The Role of a Near‐Natural Fish Bypass in Facilitating Downstream Movements and Providing Habitat for a Potamodromous Fish

ABSTRACT Fish passes, traditionally designed to facilitate upstream migration, are now increasingly recognized for their multifunctionality, including bidirectional fish movement and habitat provision. This four‐year study at the Ottensheim‐Wilhering hydropower plant on the Austrian Danube tracked the migration patterns of the common nase ( Chondrostoma nasus ) using five PIT tag antenna arrays installed in a 14 km long, near‐natural bypass system, which integrates two natural tributaries. A total of 190 nase were tagged and released in the Danube upstream of the hydropower plant, and fish movement data were collected from November 2020 to June 2024. A large percentage (52.5%) of the tagged individuals entered the fish pass from upstream. Over the years, many individuals utilized the bypass system, including the integrated tributaries, for extended periods and were observed repeatedly visiting the fish pass, highlighting its role not just as a migration corridor but also as a suitable habitat. These findings underscore the multifunctionality of near‐natural fish passes, which not only restore longitudinal connectivity but also provide critical habitats for various life stages of rheophilic fish. The study challenges the assumption that fish passes are ineffective for downstream migration and suggests that such systems can play a significant role in supporting potamodromous fish populations in highly regulated rivers like the Austrian Danube.