Abstract
The present study deals with the long-term toxic effects of the lowest sub-lethal concentration of dimethoate and malathion on a freshwater fish Catla catla. Toxicity is measured in terms of inhibition in acetylcholinesterase activity and fish behaviour. Technical grades of dimethoate (30 EC) and malathion (50 EC) were used for the present study. LC50 values for 96 hours were calculated for both pesticides. The lowest sublethal concentration (1/5 of LC50) of both pesticides was taken for chronic exposure. As organophosphates are potent inhibitors of acetylcholinesterase (AChE) activity, the AChE activity was estimated by the method of Metcalf (1957) in the brain and liver tissue of the test fishes to evaluate chronic toxicity. The fish were observed regularly to analyse the changes in behaviour. A significant increase in inhibition of acetylcholinesterase activity was observed. Experimental fishes exhibited a decreased rate of opercular movement, loss of balance, excessive mucous secretion, erratic swimming, and increased surfacing with increasing exposure time. Probable reasons for these alterations have been discussed in detail.
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