| The Yangtze Estuary ecosystem,as the largest estuarine ecosystem in China,have sufficient nutrients,and the environmental gradient changes greatly which are mixed hydrodynamic processes diluted water and the estuary adjacent waters.These characteristics produce a necessary habitat,feeding and spawning ground for different ecological fish,such as migratory and estuarine sedentary species.Especially,the unique environmental conditions provide an important migration channel and habitat for Acipenser sinensis,Neophocaena asiaeorientalis and other endangered aquatic animals.Because of human activities including water pollution,damming and overfishing,the Yangtze River biodiversity declined continually,the fish community structure in the Yangtze estuary has changed and the community stability has been strongly disturbed.To protect the aquatic biological resources,a ten-year fishing ban policy in the key waters of the Yangtze River Basin has entered from January 1,2021.Since the Yangtze estuary is an important part of these key waters,the analysis of fish community characteristics and size structure steady state in this area have great significance for understanding the changes after the fishing ban.Based on the fishery and environmental data of the Yangtze estuary obtained in 2018 and 2020-2021,changes before and after the fishing ban,including fish species composition,ecological types,dominant species,species diversity,species replacement rate and fish congregations,were studied.The fish size spectrum and stability assessment model were used to analyze the fish community characteristics.The water eutrophication index method was used to assess the ecological environment in the Yangtze estuary,and to understand effects of water environment on the stability of fish community structure.The results showed that:(1)There were 32 fish species observed in the two survey years,pertaining to 26 genera,14 families,9 orders and 1 class.Among them,12 species were observed in 2018,pertaining to 11 genera,6 families,5 orders and 1 class.30 species were observed in2020-2021,pertaining to 25 genera,14 families,9 orders and 1 class.These catches all belonged to the class of Osteichthyes,while no Chondrichthyes fish has been found.The ecological types of fish were mainly warm-temperate and warm-water fish,such as marine migratory,estuarine sedentary,anadromous and freshwater fish.In 2018,the dominant species were Cynoglossus gracilis,Tachysurus fulvidraco,Coilia nasus and Lophiogobius ocellicauda.In 2020-2021,the dominant species were Cynoglossus gracilis,Coilia nasus,Collichthys lucidus and Tachysurus dumerili.There were obvious interannual successions between species,and obvious seasonal differences in the fish composition.(2)The Shannon-Wiener Diversity index H’,Pielou evenness index J’ and Margalef richness index D had smaller interannual fluctuation.However,all indices had significant seasonal differences(P<0.05)except for Pielou evenness index J’ of 2020-2021.Results of CLUSTER clustering,n MDS ranking and ANOSIM analysis showed that,in 2018,the coefficient of community clustering Stress of autumn could not be calculated,while the Stress in other seasons were 0.01,0.04 and 0.01,respectively.There were significant differences(P<0.05)or extremely significant differences(P<0.01)among each seasonal community.In 2020-2021,the Stress were 0.08、0.12、0.02 and 0.05,respectively.There were extremely significant differences(P<0.01)among each seasonal community.SIMPER analysis indicated that there were variations and in the typical and divergent species composition in the fish communities in different seasons of two years.(3)The size classes ranged from-1 to 6,the curvature was-0.08,all the dome of all fish size spectrum curves located in the size class of 0 and 3 in 2018.Meanwhile,the size classes ranged from-2 to 12,the curvature was-0.06,all the dome of all fish size spectrum curves located in the size class of 1 and 6 in 2020-2021.The ABC curves all intersected indicated that the fish communities in the Yangtze estuary were in moderate disturbance or unstable state.There were seasonal changes in fish communities in two periods.The Sheldon size spectrum of four seasons all presented a “unimodal” pattern.Differences were found in the corresponding size interval and fish species composition between the peaks and the highest biomass in each season.In 2018,the summer had the maximum fish size spectral curvature(-0.06),the minimum in winter(-0.10),and W values was from-0.16 to 0.35.In 2020-2021,the summer had the maximum fish size spectral curvature(-0.04),the minimum in autumn(-0.09),and W values was from-0.15 to 0.08.Correlation analysis demonstrated that water temperature,salinity and dissolved oxygen were Significant correlation with the curvature of normalized size spectra(P<0.05).(4)The average stability factor in 2018 and 2020-2021 were 0.48 and 0.52,respectively,which indicated the fish community stability was different in two periods.The eutrophic coefficient indicated that the water quality of the Yangtze Estuary was severe eutrophication before the fishing ban,and there was a significant negative correlation between the eutrophication index and the stability factor.Therefore,the eutrophication may have an important effect on the deviation of the community system from the stable state.In general,it is an important work to evaluate the fishing ban effects.The fish community structure in the Yangtze Estuary have been changed after the fishing ban.The range of size interval tends to increase,and the fish community stability was low.The water state may affect the stability of the fish community system in the Yangtze Estuary.As preliminary research of the fishing ban effects,this study analyzed the changes of fish community,size structure characteristics and evaluated the stability and affecting factors of the fish community.Therefore,carrying out such research has an essential ecological significance for the conservation of fish stocks and the restoration of habitat in the Yangtze River(estuary). |
PDF Full Text Request