River infrastructures, such as hydropower plants Iron Gate I (IG; rkm 943) and II (rkm 863) represent major obstructions to fish migration in the Danube River. Knowledge about fish behaviour and movements in the vicinity of major river structures is required to build effective fish passages to protect migratory fish species, with acoustic telemetry being an useful method for observing such behaviour. From autumn 2019 to the spring 2021, the movement of 185 fish (barbel Barbus barbus, nase Chondrostoma nasus, vimba bream Vimba vimba, asp Leuciscus aspius, Pontic shad Alosa immaculata, and carp Cyprinus carpio) were monitored in the Danube River upstream and downstream of the IG II dam using acoustic telemetry. The movements of tagged fish were recorded by a combination of automatic tracking of fish passing receivers deployed in the river and manual tracking by boat. Of the 101 fish released downstream of IG II, 48% moved upstream from their release site and were detected close to IG II. The remaining 84 tagged fish were released in the reservoir between IG I and II, with 49% of fish moving further upstream in the reservoir and 18% reaching IG I, while 48% of the tagged fish moved in a downstream direction and were recorded below IG II. No fish released downstream of IG II were detected in the reservoir between two dams, which indicates that the ship locks on either side of the river do not represent viable routes for upstream migration around IG II. There was also no clear pattern in which side of the river tagged fish preferred to move along. To conserve migratory fish species, dam management strategies that take into account the behaviour of fish around such structures is necessary. Acoustic telemetry can be used to inform and guide towards construction of the safe fish passage or adapt structures to improve the movement and fish migration
