Fish Community Traits near a Large Confluence: Implications for Its Nodal Effects in the River Ecosystem

River confluences are nodes where the unique hydrological processes of two rivers meet, resulting in complex flow structure and water quality mixing processes. Thus, greater food availability and habitat complexity can occur at a confluence, making it a hotspot for fish productivity and diversity. Nonetheless, studies that relate fish community traits to specific habitat characteristics at large river confluences are limited. Two field surveys were conducted at the large confluence between the Yangtze River and the Poyang Lake outflow channel involving fish hydroacoustic detection, environmental DNA, and acoustic velocity profiling. The discharge ratios of this confluence in two surveys were similar, but the water quality conditions represented by turbidity differed greatly. The results demonstrated high spatial heterogeneity of fish density, size, and species near the confluence. In the Yangtze River with high flow velocity, the abundance of small-sized fish was substantially higher than that in Poyang Lake outflow channel with low velocity, while large-sized fish chose their habitats more freely between the two tributaries and post-confluence channel. The convergence of two tributaries created prominent spatial heterogeneity of habitat conditions in the post-confluence channel, thus highest fish abundance and species biodiversity occurred there. The species assemblage structure of the local fish community was greatly affected by the change of water quality, e.g., the intrusion of a turbid flow from Poyang Lake into the Yangtze River, and thus the exchange of fish communities between two tributaries was weakened. The present study highlights the ecological importance of river confluence in improving regional fish abundance and species diversity and provides the theoretical foundation for the conservation and management of the aquatic environment at the confluence and in the whole river ecosystem.