Evaluation of Acute Toxicity of Cassava Effluent on the African Catfish [Clarias gariepinus (Burchell 1822)] and Freshwater Clam [Egera radiata Lam (Bivalvia, Donacidae)]

The indiscriminate discharge of the wastewater generated during the processing of Manihot esculenta Crantz into the environment or public sewers remains a source of concern. In this study, the acute toxicity (96-h LC50) of cassava effluent was evaluated using Clarias gariepinus juveniles and Egera radiata. The two organisms were exposed to different concentrations of the effluent using a renewable static bioassay with continuous aeration. The LC50 of exposed C. gariepinus juveniles was found to be 1.92 ml/L with lower and upper confidence limits of 1.35 ml/L and 2.19 ml/L, while that of E. radiata was 1.17 ml/L with lower and upper confidence limits of 0.72 ml/L and 1.29 ml/L respectively. The relationship between exposure concentrations and mortality of juvenile C. gariepinus indicate a dose dependent relationship. On the contrary, the relationship between toxicant exposure concentration and mortality of the clams showed a polynomial or biphasic relationship. The biphasic mortality curve exhibited by exposed clams suggests the possibility of adaptive responses occurring at higher effluent exposures. The mean 96 h LC50 values of the freshwater African catfish and freshwater clams exposed to the cassava mill effluent indicate that the shellfish are more sensitive to the effluent.