Behavioural and respiratory dysfunction in the freshwater fish, Labeo rohita (Hamilton) under malathion intoxication

A short term definitive test by static renewal bioassay method was conducted to determine the acute toxicity (LC50) of the commercial grade organophosphate insecticide malathion (50% EC) in the freshwater edible fish, Labeo rohita. Carp fingerlings were exposed for 96 h to different concentrations (6.0 to 10.1 microL/L) of malathion. The acute toxicity value was found to be 9.0 microL/L and one tenth of LC50 (0.9 microL/L) was selected for sub acute studies. Behavioral patterns and oxygen consumption were studied in lethal (1, 2, 3, and 4 d) and sublethal concentrations (1, 5, 10, and 15 d). Carp in toxic media exhibited irregular, erratic, and darting swimming movements, hyperexcitability, and loss of equilibrium and sinking to the bottom, which might be due to the inactivation of acetylcholine esterase activity, resulting in excess accumulation of acetylcholine in cholinergic synapses leading to hyperstimulation. Variation in oxygen consumption (70.39% to 80.50%/4.45% to 21.35%) was observed in both lethal and sublethal concentrations of malathion, respectively. Such alterations in oxygen consumption may be due to respiratory distress because of impairment in oxidative metabolism. Fish at sub lethal concentrations were found under stress, but that was not fatal.