Studies On Fisheries Ecology In A Large Eutrophic Shallow Lake, Lake Chaohu

Water quality, fish community structure and fisheries resources were investigated during 2002 to 2004 in the shallow eutrophic Lake Chaohu in the low reaches of the Yangtze River. It was the first study to discuss the response of zooplanktivorous fishes to eutrophication in lakes in China, which will provide basic theory for lake restoration, development between fisheries and lake environment. The main results are as follows: Since the 1980s, Lake Chaohu was a hypereutrophic status with high total nitrogen (TN) and total phosphorus (TP) concentrations. Spatially, TN and TP concentrations were higher in the western than in the middle and eastern areas. During the survey from 2002 to 2004, 54 species of fishes belonging to 16 families were collected and identified, significantly lower than historic records of fish species in this lake. Cyprinidae comprised 64.8% of the total number of fish species. Small-sized fish species dominated the fisheries resources, comprising over 80% of the total fish yield. The catch per unit effort (CPUE) decreased in recent years due to over-exploitation with the increasing fishing boats. Diminution of the catched fish was obvious both in age-specific body length and in age structure, due to various reasons, e. g. the construction of dikes in the 1960s, and disappearance of submerged macrophytes, increased eutrophication and over-fishing. The annual gross primary production (PP) ranged from 0.18 to 9.86 g O₂ / m².d, were higher in the western than in other regions. However, PP per unit Chl a was higher in the eastern than in other regions of the lake. The correlation was positive between PP and water temperature, conductivity, pH and water depth, but significantly negative between PP and the NO2--N, PO43--P and TDP. The potential fish yield was estimated to be 2.6×104 tons in total or 374.8 kg/hm2. The fisheries resources were mostly dependent on zooplankton, whereas the high phytoplankton biomass was not exploited adequately. The feeding ecology of lake anchovy, Coilia ectenes, was studied by examining stomachs of 667 individuals and it was found this fish fed mainly on zooplankton during the young of year fish (YOY) with a total body length of less than 13 cm but shifted to shrimp larval and other larval fish. The seasonal and diel variations in the diet composition were great, and the lake anchovy was feeding throughout the year with different feeding rhythms between spring and summer. The prey item of lake anchovy were mainly large cladocerans, such as Daphnia, Diaphanosoma, Leptodora kindtii and Moina, and calanoids. The dominant zooplankton in the lake water were Bosmina and Ceriodaphnia, and Bosmina also comprised the main food items of the lake anchovy in summer. For larger lake anchovy which had a total body-length of more than 13 cm, with increased total body-length, consumption on shrimp and / or fish but selection on zooplankton decreased. Production of icefish (Neosalanx taihuensis), an important commercial fish, showed a decrease trend since the 1980s probably due to eutrophication and overfishing. The diet of N. taihuensis was also composed of large cladocerans and copepods, as well as the YOY of lake anchovy. They fed on zooplankton in the whole life, which was affected by both size and sex. N. taihuensis had obvious feeding rhythm in both spring and summer and only fed in the daytime but stopped feeding in the night, acting as a visual feeder and a particulate feeder. Like N. taihuensis, Hemirhamphus intermedius lived in the upper layer of the water column and fed on zooplankton and aquatic insects. Diel and seasonal feeding studies showed that H. intermedius had a feeding peak in the daytime with a higher food intake in summer than in other seasons. Using the Eggers, Eillott-Persson and MAXIMS models to estimate the dailyration of the above three small fishes, it was found that there was no significant difference between Egger and Eillott-Persson models to estimate on daily ration of the lake anchovy, while the MAXIMS model were suitable for estimating daily rations of N. taihuensis and H. intermedius. The daily rations of YOY of the lake anchovy were estimated to be 37.88 and 137.68 g wet weight /100 g wet body weight in spring and summer, respectively, and those of N. taihuensis 3.49 and 3.25 g w w /100 g b w in spring and summer, respectively, and those of H. intermedius 3.35 g w w /100 g b w for zooplankton and 9.78 g w w /100 g b w for aquatic insects, respectively. It is suggested that predation by these fishes had profound impacts on the structure of the zooplankton communities in Lake Chaohu.