A decadal assessment of the distribution and abundance of invasive Nile tilapia (Oreochromis niloticus) in Lake Sebakwe, Midlands Province of Zimbabwe
Résumé
Nile tilapia ( Oreochromis niloticus ) is an invasive freshwater fish species that has established self-sustaining populations in Zimbabwean water bodies since its introduction in the 1970s and 1980s. Lake Sebakwe, located in the Midlands Province of Zimbabwe, first recorded Nile tilapia in 2015 but the spread and population dynamics of the species are currently unknown. This study aimed to (i) examine historical recorded catch trends of Nile tilapia ( Oreochromis niloticus ) on gillnets in Lake Sebakwe from 2015 to 2025, (ii) evaluate seasonal variations in its catch trends, and (iii) determine its spatial distribution patterns across the lake. The study employed a mixed-methods approach incorporating secondary fisheries-dependent data from Sebakwe Aquatic Research Station (2015–2025) and primary data collected through gill net surveys and GPS mapping from January to December 2025. Non-parametric statistical methods were applied including the Mann–Kendall trend test to assess temporal trends and the Kruskal–Wallis test to evaluate seasonal variations. Spatial distribution patterns were analyzed using QGIS. As the historical dataset used here is fisheries-dependent, catch trends are interpreted as fisheries-dependent catch indices that reflect a combination of population availability, fishing effort and reporting patterns, rather than as direct estimates of population abundance. Results revealed a significant decline in annual catches (Mann–Kendall, p < 0.012), with total weight also showing a significant negative trend ( p < 0.019). The maximum fish catch recorded was in 2017 (n = 3,880), falling to a low of (n=246) fish in 2019, recovering temporarily, and declining again to n=352) fish by 2025. However, seasonal analysis indicated no statistically significant differences in Catch per Unit Effort (Kruskal–Wallis Z-value = 0.5632, p < 0.4275) or catch weight (p < 0.1287) across seasons. The distribution was spatially uneven, with fished areas (i.e., Kudu area, n = 135) more numerous than non-fished areas (e.g., Dam wall, n = 15). The results revealed significant reductions in Nile tilapia recorded catches at Lake Sebakwe over the past decade though aggregation in fished areas is present, but catch rates are not seasonally significant. The decline brings hope that native fish diversity will rebound but may also hinder small scale fisheries and livelihoods dependent on Nile tilapia as a primary catch. These findings represent vital baseline data for fisheries management in Lake Sebakwe and are essential to guide the establishment of fishing quotas, spatial management approaches, and conservation interventions.
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