Fisheries Resources Research Institute, P.O. Box 343 Jinja, Uganda.

Response of fish populations to predation, fishing and environmental impacts, pressures and changes in food webs: Lessons from Lake Victoria (East Africa)

Over-exploitation and environmental changes exert pressure on food web structure that often lead to observed changes in the Great Lakes' fish populations. In Lake Victoria, changes are manifested in biotic communities in less than five decades. Over-exploitation reduces stocks of originally abundant and commercially important fish species. Fish (Nile perch and Nile tilapia), that were introduced into the lake to boost production, have consequently depleted stocks of the remaining native species through predation and competition. Increased nutrient loading led to changes in algal communities. The food webs changed from dominance of diatoms to blue-greens. The zooplankton community changed from the larger calanoid copepods to smaller cyclopoid copepods. Macro-invertebrates, previously dominated by chaoborids, became characterised by chironomids and the prawn, Caridina nilotica, an indication of increasing oxygen deficiency. These anoxia-tolerant species and a native pelagic fish, Rastrineobola argentea became a major food base for increased stocks of Nile perch. The diverse fish fauna was reduced to three species (the perch, Nile tilapia and R. argentea). Nile tilapia stocks increased due to the species competitive advantages over other tilapiines. Other habitat flexible fish like the R. argentea and Yssichromis spp took advantage of the increased invertebrate abundance. The insectivorous haplochromine fish populations which have shown initial recovery from predation and environmental impacts. These observations suggest that resilience to human exploitation, the capacity to adjust to a changing environment, to evade and withstand predation pressure, and adjust to variations in food supply have a major influence on fish populations in large lakes. As the lakes become eutrophic and anoxic, hypoxia tolerant fish species and those, which can shift their habitat and food, become dominant. There is need to control over-exploitation and environmental degradation if fish stocks in the great lakes are to remain stable.