Spatial and Seasonal Effects of Cage Aquaculture on Water Quality in Lake Victoria, Kenya

The rapid expansion of cage aquaculture in Lake Victoria has raised concerns regarding its potential impacts on water quality and ecosystem functioning. However, information on the spatial extent and seasonal variability of these impacts remains limited. This study assessed the influence of cage aquaculture on key physicochemical water quality parameters across distance gradients and seasons in Lake Victoria, Kenya.

Water quality sampling was conducted during wet and dry seasons at 13 sites with varied distances from cage farms (0, 100, 200, 500, and 2,500 m). Physicochemical parameters were measured using a Hydrolab multiparameter probe, while chlorophyll-a and nutrient concentrations were determined spectrophotometrically. Spatial and seasonal differences were evaluated using Kruskal–Wallis tests followed by post-hoc comparisons, and Principal Component Analysis (PCA) was used to examine multivariate environmental patterns.

Results revealed clear spatial gradients associated with cage aquaculture. Chlorophyll-a concentrations were highest near cage sites and declined with increasing distance, while water transparency (Secchi depth) was lowest near cages and improved offshore. Dissolved oxygen generally increased with distance from cages. A significant negative relationship was observed between chlorophyll-a and Secchi depth (r = −0.32, p < 0.001), indicating that increased phytoplankton biomass contributed to reduced water clarity. PCA further demonstrated distinct environmental separation between near-cage and offshore sites, with chlorophyll-a, turbidity, and conductivity characterizing cage-proximal stations, whereas dissolved oxygen and water transparency were associated with offshore locations. Seasonal analyses indicated stronger associations between turbidity and chlorophyll-a during the wet season, likely reflecting enhanced runoff and nutrient transport.

These findings demonstrate that cage aquaculture produces measurable but localized effects on water quality in Lake Victoria. The study highlights chlorophyll-a, water transparency, and dissolved oxygen as sensitive indicators of aquaculture influence and underscores the need for routine environmental monitoring, carrying-capacity assessments, and sustainable cage-density regulation to support the long-term ecological sustainability of the lake.