SIMPLE BIOASSAY METHOD FOR ESTIMATION OF SAFE DISPOSAL RATES OF INSECTICIDES TO PROTECT FISH: DIMETHOATE

Abstract

Bioassays were conducted in the laboratory' using three species of fish (carp
Cyprinus carpio, singhi Heteropneustes fossilis and tilapia Tilapia mossambica) and
the insecticide, dimethoate, to evaluate a simple method for the estimation of safe
levels of insecticides. The 168-h LCD, LC50 and LClOO of dimethoate were respectively
16.0, 22.39 and 32.0 ppm for carp; 20.0, 45.71 and 80.0 ppm for singhi
and 10.0, 23.77 and 32.0 ppm for tilapia. From these data, three safe levels of the
insecticide were estimated for each fish species (11.195, 5.597 and 2.799 ppm for
carp; 11.427, 5.713 and 2.856 ppm for singhi and 7.430, 3.710 and 1.850 ppm
for tilapia). Tests were coinducted separately on three species of fish to evaluate
the safety of these estimated levels; the variables used were behavior, feeding rate,
appetite, survival, growth, reproduction, histopathology and influence on water
quality. In 5% of the exposed fish, histopathological lesions were recorded; in
other respects the estimated levels of the insecticide were found to be safe,