Distribution Patterns and Influencing Factors of Engraulis japonicus Spawning Grounds in Offshore Waters of the Liaoning Province in the North Yellow Sea

The ichthyoplankton stage is the stage that is most vulnerable to changes in the marine environment in the development cycle of marine fish. Subtle changes in the marine environment have a strong impact on fish survival, development, and growth. The abundance of fish eggs directly affects the early recruitment of fish resources and determines the vitality of generations. As one of the important spawning grounds of Engraulis japonicus, it is of great significance to understand the distribution patterns and relationship between environmental factors in offshore waters of the Liaoning Province in the North Yellow Sea. Based on the spawning grounds survey carried out in offshore waters of the North Yellow Sea from April to December, 2021, first, Garrison's distribution center of gravity was used to analyze the core spawning ground of E. japonicus and its migration route. Second, a generalized additive model based on Tweedie distribution (Tweedie-GAM) was applied to convey the main drivers of the distribution patterns of the spawning grounds. The relationship between E. japonicus egg density and six natural environment factors of seawater surface temperature (SST), seawater surface salinity (SSS), seawater surface chlorophyll a concentration, zooplankton abundance, phytoplankton abundance, and depth, as well as the factors of time (month) and space (longitude and latitude) were interpreted. Finally, the cross-validation method was used to validate the model and predict the potential spawning grounds. The results showed that the spawning period of E. japonicus was long, lasting from April to November, and the main spawning period was from May to August, with peak spawning from late May to early June, in offshore waters of the Liaoning Province in the North Yellow Sea. During the spawning season, the size and location of the E. japonicus spawning grounds showed obvious spatiotemporal variation. There was a significant nonlinear correlation between spatiotemporal factors and the density distribution of E. japonicus eggs. Although spatiotemporal factors were the main drivers of the spatiotemporal variation of the density distribution of E. japonicus eggs, these factors did not directly affect their distribution pattern. Rather, spatiotemporal factors indirectly affected the concentration distribution of E. japonicus eggs through SST, SSS, and depth. The optimal temperature range for the spawning of E. japonicus was wide, and the distribution of spawning grounds ind