Assessment Of Impacts Of Early Closed Fishing On The Change In Fish Communities And Aquatic Ecosystem In Longgan Lake Natural Reserve

There are many lakes in the middle-lower reaches of the Yangtze River basin,which have nurtured rich fish resources.However,fish resources are in serious decline due to interference from water pollution,overfishing and other factors.To alleviate the current situation of fish resource scarcity and restore the health of lake and wetland ecosystems,the Ministry of Agriculture and Rural Affairs introduced a 10-year fishing ban system on the Yangtze River in 2018.Longgan Lake,a typical shallow lake in the middle and lower reaches of the Yangtze River,was completely closed to fishing in 2020.In this study,the changes of fish communities in Longgan Lake Nature Reserve were monitored from 2021to 2022,and combined with the construction of a nutrient channel model and the analysis of fish feeding structure in reserve,the changes of fish communities in lakes closed to fishing were studied,the development trend of fish communities was predicted,and the impact of fish communities on the lake water ecosystem after the fishing ban was evaluated.The main results of the study are as follows.The main results of the study are as follows.To understand the changes in fish communities of early closed fishing,44 species of fish were found during the two years after the survey,with the highest number of Cyprinidae fish,totaling 27 species.The number of species of herbivorous,filter-feeding,carnivorous,and omnivorous fishes were 4,2,18,and 17 species,respectively.In 2021and 2022,41 and 37 fish species were investigated,respectively.The changes of fish community were mainly manifested by the relative abundance changes of dominant species.The dominant species were Culter dabryi,Culter alburnus,Carassius auratus,Coilia ectenes,and Toxabramis wwinhonis.The preponderence of C.ectenes and C.auratus increased year by year,while the relative abundance of carnivorous fishes increased,while C.dabryi gradually replaced C.alburnus as the absolute dominant species.The changes of fish community biodiversity indices（Simpson,Shannon-Wiener,Pielou,Margalef indices）in both years were not significant.It indicates that there is no significant increase in species diversity in the reserve.The total number of individuals from age 0+to 1 accounted for78.47%and 85.71%of the population size in the age composition of C.auratus and C.dabryi,respectively.The length-body weight relationship equations for both were m=0.0477 L^2.78（R²=0.963）and m=0.0218 L^2.79（R²=0.970）,respectively,with indices less than 3.It indicates that the fish in the early period of the ban have the characteristics of miniaturization and younger age.There was no significant difference between the two years’findings on the existing stock.y=-0.11 x+7.83（R²=0.622）and y=-0.08 x+7.17（R²=0.374）for the linear regression equation of the length spectrum of the fish community in 2021 and 2022,respectively,and the slope and intercept of the trend line of the length spectrum in 2022 were smaller than those in 2021,and there was an increasing trend of the fish stock.The Ecosim model was used to predict the development trend of fish community in Longgan Lake.The results showed that all fish stocks tended to increase in the first year after the ban.In the following 3-4 years,The growth of the dominant fish resources gradually reached the environmental capacity.Due to the existence of competition and predation,the resources of some fish populations will decrease to varying degrees,and then maintain a balance.In general,the resources of herbivorous fish will decrease at the end of the fishing ban cycle.Culterinae fish,Aristichthys nobilis,Xenocypris fish and the wild fish resources which is dominated by T.wwinhonis will have varying degrees of increase.After a period of increase,the resources of the C.ectenes population will begin to decline,and the resources in the end of the fishing ban will be less than in the beginning.The impact of the dominant species on the food chain of fish in the reserve was assessed using gastric and intestinal contents analysis and carbon and nitrogen stable isotope techniques.The results of gastric and intestinal contents analysis showed that phytoplankton dominated in quantity and zooplankton dominated in quality in the bait composition of T.wwinhonis,C.ectenes,C.dabryi and Xenocypris microlepis.The results of stable isotope showed that phytoplankton accounted for 56.40%of the diet composition of T.wwinhonis,followed by zooplankton（40.20%）,POM and shrimp（2.90%and 0.4%）.The trophic level of T.wwinhonis in Longgan Lake is 3.11,which is lower than that of fish of the same ecological type in other lakes in the middle and lower reaches of the Yangtze River.The stable isotope technique is based on the ratio of carbon and nitrogen stable isotopes between predator and prey and the enrichment principle,which is a more reasonable and accurate method.The results indicate that the main food source of dominant fish similar to fish is phytoplankton,which plays an important role in the consumption of primary productivity.In order to evaluate the influence of fish community on the aquatic ecosystem from the perspective of energy flow,an Ecopath model of Longgan Lake Nature Reserve was constructed,which considered the relationship between zooplankton,phytoplankton,nutrient salts and fish community as well as the general characteristics of the ecosystem.The results showed that the energy conversion efficiency of the ecosystem was only4.013%,much smaller than the Lindeman efficiency;the ratio of total primary production to total respiration（TPP/TR）was 7.47,much larger than 1,and the primary productivity of phytoplankton was excessive.The amount of debris flow that was consumed by primary consumers only accounted for 9.43%of the total debris energy flow,so too much remaining debris could cause an increase in endogenous pollution load in the lake.The Connectance Index（CI）and System Omnivory Index（SOI）were too small,0.221 and0.089,respectively,indicating a simple food web.The FCI was 0.013,which was in the low recirculation standard range of 0-0.1.This indicates that the proportion of material and energy recycling is low,the flow paths are short,and the ecosystem is immature in Longgan Lake Wetland Reserve.