rivers and the importance of tributaries as spawning and foraging habitats for large river populations are widely unknown. From June to Mai we studied migration patterns of nase and barbel within a 58 km long section of the Austrian Danube (mean annual discharge 1900 m³.s-1) consisting of an free-flowing part, called Wachau (28 km long, channel width 250-300 m, maximum depth 6-9 m, average flow velocity about 2 m.s-1.) and an impounded section (30 km long, channel with about 350 m, maximum depth 10-15 m). The only tributary within this part of the Danube providing substantial spawning grounds is the River Pielach (mean annual flow: 6,5 m³.s-1), which meets the Danube at the upper end of the study area. During March and April up to 20.000 barbel and 3.000 nase migrate into the Pielach and spawn within the lower most 2 km. In June postspawning migrants of both species (25 barbels and 25 nase) were caught in the tributary. Fishes were tagged with coded radio transmitters (life time of 399 days) by surgical implantation and immediately released. Water depth range of signals could be increased to 8 m by means of an antenna amplifier. Tracking was done by boat in the Danube and along the shore in the tributary in 2-40 days intervals depending on fish activity. Position (GPS), water depth and distance to shore were measured for individually recorded fishes. Although nase is spawning earlier than barbel, all barbel left the tributary till end of June, whereas 7 nase remained in the tributary till end of June, 3 till end of July. Maximum recorded downstream movement was 52 km, maximum upstream movement was 12 km within one survey interval. While the majority of nase stayed within the free flowing Wachau, only 3 barbels could be registered repeatedly within this area. Contrary to expectation nase not only used riparian habitats but the entire river profile (up to 8 m depth recorded). Most of the barbels obviously moved downstream to the impounded area where they could be recorded in riparian areas. Migration patterns and habitat use of nase and barbel in large rivers differ from those in small and medium-sized rivers. The longer migrations and wider spectrum of habitat use has to be considered in the development of protection strategies for these endangered fish species.