Seasonal Variation of Shoreline Fish Assemblages at Two Stations in the Southern Branch of the Yangtze River Estuary

The Yangtze River Estuary is one of the most productive estuarine ecosystems in the western Pacific, supporting diverse fish communities that sustain ecosystem functioning. This study investigated the seasonal patterns and community structure of intertidal fish assemblages to provide a baseline for future habitat assessments. Seasonal surveys conducted from May to December 2024 recorded 47 fish species belonging to 10 orders, 18 families, and 37 genera. Cyprinidae contributed the highest proportion of species (42.55%). Dominant species identified by the index of relative importance-including <i>Cynoglossus gracilis</i>, <i>Coilia nasus</i>, and <i>Lateolabrax japonicus</i>-characterized the seasonal assemblage structure. The assemblages were dominated by sedentary species (82.98%), and demersal fishes accounted for 48.94% of the species. Carnivorous taxa (57.45%) dominated the trophic guilds. Diversity indices indicated moderate diversity (<i>H</i>': 1.797-2.441; <i>C</i>: 0.788-0.892; <i>D</i>: 1.724-4.770; <i>J</i>': 0.6318-0.8642). Similarity analysis based on Jaccard's index (<i>Cj</i>) showed the highest overlap between spring and summer (<i>Cj</i> = 0.5000) and the lowest between spring and winter (<i>Cj</i> = 0.1714); spring-autumn and summer-autumn were approximately 0.30, indicating moderate overlap. ABC curves yielded slightly negative W values in spring and summer and positive values in autumn and winter (W = -0.066 to 0.276), indicating moderately disturbed assemblages in spring-summer and less disturbed communities in autumn-winter. Overall, the study provides a seasonal baseline of intertidal fish assemblages in nearshore waters of the southern branch of the Yangtze River Estuary, which can provide useful ecological context for future assessments of nursery and feeding habitats of juvenile Chinese sturgeon (<i>Acipenser sinensis</i>).