Size structure and community stability assessment of fish community in the Yangtze River estuary

The Yangtze River estuary is the largest estuary ecosystem in China. It has sufficient nutrients and provides necessary habitat, bait and breeding grounds for different ecological types of fish. With the advancement of the policy “Yangtze River Great Protection”, the key waters of the Yangtze River Basin have entered a ten-year ban on fishing. From the perspective of scientific research, evaluating the effects of fishing bans is an important research task and carrying out such research has an essential ecological significance for fisheries protection and recovery in the Yangtze River estuary. Based on the fishery and environmental survey data in the four seasons in 2018, the size structure characteristics and community stability were studied using the normalized fish size spectrum and stability assessment model to improve the understanding of fish communities in the Yangtze River Estuary. Meanwhile, the water quality of the Yangtze River estuary was evaluated using the eutrophication index method. The results showed that: ① the size classes ranged from -2 to 6, and all the dome of all fish size spectrum curves located in the size class of 0 and 3, indicating that the fish community was dominated by small fishes and was in a disturbed state; ② There were seasonal changes in fish communities of the North Branch and the South Branch, with the maximum fish size spectral curvature in summer (−0.04) and the minimum in autumn (−0.41) and W values of −0.11 to 0.12 in the North Branch, and the maximum fish size spectral curvature in summer (−0.04) and the minimum in winter (−0.24) and W values of −0.03 to 0.37 in the South Branch; ③ The average stability factor of South Branch was 0.57, while it was 0.43 in the Northern Branch, which indicated that the stability of the fish community was different in four seasons. The eutrophic coefficient indicated that the eutrophication levels of the North Branch (18.89) was higher than that of the South Branch (5.43), and there was a significant negative correlation between the eutrophication index and the stability factor, which indicated that the eutrophication may have an important influence on the deviation of the community system from the stable state. In general, the size class of the fish community is small, the community stability is weak, and the water quality is eutrophic in the Yangtze River estuary. This study explored the characteristics of fish community size structure and evaluated its community stability in the Yangtze River estuary before the implement of the fishing ban in the Yangtze River Basin, and provided a preliminary research for the effect evaluation of the fishing ban to the Yangtze River estuary ecosystem.