The effects of an aerial application of the organophosphate insecticide fenitrothion on the ecology of native fish species in a small Quebec lake.

Abstract

Fenitrothion was applied as an emulsion in water to a small lake at the rate of 420 g active ingredient per hectare. Peak residues of 21.6 ug/ℓ were present in surface waters one hour after treatment, but rapidly dispersed throughout the lake, with complete mixing within the epilinnion after 12 hours and maximum penetration into the hypolimnion (2.14 ug/ℓ) after 24 hours. All fish species in the lake rapidly accumulated fenitrothion residues with each species accumulating distinctly different residue levels. The highest residue found in each species was 1.01 mug/g in white suckers, Catastomus commersoni (Lacepede), 0.76 mug/g in fallfish, Semotilus corporalis (Mitchill), 0.44 mug/g in brown bullheads, Ictalurus nebulosus (Lesueur), and 0.34 mug/g in smallmouth bass, Micropterus dolomieu Lacepede. Rapid loss of accumulated residues was seen in all species except white suckers. The fenitrothion application had little effect on populations of fish food organisms or on the diet of native fish species, with the possible exception of cladoceran populations and their contribution to the diets of planktivorous fish species. Static bioassays in the laboratory gave 24 to 96 hour LC50 values between 1.2 and 5.4 mg/ℓ fenitrothion for seven species representing five families. Sensitivity to fenitrothion followed family lines with Salmonidae (trout) the most susceptible family and Ictaluridae (catfish) and Cyprinidae (minnows) the least sensitive. The results of the field and laboratory studies carried out indicate that fenitrothion applied at dosages registered for forest insect control does not appear to present a serious hazard to native fish populations in lakes exposed to aerial applications.