The Status of Lake Victoria Environment: Trends and Impacts to Fish Stocks

The Status of Lake Victoria Environment: Trends and Impacts to Fish Stocks

J. Gichuki (Kenya Marine and Fisheries Research Institute, Kenya), A. Getabu (Kenya Marine and Fisheries Research Institute, Kenya), C. Ezekiel (Tanzania Fisheries Research Institute, Tanzania) and O.C. Mkumbo (Lake Victoria Fisheries Organization, Uganda)
DOI: 10.4018/978-1-61520-907-1.ch020
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Abstract

This chapter discusses the environmental conditions in Lake Victoria and how they impacts on the fish stocks. Results show that in last 4 decades Secchi disc visibility decreased by about 75%. 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 as compared to historical values. Results show that the redfield ratio has decreased to 8.2:1 (N: P). Low oxygen conditions in the deep water causes rapid denitrification with subsequent loss of nitrogen. 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 changed to smaller sized species and a dominance of rotifers while Caridina nilotica has a higher abundance in inshore waters compared to offshore waters. Environmental changes have influenced changes in herbivorous and Zooplanktovore fish species resulting to increase in “Dagaa” Rastrineobola argentea, and decline of carnivore species. Changes in ecological interactions due to species introduction, predation accelerated by the environmental changes and increased fishing pressure have further complicated the ecosystem dynamics of Lake Victoria and pose serious uncertainties on the lakes future stability and sustainability of the fisheries resources. Lake Victoria’s future sustainability requires effective management of fishing effort and phosphorous loading.
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Background

Since the late 1960s disruptions of physical and biogeochemical processes have occurred in the Lake Victoria basin caused by intense human activity in the watershed caused by accelerated population growth, intense cultivation, poor animal husbandry and introduction of exotic fish species (Bugenyi and Balirwa, 1989; Hecky, 1993; Balirwa et al. 2003; Lowe-McConnel, 1997). These disruptions have lead to over fertilization (eutrophication) which affects water quality and aquatic biota including fish in Lake Victoria (Mugidde, 2001; Lungayia et al. 2000; Kolding et al. 2008). The symptoms of eutrophication as characterised by elevated nutrient concentrations; hypoxia; massive fish kills and algal blooms have become a common phenomenon in Lake Victoria in the late 1990’s (Silsbe et al. 2006). The interventions by the respective governments and some development partners have reduced the effects but still this is very far from control.

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