Invasion–food security trade–offs in African inland fisheries: multi–year evidence from native and non–native fish assemblages in a subtropical reservoir

Résumé

Small–scale inland fisheries are critical for food and livelihood security across Africa, yet many freshwater systems now contain native and non–native fish species whose relative contributions to fisheries and the trade-offs for biodiversity and household economies remain poorly quantified. This study assessed the contribution of native and non–native fishes in a subtropical reservoir using a 6–year dataset (2020–2025), complemented by a market and preference survey of 52 fishers and traders. Fish species were classified as native or non–native, and the data were further analysed using multivariate methods. The index of relative importance (IRI) integrated abundance, weight and frequency of occurrence to determine species dominance. Temporal coupling between native and non-native CPUE was tested using Spearman rank correlation to evaluate whether non-native catches compensate for declines in native catches. Twelve species were recorded: 8 native and 4 non–natives ( Oreochromis niloticus , Micropterus nigricans, Coptodon rendalli and a crustacean Cherax quadricarinatus ). Native species contributed a greater number of individuals (45–207 vs 33 654), but non-native species contributed disproportionately to more harvested biomass and market value. Native species exhibited significantly higher overall CPUE than non–native species, with pronounced differences in the hot–wet and cool–dry seasons. However, the IRI ranked M. nigricans (20.1%) and Coptodon rendalli (20.0%) as the most important species. Non–native species dominated 4–5–inch mesh catches while native species peaked in 3–inch mesh. Market survey results showed that non-native species commanded approximately twice the aggregate market value of native species (USD 45–552 vs USD 22–537 over the study period), with C. rendalli , M. nigricans and O. niloticus preferred by fishers and traders for their larger body size, higher unit price and stronger consumer demand. However, the higher numerical catch of native fish indicates that biodiversity costs of non-native dominance are real and that food-security benefits are mediated primarily through biomass and price, not abundance. Despite comparable annual yields, non–natives functioned as seasonal food–security buffers. The establishment of O. niloticus (16.0% of biomass) raises conservation concerns regarding native cichlid protection and conservation. These findings demonstrate that non–native species generate measurable but uneven food-security benefits while imposing biodiversity costs, necessitating adaptive management strategies that explicitly balance both dimensions.

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