Moi University, Department of Fisheries, P.O. Box 3900, Eldoret, Kenya. E-mail: jmanyala@hotmail.com

Survival of the Lake Victoria Rastrineobola argentea (Pellegrin 1904) in a rapidly changing environment: Biotic and abiotic interactions

Rastrineobola argentea (Pellegrin) has a fecundity of 1,800 - 3,500 for specimen weighing up to 2.8 g body weight. Lates niloticus (Linneaus) and Oreochromis niloticus have fecundity of 6 - 11 million and 340 - 6,000 eggs respectively. The reproductive potential per unit weight of the major species show that R. argentea can produce up to 12 million eggs per kg as compared to only 171,000 eggs per kg for L. niloticus, 3,370 eggs per kg for O. niloticus and 3,157 eggs per kg for Oreochromis esculentus. The foods of R.argentea consist mostly of zooplankton, food organisms that are superabundant in the environment and with few competitors for similar food item.

The reproductive potential and diet breadth alone does not seem to have enabled R. argentea to survive. Rather, changes in the environment have influenced some of the changes in fish biodiversity through their influence in water quality. For example, the chemical characteristics of Lake Victoria in 1950 showed that the conductivity was between 95-98 mS cm-1 while the pH ranged between 8.2 and 9.0. The conductivity in the lake has increased and varies from 132 to 140 mS cm-1 while the pH now range from 5 to 8 indicating an increased ionic concentration and acidification. Available data show that the densities of R. argentea were related to conductivity, temperature, secchi depth and dissolved oxygen according to the relationship: Density = -42.0 - 0.117*Conductivity + 2*Temperature - 7.82*Secchi depth + 2.26*Dissolved oxygen.

There have been shifts in diatom abundance, linked closely with changes in pH but the phytoplankton of Lake Victoria is presently dominated by the blue green algae, which form very widespread blooms in the lake during the rainy seasons. The implication of these changes in dominance is associated with increased nutrient levels in the lake (eutrophication), pollution from agricultural, municipal and industrial effluents.

There is evidence that perdition by the Nile perch alone is not the only cause of loss in fish diversity in Lake Victoria but wrong fishing gears and methods, and environmental degradation also contributes to the loss.