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

Abstract

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 indicated a synergistic effect under low temperatures and low salinity and an inhibitory effect under high temperatures and low salinity. During the early spawning season in April, the central spawning ground was located in the deep waters southeast of Haiyang Island. Alongside continuously rising water temperatures, both the size of the spawning ground and the concentration distribution of E. japonicus eventually reached their annual peaks between late May to early June, thus, constituting the peak spawning season. During this period, the central spawning ground was located around Shicheng Island and the Yingna River estuary. From June, however, water salinity decreased along the southern coast of the Liaoning Peninsula due to increasing coastal freshwater runoff. The inhibiting effect of higher temperatures and lower salinity drove the spawning fish away from the coastal waters and into deeper offshore sea areas. The E. japonicus spawning grounds, thus, migrated further offshore with SSS as the dominant factor driving this migration. As autumn and winter arrived, the spawning activity of E. japonicus gradually ended. E. japonicus eggs were sparsely scattered in the investigated sea area during October. In November, large numbers of E. japonicus began gathering in the center of the investigated sea area and gradually migrated further southward to their wintering habitat. Accordingly, no E. japonicus eggs were collected during the month of December. The main spawning grounds of E. japonicus were located in the coastal waters of Shicheng Island and the Yingna Estuary, and gradually showed a contraction trend after July with the gradual weakening of E. japonicus spawning activities. Overall, from April to December 2021, the distribution of E. japonicus eggs in offshore waters of the Liaoning Province in the North Yellow Sea showed a migration trend from the southern offshore deep water area to the northern offshore shallow water area, and then to the southwestern offshore deep water area. This study elucidated the spatial and temporal distribution patterns of E. japonicus spawning grounds and its influencing factors, which indicated that the Tweedie-GAM method could be effectively applied to analyze the relationship between the early fish resources and environmental factors. This method showed good performance in solving 0-value problems. The results provide a scientific basis for the conservation and management of E. japonicus spawning grounds in the North Yellow Sea, assist with evaluation of the current situation and development trend of E. japonicus resources in the sea area, and provide an important reference for the rational development and utilization of E. japonicus resources and research on spawning ground protection strategies.

Keywords

• engraulis japonicus
• spawning ground
• main spawning period
• tweedie-gam
• north yellow sea