Autumn fish assemblages show marked estuarine–offshore spatial heterogeneity in the Yangtze River Estuary and adjacent waters

The Yangtze River Estuary is a highly dynamic river–sea transition zone in which freshwater discharge, tidal exchange, saltwater intrusion, plume dispersion, turbidity, and shallow-water habitat mosaics jointly shape fish assemblages. To provide a synoptic autumn baseline for this estuarine–coastal system, we analysed bottom-trawl survey data collected at 89 stations during October–November 2024 across three spatially defined sectors: the Yangtze River Estuary moratorium zone (EMZ), offshore waters outside the estuary mouth (OFF), and the northern waters of Hangzhou Bay (HB). Species composition, dominant taxa, catch density, catch per unit effort (CPUE), station-level alpha diversity, abundance–biomass comparison (ABC) curves, and Bray–Curtis-based multivariate structure were compared among sectors. A total of 71 fish species belonging to 12 orders and 29 families were recorded. Perciformes was the most species-rich order, and Collichthys lucidus was the only dominant species shared by all three sectors. The EMZ showed the highest abundance density (24.49 × 10³ ind./km²), biomass density (216.03 kg/km²), and mean CPUE (7.9 kg/h), whereas OFF had lower density values and the most negative ABC pattern. Station-level alpha diversity differed significantly among sectors (Kruskal–Wallis, all P < 0.001), with the EMZ showing the lowest diversity and evenness. Cluster analysis, NMDS, and PERMANOVA revealed statistically significant multivariate separation both within and among sectors. These results indicate that autumn fish assemblages in the Yangtze River Estuary and adjacent waters are strongly structured by spatial heterogeneity along the estuarine–offshore gradient. The higher fish density observed within the EMZ should be interpreted as a spatial ecological pattern of the inner estuary rather than as direct evidence of management effectiveness. The consistently negative ABC curves across all sectors further suggest that the assemblages remain dominated by small-bodied fishes, highlighting the need to evaluate density, size structure, and community composition jointly. This study provides an updated autumn reference for future monitoring and ecosystem-based management in the Yangtze estuarine–coastal system.