Aspects of the Growth, Mortality, Population Dynamics, Stock and Exploitation Status of Hepsetus Odoe (Bloch, 1794) of Oguta Lake, Imo State Nigeria

Abstract

The study investigated aspects of the population dynamics of Hepsetus odoe of Oguta Lake with a view to ascertaining the status of the stock in the lake and providing management strategies for sustainability. A six-month sampling survey was conducted between March and August 2025, during which 5,562 specimens were examined. Sampling was carried out bi-weekly from four designated sampling stations on the lake using a combination of gillnets, seine nets, and cast nets. Data were analyzed using the FiSAT II computer software.  Von Bertalanffy growth parameters from length-frequency analysis of the catch data gave the asymptotic length, L∞ as 57.75 cm, growth coefficient, K as 0.15 year-1, the theoretical age, t0 as -0.883 year, and the growth equation as: Lt = 57.75 (1 - e-0.15(t - 0.883)). The result depicts a slow growth rate over a maximum attainable life span of 20 years. Fishing mortality F was 2.25 year-1, natural mortality M was 0.65 year-1, while total mortality estimate Z was 2.90 year-1. The estimated exploitation ratio E of 0.78 was above the recommended sustainable threshold, suggesting overexploitation of the stock and rapidly declining productivity. The selection curve revealed a preponderance of immature size classes with a reduced reproductive capacity in the lake incapable of replenishing the depleted stock. The yield-per-recruit model recommends an optimum exploitation ratio (E) of between 0.40 and 0.50 as ideal in a properly regulated fishery. The present yield of the fishery is estimated at 0.07 - 0.08 t/km2, which shows that the Hepsetus stock of Oguta Lake is depleted. ELEFAN II length frequency graphics identified six age groups (0+ to 5+ years) while the modal length-at-age of the oldest fish (5+ year individuals) was 48.0cm in total length. Regulatory measures such as prescription of mesh size limits, closed fishing seasons, protection of spawning habitats, reduction in fishing effort, and the provision of alternative sources of income for fishermen outside the fishery are advocated for the recovery of the fishery.