Effects of water mass dynamics on the structure and distribution of fish egg community in Daya bay

Hydrographic characteristics and water mass changes are essential for understanding fish egg distribution and for the protection and management of marine ecosystems. In this study, four cruises were conducted in May (spring), July (summer), September (fall), and December (winter) of 2022 to investigate early fish resources and environmental characteristics in Daya Bay waters. The K-means clustering algorithm was employed to classify water masses in Daya Bay, and the distribution patterns of temperature and salinity, along with their seasonal variations, were systematically analyzed. Additionally, based on the classification of water masses, we compared fish egg communities between seasons and water mass regions to examine the structure of fish egg communities under different hydrographic conditions. The main findings of this study were as follows: in the present study, 51 fish egg species were identified, belonging to six orders, 22 families and 35 genera. The number of fish egg species ranged from high to low was spring, summer, autumn and winter, while the average abundance ranged from high to low was spring, summer, winter and autumn. The K-means clustering method was utilized to classify Daya Bay into coastal water mass (CWM), nuclear power plant warming water mass (NWWM), high salinity water mass (HSW) outside Daya Bay, mixed water mass (MWM) and Pearl River dilution water mass (PRDWM). By integrating salinity and water temperature data, we identified associations between fish eggs and specific water masses. The mean abundance of fish eggs was highest in thermal discharge from nuclear power plants during spring and autumn; in seasonally mixed water during summer; and in littoral water of the Bay during winter. These results indicate that the thermal discharge from nuclear power plants has had specific effects on the water masses in the adjacent sea areas, forming a distinct oceanic water temperature fronts. This may played a positive impact on the spawning and nursing of some fish species, facilitating the recruitment of fish resources in Daya Bay.