Recurrent fish kills in Kisumu Bay, Lake Victoria, Kenya, threaten both ecological stability and local livelihoods. These events have been linked to eutrophication, harmful algal blooms, and declining water quality driven by deforestation, urbanization, and industrialization. Historical and recent incidents including major fish kills in 1984, 2022, and 2024, highlight the role of low dissolved oxygen, toxic runoff, and climate-related impacts, resulting in significant economic losses. In response to a recent spike in reported cases, this study seeks to identify the underlying causes of fish mortality in Kisumu Bay and propose suitable mitigation measures. Field surveys and water quality analyses were conducted across key sites, including the Kisat River mouth, Coca-Cola discharge zones, and a designated control station. This study investigates the drivers of recent fish mortality events through field surveys and water quality analysis across key stations, including Kisat River mouth, Coca-Cola discharge zones, and the 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 hy-per-eutrophic conditions marked by elevated ammonium (NH4+ > 1.5 mg L-1), total nitrogen (TN > 40–50 μg L-1), and 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, which could have impeded gas exchange and coated fish gills, intensifying respiratory stress. Nitrite peaks (21.12 mg L-1) and ammonium spikes signalled toxic pollution from wastewater, while Secchi depth measurements inversely correlated with algal biomass, confirming turbidity-driven hypoxia.
The study attributes fish kills to possible synergistic effects of nutrient pollution, climate-driven temperature rises (27.26°C on average), 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, advocating for integrated watershed management to sustain Lake Victoria’s fisheries and socio-economic resilience.
Keywords: fish kills, hypoxia, eutrophication, nutrient pollution, Lake Victoria, algal bloom