Physicochemical Properties and Fish Species Diversity of Owena Reservoir, Ondo State

Freshwater reservoirs in Southwestern Nigeria are vital aquatic resources supporting fisheries, domestic water supply, and biodiversity. This study investigated the physicochemical properties and fish species diversity of Owena Reservoir, Ondo State, South West Nigeria. Sampling was conducted at four stations bimonthly over six months. Water quality parameters including temperature, pH, dissolved oxygen, biochemical oxygen demand, conductivity, turbidity, total hardness, nitrate, and phosphate were measured following APHA protocols. Fish were sampled with gill nets, cast nets, and hook-and-line methods, and specimens were identified using standard ichthyological references. Diversity metrics including Shannon index, Simpson index, and species richness were computed. A total of 44 fish species belonging to 19 families were documented. Cichlidae and Mormyridae were the most diverse families. Oreochromis niloticus and Synodontis schall were the most abundant species. Water quality parameters were generally within acceptable ranges except for elevated phosphate concentrations at the station nearest the reservoir inlet, suggesting agricultural nutrient loading. Species diversity was highest at mid-reservoir stations and declined near the dam wall, associated with reduced structural complexity. Dissolved oxygen showed positive correlation with fish species richness. Seasonal effects were significant for turbidity and fish abundance. The study provides an updated ecological profile of Owena Reservoir and recommends a monitoring programme to detect early signs of eutrophication linked to upstream agricultural expansion. Keywords: Owena Reservoir, physicochemical properties, fish diversity, eutrophication, Ondo State.