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¹ University of Ilorin, Department of Zoology, PMB 1515 Ilorin, Nigeria Viewed : 3910 - Downloaded : 2570 Agricultural and fisheries activities around the watershed of an African tropical reservoir (Oyun Reservoir, Offa, Nigeria) were found to have contributed significantly to the eutrophication of the reservoir. This is evident from the high amount of nitrate (6.4 mg/L), phosphate (2.2 mg/L) and sulphate (16.9 mg/L) in the water body. These nutrients came from fertilizer run-offs from nearby farmlands and were found in higher concentrations in the rainy season, which was usually the peak of agricultural activities in the locality. The eutrophication altered the food web cycle with the bloom of filamentous algae and the decline in zooplankton population. The effect of this scenario affected the fish species composition and abundance with the dominance of cichlids (planktivorous group) in the fish population. Best management practices (BMP) to control and reduce the eutrophication and improve the fish assemblages should be adopted and adapted to suit the situation in the reservoir. Keywords : eutrophication, nitrate, phosphate, cichlids, management