Dynamic response of fish community to hydrologic regime alteration in the upper reaches of the Yangtze River

This study investigates the impacts of hydrological regime alterations on fish community structure and ecological types in the Yibin section of the Yangtze River. Utilizing eDNA metabarcoding data from 2021, combined with historical records, the study offers a comprehensive analysis of fish community structure, structural evolution, and shifts in dominant species. The Indicators of Hydrologic Alteration—Range of Variability Approach method (IHA-RVA) was applied to calculate the degree of hydrologic alteration after the dam impoundment. Then, the Spearman correlation method is used to identify a number of key indicators driving shifts in species composition. Results indicate that fish species richness increased during the early impoundment phase of the Three Gorges Dam, corresponding with rising water levels. However, following the operation of the Xiangjiaba Dam, species richness began to decline. The traditionally dominant Cyprinidae family has diminished in prevalence, while taxonomic diversity and community complexity have increased. Rheophilic species remain dominant, though the proportion of limnophilic species has risen, now constituting nearly 50% of the total species composition. Altered hydrological conditions, particularly increased long-term dry season flow, have emerged as key drivers of changes in fish community structure. These findings provide crucial insights for the conservation of fish resources in the upper Yangtze River and the ecological management of the Jinsha River cascade dams.