Behavioural Responses of the Freshwater Fish, Cyprinus carpio (Linnaeus) Following Sublethal Exposure to Chlorpyrifos

Common carp fingerlings were exposed to different concentrations (0.120 to 0.200 mg/L) of an organophosphate pesticide, chlorpyrifos (20% EC) for 96 h. The acute toxicity (LC50) of chlorpyrifos by static renewal (semi-static) bioassay test was found to be 0.160 mg/L. One-seventh (0.0224 mg/L) and one-fourteenth (0.0112 mg/L) of the 96 h LC50 were selected as sublethal concentrations for subacute studies. The fish were exposed to both the sublethal concentrations for 1, 7 and 14 days and were allowed to recover in toxicant free medium for seven days only after 14(th) day of exposure. Behavioural responses and morphological deformities were studied in the experimental periods. Fish in toxic media exhibited irregular, erratic and darting swimming movements, hyper excitability, loss of equilibrium and sinking to the bottom. The carp were found under stress, but mortality was insignificant in both the sublethal concentrations. Caudal bending was the main morphological alteration during the exposure periods. The behavioural and morphological changes may be due to the inhibition of acetylcholinesterase (AChE) activity. Inactivation of AChE activity results in excess accumulation of acetylcholine (ACh) in cholinergic synapses leading to hyperstimulation and cessation of neuronal transmission (paralysis). Impaired behavioural responses and morphological deformities were observed even under recovery periods. This may be a consequence due to the inhibition of brain and muscular AChE activity by chlorpyrifos-oxon via biotransformation of bioaccumulated chlorpyrifos in the tissues.