Analysis Of Fish Community Structure And Resource Utilization Of Dominant Species In Dianshan Lake

The largest freshwater lake,Dianshan Lake is the highest inland fishery production area in Shanghai.It also serves as an important source area for an aquatic life protection area,and a water source protection area,which have significant ecological and economic value.There are up to 75 different species of fish which could be identified in Dianshan Lake in the 1960s,when it had abundant bio-diversities.However,due to ongoing development and upstream environmental pollution,Dianshan Lake’s ecological environment has suffered in various degrees over the past 30 years,and fish resources had been declining.Given these circumstances and background,the "Shanghai Dianshan Lake Fishery Resources Survey Project" was initiated to design sampling stations throughout the lake and to routinely monitor fishery resources,collecting fish samples by traditional fishing gear.Biological experiment was made in the laboratory.Through the integration and processing of experimental data,the fish community characteristics of Dianshan Lake were analyzed,and the gillnet selectivity,minimum mesh size,growth parameters,fishing mortality,optimal capture length,and stock biomass were evaluated,taking the two main dominant species of Coilia nasus and Carassius auratus as examples respectively in order to provide scientific information for the management of the lake’s fisheries.During 2020 and 2021,the fish in the Dianshan Lake were sampled and collected using fixed triple gill nets,and the composition,structure,and diversity of fishes were determined by various mesh sizes.Results showed that 38 species of fish were identified belonging to 30 genera,6 orders,and 8 families,It is noted that 73.7%of fish species belonged to the Cyprinidae family.According to individual size,52.5%of fish species were small-size,mainly consisting of Coilia nasus,Acheilognathus macropterus,and Carassius auratus.Similarity of fish catch composition occurred by using different mesh-size gill net.The dominant species captured by using the largemesh were Hypophthalmichthys molitrix,Cyprinus carpio.The values of ShannonWiener diversity index(H’),Pielou evenness index(J’),Margalef species richness index(D)and Simpson dominance index(C)showed a general decreasing trend with gillnetmesh-size increase.Using methods of Cluster analysis and Non-metric multidimensional scaling,the fish community can be clearly divided into two groups based on the size of the gillnet mesh:small-mesh-size(20,40 mm)group I and largemesh-size(60,80,100 mm)group Ⅱ.Coilia nasus,Acheilognathus macropterus,Carassius auratus were the main characteristic species which resulted in variability between the groups.Compared with the years at the beginning of fish stock enhancement activities,the number of species in this survey increased,but the composition of the fish still dominated in small-size fish.During the 12 sample surveys period in 2020-2021,7031 fish samples were examined using different gillnet mesh sizes(20,40,60,80 and 100 mm),of which 1885 individuals of Coilia nasus and 454 individuals of Carassius auratus attributed to the main dominant species.The selectivity parameters for gillnets for each mesh size were calculated using the SELECT model.The results indicated that the Normal Sacle model could more accurately capture the selectivity of gillnets for both Coilia nasus and Carassius auratus,and the expressions were presented as:Slm=exp(-(l-63.82m2)/152.12m2 and Slm=exp(-(l-56.30m2)/168.48m2).Based on the model curve and body length,the optimal mesh size of Coilia nasus and Carassius nasus were determined to be 72.64mm and 45.96mm respectively.In determining the optimum capture body length of Coilia nasus and Carassius nasus,the length of the fish body should be taken into account to spawn at least once,and the principle of protecting immature fish for recruitment groups must be considered.Therefore,setting the minimum mesh size of the triple gillnet for Coilia nasus as 80mm and Carassius auratus as 60mm would be conducive to protecting the current situation of fishery resources.The growth and mortality parameters were calculated using the body length frequency distribution based on the biological data of the body length and body weight of Coilia nasus and Carassius auratus,and the status of the utilization of fisheries resources was examined.The findings revealed that the Coilia nasus’s total length-body weight showed power function relationship parameters a=0.000005 and b=2.876 that showed negative anisotropic growth,while the Carassius auratus’s total length-body weight showed power function relationship parameters a=0.00002 and b=3.079 that demonstrated uniform growth.For Coilia nasus,Linf is 444.3mm,and growth coefficient K is 0.15,and natural mortality coefficient M is 0.183,and total mortality coefficient Z is 0.968,and fishing mortality coefficient F is 0.785,and exploitation rate E=0.811.For Carassius auratus Linf is 304.73mm,and growth coefficient K is 0.33,and natural mortality coefficient M is 0.275,and total mortality coefficient Z is 1.49,and fishing mortality coefficient F is 1.215 and exploitation rate E=0.815.Biomass for Coilia nasus in Dianshan Lake is presently estimated to be in number 3,317,635,000 and weight of 363,724 tons.Biomass for Carassius auratus is assessed to be in number 617,973,000 and in weight 52,956 tons,.Based on the analysis of the Yield Per unit Recruitment model,the capture age of the Coilia nasus was 2.864,and the capture body length was 191.8mm,Fcur=0.785,and YPRcur was 151.563.The capture age of the Carassius auratus was 2.149,and the capture body length corresponding to that was 175mm,Fcur=1.215,and YPRcur was 1.215.To maximize profits and conserve the above-mentioned two fish stocks,it is recommended fishing mortality should be reduced to to 0.281 for Coilia nasus and 0.481 for Carassius auratus respectively.In order to enhance biodiversity of the lake,suggestions was made as following(1)Fish stock enhancement for targeted species should be carried out,such as enhancement of species with low stock biomass.(2)Periodical lectures on environmental protection and the detriment of alien species should be held to promote conservation consciousness and(3)Environmental protection in Dianshan Lake should be strengthened.Strictly control—sewage discharge and wastewater from the neighborhood.To preserve the habitat of benthic organisms,bottom trawling and river clam harvesting gear should be banned.