The need for improved fishery management in a shallow macrophytic lake in the Yangtze River basin: Evidence from the food web structure and ecosystem analysis

There are numerous shallow macrophytic lakes distributed in the middle reaches of the Yangtze River basin, which are an important fishery resource for this part of China. However, there is limited knowledge about the food web structure and energy flows of these highly disturbed ecosystems, mainly due to lack of suitable ecosystem approaches applied to the abundant but isolated ecological data from these lakes. To better manage the important ecosystems, Ecopath with Ecosim was applied to establish a mass-balance model for a typical shallow macrophytic lake (Bao’an Lake) as a case study, with the aim of describing the food web structure and the properties of the ecosystem to evaluate the ecological implications for fishery resource management and the protection of the aquatic ecosystem of these lakes. Given that there were extensive first-hand data available for the target lake, a credible trophic model including 23 functional groups was constructed. The results showed that all the commercial fish groups suffered from high fishing pressure for their higher ecotrophic efficiency (EE) values, normally more than 0.5. On the contrary, forage resources such as attached algae, submerged plants and molluscs were not fully utilized by the lake fishery, with EE values even as low as 0.089, 0.120 and 0.126 respectively. The discrete trophic level of large culters was highest (3.143) in the lake ecosystem, followed by mandarin fish (3.138) and snakehead fish (3.131). For the transfer efficiencies in the food web structure, a mean value of 8.68% was calculated for the lake ecosystem. Ecosystem maturity indices such as TPP/TR (1.640), TPP/TB (6.993), as well as ascendency (0.387) which were derived from the network analysis together with the revealed detritus-based trophic flow, illustrated that the Bao’an Lake ecosystem was a mature system according to Odum's theory. However when compared with some other lake ecosystems, the Bao’an Lake ecosystem, as well as some China lake ecosystems, showed extremely low values of CI (Connectance index), FCI (Finn's cycling index) and SOI (system omnivory index), indicating that the food web structures of these Chinese lake ecosystems tended to be simpler and more linear than lake ecosystems in other countries. Consequently, this study established the first food web model for a shallow macrophytic lake and provided overall insights and ecosystem knowledge for this kind of shallow macrophytic lake, and indicated an urgent need for fishery resources management to shift from traditional population-based to ecosystem-based models.