The ecological health of Lake Victoria (Kenya) in the face of growing cage aquaculture

The rapid growth of cage aquaculture of introduced Nile tilapia (Oreochromis niloticus) in Lake Victoria has raised urgent environmental concerns. This study aims to compare local environmental conditions, fish species abundance and diversity between paired cage and control stations at four locations over 9 months (n = 72) in Lake Victoria, Kenya. The study sites were arrayed along a limnological gradient from eutrophic waters within Winam Gulf, to more oligotrophic waters off Mfangano Island. It was hypothesized that nutrient loading, fish biomass, and species diversity would be higher at the cages than within control areas. Water quality variables were measured at 1 m depth and fish sampled using replicate sets of monofilament gillnets. A principal components analysis reduced our dataset into two significant PCs; PC1, which accounts for eutrophication and PC2, which reflects seasonal variability. Two-way analysis of variance tests on PC1 and PC2 scores found no significant differences between paired cage-control stations at any site. Significant differences between sites confirmed a strong gradient in eutrophication and variability, decreasing from the inner gulf to the open lake (PC1, F-Site = 139.426, p < .001; PC2, F-Site = 10.675, p < .001). Water transparency ranged from 0.38 (SD 0.09) to 2.65 (SD 0.4) m, increasing with distance from the inner gulf. Mean monthly fish biomass was higher near cages than controls (F-Station = 6.517, p < .05). Seventeen species across eight families were recorded. Fish species were more abundant and diverse in the inner gulf with some species significantly more abundant near cages than near controls. The findings failed to show significantly degraded water quality near cage aquaculture farms compared to control areas. This suggests that cage farms positioned in deep high-flow lake areas can act as both a refugia from fishing pressure and a submerged structure for shelter and foraging for threatened endemic fishes.