Pesticides and drugs used in agriculture and veterinary medicine may end up in aquatic environments and bioaccumulate in the food chain, thus posing serious problems for fauna and human health. The lethal concentrations of 50% of sample (LC50) and 25% (LC25) were determined using semi-static method. Eighty-four adult catfish were used for the toxicity study. Seven fish per test concentration in 2 replicates were exposed to varying concentrations of diazinon (0, 1.62, 3.24, 6.48, 12.96 and 25.96 ppm) in water. The effect of diazinon (LC50) 6.6 ppm was further studied to determine the effect on behavioural, blood parameters and plasma biochemistry. The fish in the experimental group showed restlessness, increased reaction to exogenous stimuli, incoordination of movement and postural orientation before death. It is obvious from this study that diazinon negatively affects respiration and produces nervous signs with decrease acetylcholinesteraseactivities in fishes.
Key words: African catfish, Clarias gariepinus, acute toxicity, lethal dose.
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