Acute toxicity of the insecticide, Dimethoate on juveniles of Oreochromis niloticus (Trewavas) and Clarias gariepinus (Teugels)

Abstract

Juveniles of Oreochromis niloticus and Clarias gariepinus were exposed in glass aquaria to acute concentration of dimethoate for 96 hours. The various concentrations used for O. niloticus were in the following order; 15.00, 17.50, 20.00, 22.50 and 25.00 mg.1^-1 while 32.00, 35.20, 38.40, 41.60 and 44.80 mg.1^-1. Behavioral reactions in order to their appearance were agitated swimming, loss of equilibrium, air gulping, periods of quiescence and death. Copious accumulation of mucus was observed in the gill filaments and body surfaces of dead fish in both species. However, the intensity of mucus production was more pronounced in C. gariepinus than in O. niloticus. Possible reasons for the differences are suggested. Also, blood was observed around the gill coverings of the dead fishes. These reactions were more pronounced in the tanks containing higher levels of toxicants. Mortalities and 96h-LC₅₀ values for O. niloticus (20.70 mg.1^-1) were lower than that of C. gariepinus (39.40 mg.l^-1). This infers that O. niloticus is more sensitive to dimethoate than C. gariepinus. Possible reasons for the differences in susceptibility of these two species to dimethoate are suggested. Acute exposure of the toxicant showed altered behaviors and mortalities in both fishes.


Key Words: Acute toxicity, dimethoate; Oreochromis niloticus; Clarias gariepinus


Journal of Aquatic Sciences Vol.19(1) 2004: 5-8