Study On Fish Community Structures And The Biology And Biomass Estimation Of The Main Fish Species In Dianshan Lake

As the largest freshwater lake on the outskirts of Shanghai, Dianshan Lake is one of the aquatic production bases and tourist scenic area. Among Shanghai inland lakes, it occupies an important position which has the close relationship to Shanghai citizens’ life and economic construction. However, in recent years, due to various natural and man-made causes, it’s getting more and more concern about the ecological environment quality. Fishery resources have been reduced from 62 species among 42 genera in 1980 s to 22 species in 2005. Since 2004, large-scale stock enhancement has been carried out which releasing economic juveniles such as Hypophthalmichthys molitrix, Aristichthys nobilis, Cyprinus carpio and Carassius auratus annually in Dianshan Lake in order to guarantee fishermen’s income and the ecological fresh water.In order to assess the effect of stock enhancement reasonably, this research track investigate to it and carry out the fishery resources monitoring regularly and continuously. Through detailed data collection and analysis to master: 1.the fishermen demand for fishery resources, 2.the impact of fishing gear and fishery law on fishery resources and 3.biological condition of enhancement juveniles which to evaluate the condition of fishery resources scientifically. To provide decision support to the next step of stock enhancement better, we also analyzed the main fish species resources in the lake via model to provide data reference for fishery management in Dianshan Lake and provide the basis for the follow-up researchers who carry out the resource assessment.The study results showed that: 28 fish species were collected by gillnet investigation from January to October in 2011, values of IRI pointed out it mainly caught Xenocypris microlepis(5500), Acheilognathus taenianalis(3842), Coilia ectenes(1999), Carassius auratus(1355), Hypophthalmichthys molitrix(1019) Cultrichthys erythropteru(728)etc., capture weight of economic species and enhancement species accounted for the major part in the total weight of the gillnet captures. 35 fish species were collected by trawl survey, a total of 15149 tails; 106.02 kg, mainly caught Coilia ectenes(545.89), Acheilognathus taenianalis(388.86) etc., and small-sized indigenous species are the major component of the trawl capture weight. Shrimp trawl survey showed that Macrobrachium nipponense and Exopalaemon modestus are the main shrimp species in Dianshan Lake. Macrobrachium nipponense captured 10001 tails total for 16.11 kg and Exopalaemon modestus captured 7272 tails total for 8.14 kg. Through diversity analysis found out that gillnet community diversity index is relatively high in January to February compared with other months, abundance index is highest in May. ABC curve shows that the abundance curve is always above the biomass curve indicate that fish species might be overfishing in 2011. Small-sized fishes of less weight accounted for the major part in the community what are highly affected by external interference.Through the research on the growth of enhancement species as Hypophthalmichthys molitrix and Aristichthys nobilis, fitting out the length- weight relationship of Hypophthalmichthys molitrix is W= 0.0071×L3.131(n=475,R2 = 0.8988)and Aristichthys nobilis is W= 0.009×L3.094(n=130,R2 = 0.9302)indicates that both of them are uniform growth type. Von Bertalanffy growth equation of Hypophthalmichthys molitrix fitted out that it is Lt=96.08×(1-e(-0.22×(t+0.014))) and Aristichthys nobilis is Lt=108.54×(1-e(-0.16×(t+0.29))),The average weight of 3-age Hypophthalmichthys molitrix is 1089.90 g, which grows up to 750 g for reaching commercial specifications. The average weight of 3-age Aristichthys nobilis is hard to reach 1500 g, 4-age of them then reach the commercial specifications.By analyzing the biological parameters of the main dominant species of Coilia ectenes, Carassius auratus and Pelteobaggrus nitidus, we find out the exploitation rate of Carassius auratus and Pelteobaggrus nitidus are less than the loose index(0.5) what are in the development state, while Coilia ectenes are in the overexploited state. Virtual population analysis estimate the average annual quantity of resources in Dianshan Lake shows that Carassius auratus are 5561634 tails,Coilia ectenes are 8399415 tails and Pelteobaggrus nitidus are 1804906 tails. Through market survey of the shrimp in Dianshan Lake, a preliminary estimate of shrimp resource is 0.9 t in 2011 whereover might be an underestimate.By biology and resources research of shrimp and fishes in Dianshan Lake, we suggest fishery resource managers should take measures to achieve the proliferation of fishery resources such as to reduce lower-in-age and miniaturization of population structure, strengthen the management of natural capture, strictly limit the fishing time and gear sizes, strengthen the system of closed season in spring, strictly control of fishing effort and protecting the ecological environment etc.. We recommend protecting the juvenile shrimp for shrimp resource management which should strengthen moratorium of the shrimp breeding season in April and May. We proposed to increase the quantity of filter-feeding fish in enhancement activities, to increase enhancement size of larvae and juveniles, to extend the moratorium period, to control the pollution resources, to do ideological work for fishermen and transform fishery management pattern.