ANTICHOLINESTERASE EFFECTS OF FORMULATED METHYL PARATHION TO JUVENILES OF ODONTESTHES BONAERENSIS (TELEOSTEI, ATHERINIDAE)

In the present work, toxicity and cholinesterase inhibition induced by formulated methyl parathion (FOLTDOL 600(R), Bayer) on juveniles of O dontesthes bonaerensis (Teleostei: Atherinidae) were studied. Signific ant cholinesterase inhibitions were registered after 96 hr of exposure to the two higher methyl parathion concentrations (75 or 750 mu g/L) tested. In these cases, the mean enzyme activities were 74.2 and 86.3% lower than that registered in the control fishes (0.226 +/- 0.018 mu moles.mg protein(-1).min(-1)), respectively. Despite the higher enzyme inhibition, only one fish death at 0.75 and another at 75 mu g/L were registered Considering nominal concentrations, EC50 and MATC values w ere estimated as 29 mu g/L and 1.16 mu g/L of methyl parathion, respec tively. Gas chromatography indicates that the measured concentration o f formulated methyl parathion was 12 % lower than the nominal one. Com parisons based on exposure units show that the sensibility of Odontest hes bonaerensis cholinesterase to organophosphates is intermediary in relation to that of other fishes.