Fish community structure and biodiversity in the offshore waters of Zhoushan Islands in spring and autumn

In order to understand the species composition, spatial distribution, biodiversity, and community structure characteristics of fish resources in the waters off the Zhoushan Islands, a trawl survey was conducted in the waters off the Zhoushan Islands in April (spring) and October (autumn) 2018. Studies have shown that: (1) there are 106 species of fish in the waters off Zhoushan Islands, belonging to 12 orders, 47 families and 80 genera, including 63 species in spring, belonging to 12 orders, 34 families and 49 genera, and 85 species in autumn, belonging to 12 orders, 45 families. (2) the number of fish species in this survey is 63 in spring and 85 in autumn. Compared with the historical survey results, the total number of fish species significantly reduced in spring, but slightly increased in autumn. This may be due to the previous use of beam drag shrimp nets, and bottom trawl nets were used in this investigation. The difference in nets may cause differences in the main fishing targets, or it may be due to the different layout of survey stations; (3) the dominant fish species in the two seasons changed. Obviously, the dominant species in spring are Lophius litulon, Lepidotrigla japonica and Apogon lineatus, while in autumn the dominant species are L. litulon and A. japonicum; (4) The value of fish biodiversity index is greater in autumn than that in spring, and the evenness index (J') of spring and autumn is relatively small compared to diversity index (H') and richness index (D), and the value of biodiversity index in spring and autumn basically increases with the increase of water depth, and the water depth interval is higher than 80 m. In spring and autumn, the horizontal distribution of D, H', and J' values is lower in the western part of the survey sea area and lower in the sea area near the island. The open sea area far from the islands in the south is greater. However, since there are only 1 and 3 stations with a water depth greater than 80 m and less than 50 m, and it is also related to factors such as seasonal fish activities, and the water depth in the outer waters The degree of impact on biodiversity and the specific reasons need to be further explored; (5) through cluster analysis and non-metric multi-dimensional scaling analysis, it can be obtained that 3 communities are divided into 3 communities at a similarity level of 35% in spring and 45% similarity in autumn. It is divided into two communities at 45% level. The species similarity in autumn is higher than that in spring, and the biomass and species level in spring are lower than those in autumn. The composition of fish communities in waters where the depth gradient does not change greatly depends on temperature. The changes in water temperature caused changes in the dominant species and dominance of fish in the sea, and the migration of migratory fishes caused changes in the concentration of fish populations, which may be one of the main reasons affecting the stability and changes of fish communities in the waters off Zhoushan Islands. (6) the ABC curve shows that the spring fish community structure outside the Zhoushan Islands is undisturbed, which may be caused by the small temperature difference between different stations in the sea area in spring and the relatively stable environment. The biomass dominance curve in autumn has always been below the abundance dominance curve, which indicates that fish individuals are generally small, mainly small fishes, and some large individuals with obvious biomass dominance are almost non-existent. They are perennial and large in size. There are very few fishes, and the fish communities off the Zhoushan Islands are seriously disturbed. There is a large number of supplementary groups in autumn, which has a certain impact on the size of fish communities. Therefore, the ABC curve to determine the stable state of the community needs further discussion.