Fish diversity and community structure of Ona lake, Asaba, Nigeria

Abstract

Freshwater ecosystems in West Africa are threatened by anthropogenic disturbances, necessitating baseline biodiversity assessments for conservation planning. This study evaluated fish diversity and species composition in Ona Lake, Asaba, Delta State, Nigeria. Five stations representing habitat gradients were sampled: S1 Inlet, S2 Vegetated, S3 Open Water, S4 Shallow, and S5 Outlet. Monofilament gill nets (30-70 mm), cast nets (6 mm mesh), and baited traps were deployed for 12 hours overnight during sampling. A total of 2,811 individuals representing 18 species across 14 families were recorded. Cichlidae (815) and Clariidae (507) were dominant, with Oreochromis niloticus and Clarias gariepinus the most abundant species. Shannon–Wiener diversity (H′) ranged from 1.34 to 2.04, indicating moderate diversity, with the highest value at S2. Simpson’s index ranged from 0.73 to 0.87, while evenness (0.96-0.98) indicated balanced species distribution. Dissolved oxygen was positively associated with species richness (r = 0.89, p < 0.0001), whereas nitrate was negatively associated with evenness (r = -0.93, p < 0.0001) and diversity (r = -0.91, p < 0.0001). Reduced oxygen and elevated nutrients were associated with dominance by tolerant taxa, particularly Oreochromis niloticus and Clarias gariepinus. Analysis of variance showed significant station differences in mean length (F = 13.505, p < 0.0001), mean weight (F = 3.035, p = 0.0188), and abundance (F = 5.513, p = 0.0003), while month was not significant during February–April 2025. Despite healthy biodiversity indices, spatial variation suggests emerging anthropogenic pressures. These findings provide baseline data for conservation planning and sustainable fisheries management in Ona Lake.