Ecosystem structure and functioning of Lake Taihu (China) and the impacts of fishing

Lake Taihu is the third largest freshwater lake in China and has provided local communities with valuable fisheries for centuries. However, we have only a limited knowledge of its ecosystem. In this study, a trophic model was constructed for the Lake Taihu ecosystem. This model was used to evaluate and analyze the food web structure and other properties of this ecosystem using data covering the period from 1991 to 1995. Using the model, we evaluated the impacts on local fisheries of various management scenarios comprising two basic management regimes: (1) setting fishing mortality for the top predator (large culters, Erythroculter mongolicus and Erythroculter ilishaeformis) to 0, 0.3, 0.6, 0.9 and 1.2, and (2) adjusting overall fishing effort to 0.25, 0.5, 0.75 and 1.25 times the current level. For both scenarios, fishery profit and cost were evaluated to provide an understanding of how components of the ecosystem interact. We identified possible causes of fishery overexploitation in the lake ecosystem and described the necessity of developing ecosystem-based management. The results showed that Lake Taihu had six theoretical trophic levels (TLs), with the trophic flows primarily occurring through the first five TLs. System properties such as transfer efficiency, Finn's index, Finn's mean length, connectance index, system omnivory index, primary production/respiration ratio, and net primary production all indicated that Lake Taihu was an immature, fairly simple ecosystem in which a relatively low fraction of total primary production was utilized. At the same time, the ecosystem was also experiencing high fishing pressure. Yet despite this, the low ascendency index (25.9%) and high system overhead ratio (74.1%) indicated that the system was highly developed and relatively stable, a condition that might result from the high degree of recycling in the system. Among the harvesting strategies considered, a strategy of either decreasing the fishing mortality of the top predator (large culters) to 0.3 or, alternatively, reducing the overall effort on the system by a factor of 0.75 appeared to be most effective at increasing the efficiency of the fisheries.