Preliminary insights into fish movements beyond the massive Iron Gate dams on the Danube River using acoustic telemetry

Upstream fish movement in the Danube River at the Iron Gate is blocked by the massive hydropower dams and ship locks, as shown by tracking six fish species (vimba bream Vimba vimba, common nase Chondrostoma nasus, barbel Barbus barbus, asp Leuciscus aspius, Pontic shad Alosa immaculata and common carp Cyprinus carpio). In the absence of effective fish passage systems, the current level of river connectivity is insufficient to support upstream movement and migration for this diverse, multispecies fish community. The tagged cyprinids displayed evidence of migratory behaviour. Individuals of vimba bream, barbel, asp and common nase that were transported across the lowermost dam and released into the lower reservoir section showed rapid upstream movement, suggesting that the reservoir itself did not present significant obstacles to migration. Some covered the entire 76-km long reservoir within a few days to 2 weeks but were ultimately blocked by the next dam. Cyprinids released below the dams were recorded at varying depths and on both sides of the river. Asp, barbel, common nase and vimba bream moved both upstream and downstream relatively close to the surface below the Iron Gate II dam, averaging 2-3 m below the surface, but also diving down to about 10-20 m. Future studies combining three-dimensional telemetry methods and detailed information on hydrology below the dams could provide further information on the behaviours of the different fish species, which is needed to design efficient fish passage solutions.