Effects of Cage Aquaculture on Biodiversity of Lake Victoria, Kenya

Abstract

Cage aquaculture is rapidly expanding in Lake Victoria, Kenya, yet its effects on native biodiversity remain poorly understood. This study assessed the impacts of cage aquaculture on wild fish and zooplankton communities by comparing biodiversity indices between experimental cage and control sites between March and August 2024. This study was conducted at the Rasira cages site in Lake Victoria, Kenya. Wild broodstock of Oreochromis esculentus were collected from Gesebei, Nyamira County whereas Oreochromis variabilis were collected from Oki and Kanyaboli dams in Homa Bay and Siaya County. They were transported to the Kenya Marine and Fisheries Research Institute where they were quarantined for 30 days followed by stocking and pairing in a ratio of 3:1 in 200 m 2 multiplication pond. After three weeks, fingerlings were collected, stocked in nursery ponds, and fed 45% CP starter feeds until they attained 5 ± 0.01 g where they were conditioned, transported and stocked in the cages at a density of 83 fish/m 3 . Wild fish were sampled using experimental gill nets, while zooplankton were collected using standard plankton nets to collect data on species biodiversity. Shannon-Wiener diversity indices were calculated for both communities. Results showed that Two-Way ANOVA presented a significant difference between the sampling periods for the two communities ( p < 0.05). Fish diversity was higher at cage sites (H’ = 1.33 ± 0.58) compared to control sites (H’ = 0.99 ± 0.43) during March, but this pattern shifted in August with cage sites showing lower diversity (H’ = 0.65 ± 0.27) dominated by haplochromine cichlids (81%). Zooplankton communities were dominated by copepods at both sites, with Cyclopoida comprising 51% - 78% of total density. Species composition shifted between sampling periods, with Thermocyclops showing increased dominance near cages. These findings suggest that cage aquaculture alters local biodiversity patterns, with seasonal variations in fish community structure and potential nutrient enrichment effects on zooplankton. Management strategies should incorporate biodiversity monitoring to ensure sustainable cage aquaculture development in Lake Victoria.