Abstract: This study evaluated fish community structure and the invasion status of alien fish in the middle and lower reaches of the Zengjiang River, with a focus on the distribution pattern of alien fish and their relationship with environmental factors, aiming to reveal the invasion mechanism and provide a scientific basis for ecological management. Fish samples and environmental factors were collected from 20 sampling sites in the middle and lower reaches of the Zengjiang River in the summer and winter of 2024. Fish community composition and dominance were quantified using the index of relative importance (IRI), the abundancebiomass comparison (ABC) curve, and Jaccard similarity index. Principal component analysis (PCA) was applied to identify dominant environmental drivers, while redundancy analysis (RDA) was employed to elucidate the relationships between fish assemblages and environmental factors. A total of 25 fish species, comprising 753 individuals with a total biomass of 47.88 kg, were recorded. Exotic species accounted for 74.37% of the total abundance and 74.40% of the total biomass. The Redbelly tilapia (Coptodon zillii) and Amazon sailfin catfish (Pterygoplichthys pardalis) were the dominant species throughout the year, replacing the previously dominant Nile tilapia (Oreochromis niloticus). Fish communities in the winter showed higher similarity compared to summer. PCA results indicated that flow velocity and specific conductivity were the key environmental factors. RDA further revealed that the distribution of Redbelly tilapia was negatively correlated with specific conductivity, while Amazon sailfin catfish was positively correlated with flow velocity. This study demonstrates a rapid replacement of dominant exotic species in the Zengjiang River and clarifies the coupling mechanisms between their distributions and key environmental factors. Our findings highlight the importance of implementing targeted management measures based on the major environmental drivers influencing exotic fish distribution, in order to curb the spread of high-risk invasive species and promote ecological restoration and sustainable fisheries in the Zengjiang River Basin.

Key words: Zengjiang River, alien fish, fish community structure, redundancy analysis, Redbelly tilapia, Amazon sailfin catfish