Asian Journal of Fisheries and Aquatic Research

Estuarine ecosystems are among the most biodiverse yet increasingly threatened environments worldwide, particularly in tropical coastal regions where river and marine systems interact. This study investigated the structure, species composition, and seasonal variation of fish assemblages in the Han and Cu De Rivers in Da Nang City, central Vietnam. Field surveys were conducted during the rainy season (September–December 2024) and dry season (March–June 2025) at major fish landing sites and aquatic product collection points along the two river basins. A total of 957 specimens were collected and analyzed using standard morphological taxonomy, and community patterns were analyzed using Bray–Curtis similarity and Principal Component Analysis (PCA). Overall, 166 fish species belonging to 122 genera, 64 families, and 21 orders were recorded. Species richness differed between rivers, with 109 species in the Han River and 99 in the Cu De River, and only 42 shared species, indicating clear spatial differentiation. The Han River assemblage was dominated by estuarine and marine-associated taxa (e.g., Acanthuriformes and Carangiformes), whereas the Cu De River was characterized by freshwater taxa, particularly Cypriniformes (28.3%). Seasonal variation was pronounced, with 121 species recorded in the dry season compared to 75 in the rainy season. These findings demonstrate that the study area functions as an estuarine transition zone with strong environmental gradients shaping fish communities. The results provide essential baseline data for biodiversity conservation and fisheries management, while highlighting the importance of hydrological connectivity and seasonal dynamics in coastal river ecosystems.