Physicochemical Characteristics, Fish Species Composition and Diversity of Orakpa River, Kogi State, Nigeria

Abstract This study assessed the physicochemical characteristics, fish species composition, and diversity of Orakpa River, a tributary of the Lower River Niger in Kogi State, Nigeria. Water and fish samples were collected from three sampling stations (upstream, midstream, and downstream) over a three-month period (July-September 2022). Physicochemical parameters were analyzed using standard methods, while fish species were identified using taxonomic keys. Fish diversity was evaluated using Simpson’s Index. Results showed that most physicochemical parameters, including pH (7.60-7.67), temperature (26.67-27.00°C), dissolved oxygen (5.03-6.30mg/L) and conductivity (95.25-140.72µS/cm), were within acceptable limits for tropical freshwater fish species. However, dissolved oxygen decreased downstream (5.03 mg/L), indicating localized environmental stress. A total of 646 fish individuals comprising 20 species from 10 families were recorded. The family Bagridae, particularly Bagrus bayad (23.2%), was the most dominant, while Schilbe mystus (0.7%) was the least abundant species. Fish abundance was highest at the midstream station and lowest downstream, reflecting variations in environmental conditions. The Simpson’s Index of Diversity (1 - D = 0.90) indicated high species diversity, suggesting a relatively stable ecosystem despite moderate dominance by a few species. The study concludes that Orakpa River maintains good ecological status but shows signs of downstream environmental stress. These findings provide essential baseline data for sustainable management, conservation planning, and future monitoring of the river system.