Assessment of the Effects of Agricultural Runoff on Fish Communities in Anambra River

This study assessed the effects of agricultural runoff on fish community structure and water quality in the Anambra River, Anambra State, South East Nigeria. Intensification of crop agriculture in the Anambra River basin has increased the loading of fertilisers, pesticides, and sediment into the river system, with potential consequences for aquatic biodiversity. Fish samples were collected using drag nets and gill nets at four stations, two of which were located near actively farmed riparian zones and two in relatively undisturbed upstream sections. Water quality parameters, including pH, dissolved oxygen, nitrate, phosphate, and pesticide residue levels, were simultaneously measured. A total of 19 fish species from 11 families were recorded. Species richness and diversity were significantly lower at agriculture-impacted stations compared to reference stations. Cyprinidae and Mochokidae, which are pollution-sensitive, were absent from downstream agricultural stations. High phosphate concentrations correlated with algal blooms at two stations, contributing to oxygen depletion during nighttime hours. Organophosphate pesticide residues were detected in water samples at levels above safety thresholds set by the United Nations Environment Programme. Fish sampled near agricultural stations exhibited visible signs of stress, including erosive skin lesions and abnormal swimming behaviour. The study underscores the need for buffer zone regulations, integrated pest management, and community-level conservation education to protect the fisheries resources of the Anambra River from agricultural pollution. Keywords: agricultural runoff, fish community, water pollution, biodiversity, Anambra River