[6], and the mosquito fish, Gambusia affins (8.4 mg L1) [32]. The
LC(50) 50–96 h of methyl parathion was 3.97 mg L1 for larvae
and 9.89 mg L1 for juvenile Piaractus mesopotamicus [33]. Behavioral
changes caused by methyl parathion were reported for
B. cephalus [6] and Rhandia quelen [34], mostly related to erratic
swimming and hyperexcitation in the latter.
There is evidence that larger or older fish are less susceptible to
pesticides than smaller or younger fish. Mount [35] reported Pimephales
notatus 96 h LC50 for endrin of 0.27 lg L1 in 30 mm-long
fish and 0.47 lg L1 in 60 mm-long fish. Pickering et al. [36] conducted
a variety of acute toxicity tests with 13 organophosphate
pesticides and concluded that Bluegills averaging 10 g were
slightly more tolerant than those weighing 2 g to dioxathion, parathion
and malathion.
Most fish species in both fresh and sea water have LC50s of
methyl parathion between 0.25 and 25 mg L1, with a few species
substantially more sensitive [36–38]. The value obtained in the
present study of 2.7 mg L1 is in general agreement with previously
reported values for O. niloticus of 2.2 mg L1 [5,39]. The