An assessement of the ecosystem health of Lake Victoria (East Africa): Current status, trends and impacts to fishery

Abstract

The paper presents results on the current environmental conditions in Lake Victoria in order to improve understanding on the limnological changes that have taken place over the last four decades and their impacts on fish stocks. Results show that in last four decades, Secchi disc visibility decreased by about 75% (range: 0.5m in the shallow bays around Nyanza Gulf to 5.7m in deep waters). Oxycline depth decreased by 50% indicating that a large body of the lake water in the deeper waters cannot support life. Chlorophyll a has increased three times compared to historical values. Results also show that the redfield ratio has decreased to 8.2:1 (N: P)  indicating more P and less N in the lake water. Low oxygen conditions in the deep water causes rapid denitrification with subsequent loss of nitrogen. In addition, primary productivity has doubled over the period and algal biomass increased by 8-10 folds. The algal biomass is currently dominated by Cyanophyta. Zooplankton communities have also changed to smaller sized species and a dominance of rotifers while the micro-invertebrate Caridina nilotica has a higher abundance in inshore waters
compared to offshore waters. Changes in phytoplankton and zooplankton biomass and species composition following eutrophication has influenced changes in herbivorous fish species as well as zooplanktivorous resulting in increase of zooplankton fish species such as “Dagaa” Rastrineobola argentea, and decline of other species. Changes in other ecological interactions due to species introduction and predation accelerated by the environmental changes and increased fishing pressure have further
complicated the ecosystem dynamics of Lake Victoria and pose serious uncertainties about its future stability and sustainability of the fisheries
resources. Lake Victoria’s future sustainability requires effective management of fishing effort and phosphorous loading. Future research should endeavour to determine an acceptable nutrient loading rate that will sustain/ improve the fishery.

Key words: Deoxygenation, Phosphorous, NP ratios, Eutrophication, Fishery