Exploring the structural and functional properties of the Lake Victoria food web, and the role of fisheries, using a mass balance model

Human and environmental factors have greatly challenged Lake Victoria ecosystem, especially in the last four decades. However, the lake continues to support the World's largest freshwater fishery, currently producing ca. one million tons of fish per year and directly supporting livelihoods of ca. four million people in three riparian countries. We used the Ecopath component of Ecopath with Ecosim modelling software to re-parameterise two existing mass balance models to reflect ecosystem state of Winam Gulf in 1971-1972 and 1985-1986, and construct a new model for the whole lake to reflect ecosystem state in 2014. The aim was to understand the structural and functional properties of Lake Victoria food web and the role of fisheries on the ecosystem. We found a decrease over time in productivity in relation with biomass and respiration, and food web connectivity, and an increase in biomass cycling. The total system throughput, decreased fivefold between 1971 and 1972 and 1985-1986, but was slightly higher in 2014 with a moderate shift from herbivory to detritivory. The implication of these changes on system maturity and resilience are discussed. The trophic level of catches increased between 1971 and 1972 and 1985-1986 due addition of high trophic level catches from the introduced piscivorous Nile perch (Lates niloticus) i.e. “fishing up”. However, the decline in trophic level of catches between 1985 and 1986 and 2014 seems to have been due to sequential addition of low trophic level catches, especially from the native Silver cyprinid (Rastrineobola argentea), a phenomenon termed “fishing through”, as opposed to a decline of high trophic level catches (or “fishing down”). Currently, exploitation is unbalanced and skewed to the least productive species at higher trophic level, with significantly less fishing occurring at the most productive species at lower trophic level, and the causes are discussed.