The Yuanjiang River is one of the most significant rivers in Yunnan, characterized by abundant fish resources; however, in recent years, human activities have led to a decline in these resources. We employed environmental DNA (eDNA) technology to investigate the fish species in the Yuanjiang River, providing baseline data for fish resource management and conservation. In October 2023, 16 sampling stations were established along the river to collect water samples, and fish diversity was analyzed using eDNA techniques. A total of 47 fish species (5 orders, 15 families, and 40 genera) were detected, including rare and endangered species such as Bagarius rutilus and Hemibagrus guttatus, as well as 8 invasive fish species. Alpha diversity analysis shows Shannon-Wiener index of 0.92–2.41, Simpson index of 0.44–0.88, Pielou evenness index of 0.35–0.60, and Margalef index of 0.30–1.16. Hierarchical cluster analysis and non-metric multidimensional scaling (NMDS) were applied to examine spatial distribution patterns of fish communities, showing distinct differences among stations that grouped into three clusters from downstream to upstream, with geographically proximate stations clustering together. Similarity percentage analysis (SIMPER) identified species contributing to inter-group differences, with Culter alburnus, Channa gachua, Cirrhinus molitorella, and Abbottina rivularis as the primary contributors. Redundancy analysis (RDA) assessed relationships between fish diversity and environmental factors, indicating that the Yuanjiang River fish community was primarily influenced by altitude, salinity, conductivity, total dissolved solids, redox potential, and total phosphorus. This study demonstrates that eDNA technology is effective for analyzing fish species composition and distribution in the Yuanjiang River and can serve as a crucial tool for biodiversity monitoring and conservation.

The Nansha Reservoir in Yunnan Province is a key hydraulic project in the cascade development of the Red River Basin, but the response of its fish resources to seasonal environmental changes in the reservoir environment remains unclear. This study aims to clarify the dynamic characteristics of fish resources during the breeding season (May) and the overwintering preparation period (October), thereby providing a scientific basis for the reservoir's ecological management, exotic species control, and native fish conservation. In May and October 2024, we integrated hydroacoustic surveys with traditional fishing methods to systematically analyze the spatiotemporal variations in species composition, body length structure, and spatial distribution of the fish community. The results show that: 1) Regarding species composition, 26 fish species were identified in May, with Cypriniformes dominating in both relative abundance (70.92%) and biomass (55.93%). In October, 43 species were identified; the relative abundance of Cypriniformes decreased to 60.44%, while Siluriformes increased significantly in both abundance (25.26%) and biomass proportion (31.19%). The total biomass proportion of non-native species reached 28.19%. 2) Regarding body size structure, target strength (TS) inversion indicated that the proportion of small individuals (<10 cm) was notably high in May (78.81%) but decreased significantly to 61.43% in October. Concurrently, the proportion of medium-sized individuals (10–30 cm) increased substantially to 30.46%. 3) Regarding spatial distribution, high-density areas in May were concentrated in shallow waters from the reservoir embayment to the tail area [Average density: (0.2065±0.1858) ind·m⁻³], whereas in October, they shifted to the transition zone between the central reservoir and the embayment [(0.1940±0.1720) ind·m⁻³], with enhanced spatial continuity. Vertically, fish were primarily concentrated in water layers≤10 m during the breeding season (74.88%) and showed significant descent during the overwintering preparation period. The study is the first to systematically reveal the coordinated seasonal shifts in the Nansha Reservoir fish community, characterized by "increased species richness, succession of dominant taxa, growth of juvenile cohorts, and distributional descent". Meanwhile, it is found that exotic species have formed an ecological niche pattern of "low abundance-high biomass".