Mortality Rate and Recruitment Patterns of Labeo coubie (Rüppell, 1832) and Labeo senegalensis (Valenciennes, 1842) (Cypriniformes: Cyprinidae) in the Orashi River, Engenni, Ahoada-West, Rivers State, Nigeria

Abstract

Estimates of mortality and recruitment underpin assessment of exploited fish stock sustainability, yet such data remain scarce for Niger Delta river systems. This study assessed mortality and recruitment patterns of Labeo coubie and Labeo senegalensis (Cyprinidae) in the Orashi River, Engenni, Ahoada-West Local Government Area, Rivers State, Nigeria, using length-frequency data collected monthly from four stations (July 2022 to June 2023). A total of 880 L. coubie and 491 L. senegalensis were measured and analysed in FiSAT II to estimate mortality, exploitation rate and recruitment parameters. Length-converted catch curve analysis gave total mortality (Z) of 2.62 year-1 for L. coubie and 1.40 year-1 for L. senegalensis; natural mortality (M), estimated from Pauly's empirical formula, accounted for most total mortality in both species (1.92 and 1.19 year-1), while fishing mortality (F) was low (0.70 and 0.21 year-1). Exploitation rates (E = 0.27 and 0.15) fell below the theoretical optimum of 0.50, indicating both stocks are under-exploited. Recruitment was continuous with two unequal annual pulses: a major peak in November (17.69%) and minor peak in December (15.63%) for L. coubie, and a major peak in February (25.18%) and minor peak in January (20.66%) for L. senegalensis. Length at 50% capture probability (Lc) was 16.50 cm for L. coubie and 15.32 cm for L. senegalensis. These findings indicate Labeo stocks in the lower Orashi River are within sustainable limits, but continued monitoring of mortality, recruitment and exploitation is recommended to detect early signs of overexploitation as fishing pressure increases.