Unravelling the Causes of the Current Wild Fish Kills in Kisumu Bay Fishery of Lake Victoria, Kenya

Recurrent fish kills in Kisumu Bay, Lake Victoria, threaten both ecological stability and local livelihoods. This study investigates the drivers of recent fish mortality events through field surveys and water quality analysis across key stations, including the Kisat River mouth, Coca-Cola discharge zones, and a control station. Results revealed severe hypoxia (dissolved oxygen [DO] < 2.5 mg L⁻¹), far below the 6–8 mg L⁻¹ threshold required for fish survival, alongside hyper-eutrophic conditions marked by elevated ammonium (NH₄⁺ > 1.5 mg L⁻¹), total nitrogen (TN > 40–50 µg L⁻¹), and total phosphorus (TP > 700 µg L⁻¹). Chlorophyll-a concentrations exceeded 100 µg L⁻¹, indicating prolific algal blooms linked to nutrient overloading, possibly from industrial effluents, agricultural runoff, and untreated sewage. Spatial-temporal analysis identified the lowest DO levels during algal decomposition phases, exacerbated by an oil layer near docking sites that may have impeded gas exchange and coated fish gills, thereby intensifying respiratory stress. Nitrite peaks (21.12 mg L⁻¹) and ammonium spikes signaled toxic pollution from wastewater, while Secchi depth measurements inversely correlated with algal biomass, confirming turbidity-driven hypoxia. The study attributes fish kills to the possible synergistic effects of nutrient pollution, climate-driven temperature increases (average 27.26°C), and inefficient waste management. The study recommends enforcing industrial effluent regulations, expanding wastewater treatment infrastructure, and reclaiming wetlands to mitigate runoff. This work provides a framework for addressing eutrophication-driven fish mortality in tropical freshwater ecosystems and advocates for integrated watershed management to sustain Lake Victoria’s fisheries and socio-economic resilience.