Structure and Health of the Fishery Community in the Northern Shandong Peninsula Sea Area

The waters of the northern Shandong Peninsula are spawning and nursery grounds for many fishery organisms from the Bohai Sea and Yellow Sea and play an important supporting role in the supplementation and reproduction of fishery resources in these seas. In recent years, with global warming and intensification of human activities, the fishery resources in Shandong offshore have generally declined, with obvious species miniaturization, low quality, and reduction of species diversity. Important fishery resources have been unable to achieve the minimum amount for the fishing seasons, and its support function in the Bohai and Yellow seas is increasingly diminishing. The fishery organisms research in the northern part of the Shandong Peninsula is less focused on their composition, community structure, and fishery biological health evaluation. Therefore, based on the northern Shandong Peninsula fishery resource survey data of May–June 2021, this study analyzed the fishery's biological diversity and identified the dominant species in these waters. Key species were evaluated through food web topology and social network analysis, and the fishery's biological health status was provided based on the survey data of 2011 and 2013. The study showed that the fishery resources of the Shandong Peninsula would be conserved and managed in 2021. The proportion of pelagic fish in spring decreased in 2021, and the turnover of dominant species showed a gradual decrease in pelagic fish, mainly small and less economically valuable. In the past 40 years, the spring fish diversity and evenness indices in the northern Shandong Peninsula sea area have been continuously reduced, reaching values lower than the reasonable range of the proposed diversity index, which indicates that the species in this area are poorly homogeneous concerning their horizontal structure. The higher number and extremely uneven distribution of species can lead to a lower diversity index. Compared with the late 20th century to the early 21st century, Engraulis japonicus was a dominant resource based on the results of this study, and the lower number of other fishes may be a factor for the lower diversity index. The significant increase in the richness index may be related to the occurrence of succession in the community structure, resulting from the mutual replacement of small, low-quality species with short life cycles, rapid resources renewal, and small individuals. The dominant species E. japonicus is often found among the catch. Shandong offshore fisheries development history corresponds to a significant number of large, high-value class invertebrates fishing, including cephalopods such as Loligo chinensis, in an unhealthy state. Crangon affinis is the absolute dominant species of invertebrates. In the spring of 2021, the fish food web in the northern Shandong Peninsula showed correlation values ranging from 0.03 to 0.30, excluding a possible specific relationship between the population feeding and the community. Moreover, external factors may disturb the community, which is consistent with the community interspecific feeding relationship under natural conditions. The key community species were Liparis tanakae, Lophius litulon, E. japonicus, Larimichthys polyactis, and Chaeturichthys stigmatias. E. japonicus was both the dominant and key species in the fish community in the northern Shandong Peninsula waters in 2021. The regulating effect of E. japonicus on the food web was related to its upward control effect through changes in resource abundance that affects the changes of other fish that use this area as bait. L. tanakae, L. litulon, and L. polyactis were important predators in the sea through the downward control effect, affecting the stability of the whole food web. Fishery biological density, fish density, and crustacean density health index were found within healthy parameters, while the cephalopod density health index was in a sub-healthy state. Compared with 2011–2013, the densities of various economic species have increased significantly, and the overall health of fishery organisms in the northern part of the Shandong Peninsula is good. Overall, the results of this study can provide a reference for conserving and managing fishery resources in the Shandong offshore region