Abstrat One of the major challenge of aquatic environment particularly in agricultural areas is the discharge of toxic effluent. Most cassava varieties contain a substance called cyanide that can make the crop toxic to the aquatic ecosystem if they are inadequately processed. Untreated effluents from cassava factories and local processing sheds have effects on fish population. The study aims at addressing the problem of cassava effluent on aquatic organisms especially fish. Bio-assay test was

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... rried out in the laboratory in order to assess the toxicity of cassava effluent on aquatic organisms such as fish. Different concentrations of cassava effluent (100, 120, 130, and 140 ml/l) were administered to the various treatments for 96 hours. The LC50 of Cassava effluent to Clarias gariepinus was determined using probit analysis. Data collected were analyzed using two way analysis of variance (ANOVA) at 0.05 level of probability using SPSS (version 14.0). After 96 hours, no mortality was observed in the control (0.0 ml) and 20-100% mortality was recorded during the 96-hour experiment. Behavioural responses were dose dependent, it increased with increase concentrations. The 96-hour Lethal Concentration (LC50) value was 116.62 mls. Histological changes in the gill, liver, and kidney of fish treated with different concentrations of cassava mill effluent for 96h showed gill necrosis, infiltration and degeneration of cells, gill lamellae. The liver showed fibrosis, pyknosis, and vacuolation, while the kidney showed destruction of tubule, fusion of tubules, pyknosis and condensation of glomeruli content. In conclusion, the alteration observed in the gill, liver and kidney structures of fish showed that cassava mill effluent is toxic to the fish and had profound impact on behavior and respiration of C. gariepinus in both lethal and sub-lethal concentrations; therefore their use near aquatic environment should be discouraged. Résumé L'un des princi [...]