Methoxychlor Residue Studies in Caged and Wild Fish from the Athabasca River, Alberta, Following a Single Application of Blackfly Larvicide

Abstract

On June 4, 1974, the Athabasca River was treated with methoxychlor for control of blackfly larvae. Pesticide was introduced into the water in a manner calculated to generate a 15-min pulse of treated water containing 300 μg methoxychlor/ℓ. A preliminary experiment with rainbow trout (Salmo gairdneri) indicated that liver or kidney residues of methoxychlor would be suitable for monitoring likely times and places to expect toxic effects on fish in the river. Fish (Salmo gairdneri, Catostomus commersoni, C. catostomus, and Hybopsis gracilis) were caged in order to observe residues and mortality. Caged fish were not killed by methoxychlor but they contained lower liver residues than wild fish of the same species captured near cages. Contaminated wild fish were found upstream of the treatment site after treatment. Residues were below detection limits by June 16. Residue measurements suggested that the most likely times to expect fish poisoning were the 2 wk following treatment and that the most likely places were those river areas extending a few kilometres in both directions from the point of application. Evidence of fish movements and low pesticide residues in caged individuals reduced confidence in residue methods as monitoring tools.