The acute toxic effect of industrial effluents from Agbara Industrial environs of Ologe lagoon was investigated in a static renewable lethal bioassay using fingerlings and Juveniles of African catfish (Clarias gariepinus). Twenty (20) pieces of the fingerlings were stocked per tank and each treatment was in triplicate. Physico-chemical parameters: temperature, pH, Dissolved Oxygen, DO and conductivity in treatment tanks were monitored for 96 hours. Behavioural responses were studied; mortality data recorded, and histopathological analysis was also done. Except for dissolved oxygen, other physico-chemical parameters monitored did not show significant differences (p>0.05). The lethal bioassay showed that as the concentration of effluent increased, more mortality was recorded; but as the time of exposure increased, mortality reduced. The LC50 obtained at all the different time intervals for C. gariepinus fingerlings in 24,48, 72 and 96 hrs were: 69.45,46.39, 40.81 and 34.03(%). While in juveniles, the values were: 64.52,49.21, 32.50, and 19.63(%) respectively. Abnormal behaviour was observed; they showed repeated darting movement within an hour of introduction, darkening in the eye and on the skin, haemorrhage in the gills, spiral swimming and death. Histopathological examination of the gills and liver of the fishes showed lesions which increased progressively with increasing level of toxicants. Observed changes in the gills were mainly: epithelial lifting, swollen lamellae, necrosis and mass degeneration, fatty and vacuolar degeneration, loss of lamellae and marked disorganization in gill structure and arrangement. In the liver, there were vacoulation, portal congestions, pancreatic necrosis, fatty degeneration and severe disruption of the hepatic cord. Fishes in the control treatments showed no visible lesions throughout the experiments. This information confirms that histopathological alterations are good biomarkers for toxic impact assessment of industrial effluent on fish. Therefore, an indiscriminate discharge of this effluent to surrounding water should be discouraged
