Toxicity assessment of insecticides commonly used in rice pest management to the fry of common carp, Cyprinus carpio, a food fish culturable in rice fields

Toxicity of four insecticides commonly used in rice pest management, chlorpyrifos, dimethoate, carbaryl and carbosulfan, to the fry of common carp was assessed through median lethal concentrations (LC50) and in vivo inhibition of the brain acetylcholinesterase (AChE) enzyme at sublethal concentrations. The 96-h LC50 values for these four insecticides were determined to be 0.008, 26.11, 7.85 and 0.60 mg L-1 respectively. Exposure of fish to a series of sublethal concentrations (0.5-5% LC50) of each insecticide for 14 days resulted in concentration-dependent inhibition in AChE activity in comparison with the controls. AChE activity was greatly inhibited in the fish exposed to sublethal concentrations of chlorpyrifos. Upon transfer to insecticide-free water, AChE activities in fry exposed to 0.5 and 1% LC50 concentrations of carbaryl and carbosulfan were restored to the control level within 7-21 days whereas the fish exposed to chlorpyrifos or dimethoate did not fully recover from the insecticide-induced anticholinesterase action. Of the four insecticides tested, chlorpyrifos was the most toxic for the fry of common carp. Although dimethoate was least toxic for the fish under acute exposure, the restoration level of normal AChE activity was slower under chronic exposure in comparison with carbaryl and carbosulfan. Hence, the use of carbamates, especially carbaryl, to control insect pests of rice in rice-cum-carp culture systems is recommended when considering survival, restoration of the normal AChE activity and stamina of the cultured fish.