Toxicity imposed by organophosphate pesticides to the freshwater cultivable fish species mrigal (Cirrhinus mrigala) was assessed under laboratory conditions. Healthy juveniles were exposed to chlorpyrifos, dichlorvos, and their equitoxic mixture in geometric series. Median lethal concentrations of chlorpyrifos were found to be 0.906 (0.689-1.179), 0.527 (0.433-0.633), 0.435 (0.366-0.517) and 0.380 (0.319-0.450) mg/L and dichlorvos were found to be 38.432 (33.625-47.866), 22.477 (19.047-26.646), 12.442 (9.619-14.196) and 11.367 (9.496-13.536) mg/L after 24 h, 48 h, 72 h and 96 h of exposure respectively. Surprisingly, the joint toxicity of these organophosphates in the binary mixture was less than additive during most of the exposure periods. Behavioral changes exhibited by individual as well as mixture pesticide treatments were loss of schooling behavior, aggregating at corners of the test chamber, elevated opercular beatings, surplus mucus secretion, slight color changes and sudden and rapid body movements before death. Loss of fish equilibrium was noticed only in chlorpyrifos treated fish, whereas sluggish behavior was noticed only in mixture pesticide treatment. Such behavioral studies can be applied as a non-invasive bio-monitoring tool for water quality assessment for fish growth and development. Despite the same mode of action of both pesticides, the antagonistic action in the binary mixture is an interesting outcome of this research that requires further investigation for a lucid understanding of the joint toxicity mechanism of such pesticides.
Keywords: Additive index; Fish behavior; Growth; Organophosphate; Pesticide mixture.
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