Seasonal dynamics and habitat preferences of juvenile fish assemblages in the Wanquan Estuary, South China: Implications for conservation and management

Tropical estuaries encompass diverse habitats, which are crucial for supporting juvenile fish by providing nurseries, refuges, and migration routes. In the present study, we investigated the seasonal and spatial dynamics of juvenile fish assemblages in the Wanquan Estuary, Hainan Island, China, examining variations during the wet and dry seasons across four types of habitats. Our research aimed to test the hypotheses: (1) the wet season supports a greater abundance and species richness of juvenile fish compared to the dry season, and (2) juvenile fish showed varying habitat preferences across seasons. We recorded a total of 3617 individuals belonging to 33 families, 50 genera, and 78 taxa. Our findings revealed significant seasonal fluctuations in the assemblage structure, with peak abundance during the wet season in August, primarily consisting of diadromous species. A notable shift towards marine species was observed in March. Spatially, juvenile fish exhibited significant habitat preferences, with a strong preference for sand substrates for most species. Contrary to common expectations, mangrove fringe habitats were not identified as the primary nursery grounds for juvenile fish. Instead, our study highlights the importance of habitat diversity and connectivity in enhancing nursery functions. Water temperature and salinity were the key drivers influencing the seasonal and temporal patterns of juvenile fish assemblages in the Wanquan River estuary The pronounced temporal dynamics of these assemblages, driven by seasonal environmental alterations affecting water variations and habitat connectivity, were more significant than spatial differences. The findings emphasize the need for focused connectivity of various habitats to support the conservation of diverse fish communities in tropical estuarine ecosystems.