Community structure of fishery organisms in offshore waters of the North Yellow Sea

The offshore waters of the North Yellow Sea is a traditional fishery area in northern China, with sufficient bait organisms and high marine primary productivity, which meet the habitat conditions required for the growth and development of various marine fishery resources and are important spawning grounds and habitats for fishery taxa. In order to understand the current community structure of fishery organisms and reveal the main environmental drivers of its spatial and temporal changes, six monthly voyages of the resource bottom trawl and its habitat surveys were conducted from April to November 2021. Abundance-biomass comparison curves(ABC), spatial interpolation analysis, diversity Analysis, CLUSTER analysis and non-metric multidimensional scale ranking(NMDS) were used to analyze the species composition, dominance, relative abundance, diversity, stability and intra-annual succession of fishery organisms community structure; the relationship between spatial and temporal variation of fishery organisms ommunity structure and environmental factors was analyzed by Canonical Correlation Analysis(CCA). The results showed that 89 species were collected, including 50 species of fish, 19 species of crustaceans, 6 species of shellfish, 6 species of cephalopods, 6 species of echinoderms and 2 species of coelenterates. The dominant species from April to November were Engraulis japonicus and Crangon affinis, while the important species were Actiniaria, Liparis tanakae, Loligo japonica, Todarodes pacificus, Metapenaeopsis dalei and Hexagrammos otakii. Compared with past surveys, the pelagic species Konosirus punctatus and Sardinella zunas had a significantly lower catch proportion in this survey. During the survey, many kinds of fishery biological parents and larvae appeared, showing that the sea area has the property of good spawning and nursery grounds. In addition, under the long term influence of protection measures such as fishing moratorium, juveniles of important economic species Pagrus major were found in this survey, and juveniles of Decapterus maruadsi also showed obvious clustering phenomenon, which to a certain extent reflects the development trend of the resource for the better. The community was unstable for most of the survey period. In April, the selective species number in the fishery community began to increase, the community structure was in a state of moderate interference. From May to June, there was a large-scale outbreak of anemone in southern waters of the Zhangzi Island and Haiyang Island, and the marine environment was under great pressure. Therefore, the community was in a state of serious disturbance. In July, the community showed a state of severe disturbance. Since spring and summer are the peak periods of spawning migration, the spawning migration of a large number of fishery organisms formed a relatively continuous migration disturbance. Due to this and the drastic changes in the sea area habitat, the disturbance to the community in July was further intensified. The community in August was in a severely disturbed state, and the diversity level showed a decreasing trend compared with the previous month. The reason for this is that a large number of jellyfish communities appeared in the sea in August, which greatly occupied the ecological niche in the upper ocean space, leading to a significant increase in natural disturbance and a certain degree of damage to the marine habitat. But as was in timing of the peak spawning migration, when migratory fish species gathered, the nutrient channels became more complex, and the community was more resistant to disturbance, so its stability was improved. After four months of fishing moratorium, the community richness and diversity were significantly improved from September to November, but with the end of the moratorium, the ABC curve showed that the community structure was severely disturbed from September to November due to the sudden increase of anthropogenic fishing disturbance. The CCA results showed that among the selected environmental factors, sea surface temperature (SST), sea bottom temperature (SBT), sea bottom salinity (SBS) and chlorophyll-a (Chl.a) were the main environmental factors causing spatio-temporal changes in community structure, among which SST and SBT had more significant effects. During the transition from spring to summer, the offshore waters of the North Yellow Sea gradually entered the warming period, and the warm-temperature species began to migrate into the sea area. The typical species of fishery biological community in the sea area around the southern Yalu River estuary and the central sea area of the Haiyang island gradually transited from cold-temperature species to warm-temperature species such as E. japonicus and T. pacificus. Possibly affected by the edge of the cold water mass in the North Yellow Sea, the communities in the deep water area south of Zhangzi Island and Haiyang Island gradually evolved into cold-temperature species, such as H. otakii and L. tanakae. In autumn, the sea area was still characterized by warm water, and the fishery biological communities in different geographical locations and different water layers of the sea area were basically replaced by warm temperature and warm water species. In addition, according to the Jaccard similarity index and species turnover rate, the species turnover rate of fishery biological community was relatively large during the transition from spring to summer, while the species turnover rate was significantly decreased during the transition from summer to autumn. Traditional resources such as Larimichthys polyactis, Scomberomorus niphonius and Scomber japonicus were still in the process of decline, and the phenomenon of low quality of marine fishery resources was still more significant. The relationships between species composition, quantitative distribution, community structure changes and stability, biodiversity changes and spatial and temporal changes in community structure and environmental factors in the marine area were also elucidated preliminarily. It was found that reproduction, feeding migration, overwintering migratory behavior, human activities and environmental changes of each fishery taxon all have important effects on the turnover of community species, and the resource dynamics of the early replenishment group is particularly critical, while external environmental factors and internal mechanisms of its changes are complex. It is necessary to carry out further study on the early life stage of marine fish and the recruitment mechanisms should be discussed in terms of environmental conditions. This study systematically clarifies the structural characteristics and seasonal succession of fishery biological community in offshore waters of the North Yellow Sea, providing reference and data support for further understanding the current situation of fishery biological community structure and its response to environmental factors.