Assessment of Non-Native Species Impact on Fish Diversity in the Ćelije Reservoir: Implications for Conservation and Management

The Ćelije reservoir was built to prevent flooding and sedimentation in the Đerdap reservoir, but now serves as an important water source for the Rasina district. Due to sediment deposits, its length has been reduced by about 1 km and is now about 31 km at normal water levels. The Lake Ćelije ichthyofauna has been monitored for over a decade. For this study, the ichthyocoenosis was analyzed in 2011, 2016 and 2023. Sampling was carried out using electrofishing and fishing nets with different mesh sizes. In 2011, eleven fish species were recorded, including strictly protected tench, Tinca tinca, and protected species, chub Squalius cephalus, which were only detected in that year. In 2016, nine fish species were found, with Wels catfish, Silurus glanis missing in 2023. The non-native species the pumpkinseed, Lepomis gibbosus and the Gibel carp Carassius gibelio were evenly represented each year. Even with the highest number of fish species in 2011, the Shannon diversity index was lowest at 1.33, likely due to the high abundance of freshwater bream, Abramis brama. In 2016, the Shannon index increased to 1.68, and in 2023 it increased further to 1.76. The ABC curve was also calculated. In 2011 and 2023, biomass exceeded abundance, indicating a stable biocoenosis with low fishing pressure. In 2016, the curves overlapped, indicating higher fishing pressure. Only two non-native species, the Gibel carp and the pumpkinseed, were detected in this ichthyocoenosis. Another invasive species spiny cheek crayfish Faxonius limosus has been recently detected and reported to be abundantly present by locals. While non-native species do not outnumber native species in Ćelije reservoir, the decrease in species diversity since the first year of sampling underscores the growing importance of conservation and monitoring efforts.