Long-Term Changes in Fish Community Structure in the Regulated Đerdap Section of the Danube River

The Đerdap/Iron Gates section of the Danube River has undergone a profound hydromorphological transformation following the construction of the Iron Gates hydropower system, affecting flow conditions, habitat structure, connectivity, and fish assemblages. This study assessed long-term changes in native and non-native fish communities within the Đerdap Reservoir using monitoring data collected over 18 years. Fish were sampled at three sites (Dobra, Donji Milanovac, and Tekija) in 2005, 2010, 2017, 2019, and 2023 using electrofishing and gillnets. Temporal patterns were evaluated using complementary univariate and multivariate approaches to assess changes in community structure, diversity, disturbance status, and relationships with hydrological variability. The biomass contribution of non-native fish increased at all sites, with the most pronounced changes observed at Donji Milanovac and Dobra. These changes were accompanied by clear shifts in community composition, indicating progressive restructuring across the reservoir. Although diversity indices remained relatively stable, abundance–biomass comparison (ABC) analyses suggested moderate disturbance. Overall, long-term hydromorphological changes in the reservoir were associated with shifts in fish community structure and an increased contribution of non-native species, potentially influencing assemblage stability.