In situ captive growth performance and conservation potential of native tilapiines in Lake Victoria, Kenya a case study of Rasira Beach

The endangered native tilapiine cichlids Oreochromis variabilis and Oreochromis esculentus have declined dramatically in Lake Victoria due to introduced species, overfishing, and habitat degradation. This study evaluated the potential of cage aquaculture as an ex-situ conservation tool for these species by assessing growth performance, survival, natural reproduction, and water quality impacts under controlled cage conditions in Lake Victoria, Kenya. A six-month completely randomized experiment was conducted using 8 m³ cages stocked with 30 fish m⁻³ (initial weight 5 ± 0.01 g) at Rio Fish Farm, Homa Bay County. Oreochromis niloticus served as a comparative reference. Growth performance differed significantly among species (two-way ANOVA, species effect: F (2, 24) = 124.7, p < 0.001). O. niloticus attained the highest final weight (185.39 ± 0.93 g), whereas O. variabilis (47.26 ± 5.25 g) and O. esculentus (46.03 ± 1.99 g) grew more slowly (Tukey’s HSD, p < 0.001 for both comparisons). Feed conversion ratios (FCR) were significantly lower for O. niloticus (1.77) compared to O. variabilis (5.26) and O. esculentus (5.48) (Tukey’s HSD, p < 0.001). Survival rates were high across all species: O. niloticus (88.55%), O. esculentus (85.55%), and O. variabilis (75.9%), with significantly lower survival in O. variabilis compared to O. niloticus (Tukey’s HSD, p = 0.018). Critically, fry of both native species were observed throughout the trial, indicating successful natural spawning under cage conditions a key finding for conservation breeding. Water quality monitoring revealed significantly elevated total nitrogen (up to 463.5 ± 54.7 µg L⁻¹; p = 0.008) and total phosphorus (up to 41.5 ± 2.2 µg L⁻¹; p = 0.042) at cage sites compared to open water controls. We conclude that cage aquaculture offers a viable pathway for ex-situ conservation of O. variabilis and O. esculentus, particularly through captive breeding and stock enhancement. However, species-specific feed development and strict nutrient management are required to overcome current growth limitations and mitigate eutrophication risks.