| Dianshan Lake is the largest freshwater lake in Shanghai.In recent years,with the development of fishing gears and methods,marketization of aquatic products,human and environmental activities(such as pollution,eutrophication,overfishing etc.)intensified in and around the Lake.These factors greatly contributed to the overfishing of most resources in the Lake causing a continuos decline of their stocks;thereby modifying the fish community structure of these resources.Therefore,from data surveied every month in 2016,the main objective of this study was to: 1)evaluate the distribution of fishery resources,2)provide reference values for species,and 3)determine suitable enhancement and release locations.In addition,due to long-term overfishing and environmental pollution etc.,stock biomass of some fish species of high trophic levels continuously declined,causing the abundance of some fish species of low trophic levels in the Dianshan Lake.Changes of such nature raises great ecological problems which need to be looked upon.To ensure the sustainable development of fishery resources and ecosystem balance,it is very important to study food web structures,which is of great significance to ecosystem dynamic reseraches.According to the data analyzed during our survey period(May,July,September,October and November in 2016)in the Dianshan Lake,the results showed that 39 fishes(belonging to 12 families and 30 genera)were indentified in the trawl survey,and 33 fishes(belonging to 12 families and 30 genera)were indentified in the gillnet survey.According to the Index of Relative Importance(IRI)analysis,the dominant fish species were mainly divided into: enhancement and released fish species(Hypophthalmichthys molitrix,Aristichthys nobilis etc.)and small wild fish species(Rhinogobius giurinus,Acheilognathus macropterus,Coilia nasus etc.).The fish community diversity index(such as Shannon-Wiener index(H ’),Margalef index(D),Pielou index(J’)and Simpson index(1-λ)).Shannon-Wiener index(H ’),Margalef index(D),Pielou index(J’)commonly used to reflect the stability of fish community structures reached a peak at station three,and lowest recorded value at station seven.However,the Simpson index(1-λ)was opposite,its lowest value was recorded at station three,and its peak at station seven.The results showed that the species at station three were the most abundant but not obviously dominant.Overally,species abundant and diversity of the upper half zone of the Lake was lower than the downer half.Spatially,the fish resources are lower in the coastal zones than in the opened waters in Lake Dianshan.Especially,the resources of station twelve located at the water inlet point of the lake and station nine found nearby the water sports center were extremely low.Temporally,the fishery resources reached its peak(1.872 kg·boat-1·h-1)in May,then dropped to(0.826 kg·boat-1·h-1)in July,and later rose to(1.427kg·boat-1·h-1)in September,and again dropped to its lowest(0.536kg·boat-1·h-1)in October.Temporal and spatial distrubition of catch per unit effort of the main dominant economic fish species(Carassius auratus,Pelteobagrus nitidus and Coilia nasus)were recorded as follows: For spatial distribution,Carassius auratus and Pelteobagrus nitidus had similar trends;their trend was lower in the upper zone than in the downer zone,however,Coilia nasus hardly had differences in both zones(p>0.05).The CPUE of Carassius auratus were slightly higher in the coastal zones than in the opened waters,however,Pelteobagrus nitidus and Coilia nasus do not have obvious differences in Dianshan Lake(p>0.05).Temporally,Carassius auratus,Pelteobagrus nitidus and Coilia nasus have strength swimming ability,Carassius auratus,Pelteobagrus nitidus and Coilia nasus began to move from the upper zone to the downer zone,the first two started to move into the upper zone untill October,however,the last one started to move into the upper zone untill September.According to studies on temporal and spatial variations of the freshwater shrimp resources from the survey data collected during the period August 2015-August 2016 in the Dianshan Lake.The results showed that the range of carapace length of M.nipponense was 7.1~47.1 mm,average length(22.19±1.65)mm,advantaged carapace length was 15.5 ~30.5mm,accounting for 82.98 % of the whole carapace length;the range of carapace lengths of E.modestus was 9.1~38.7mm,average length(21.22±1.60)mm,advantaged carapace length was 15.5 ~25.5 mm,accounting for 80.49 %of the whole carapace length;Resource density for M.nipponense reached a peak in September and May,while the lowest resource density was in March.Resource density for E.modestus reached a peak in October and April,and the lowest resource density was in June.Spatially,the biomass of M.nipponense and E.modestus witnessed large fluctuations in different areas.The biomass of freshwater shrimp was higher in the opened waters than in the coastal zones during winter and spring.However,the biomass of these freshwater shrimps was lower in the opened waters than in the coastal zones during summer and autumn.Besides,biomass of the upper half of the Dianshan lake was overall higher than that of the downer half of the lake.Stable isotopes analysis,especially carbon and nitrogen,are now commonly used to study material circulation and energy flow within ecosystems.In this study,33 fish species from Dianshan Lake were used as samples and their stable carbon and nitrogen isotope ratios were analyzed in the laboratory.The results showed that,there was significant variation in carbon isotopes which ranged from-30.71‰ ~-21.34‰,and the phase contrast was 9.37‰.There was hardly significant variation in nitrogen isotopes;it was in the range 6.70‰ ~ 21.23‰,and the phase contrast was 14.43 ‰.The trophic level of the 33 fish species were estimated based on the δ~15N value among which carnivores Chanodichthys erythropterus seized the superme trophic level;most of them were at the third trophic level. |
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