Unusually high concentrations of persistent organochlorines were found in lake trout muscle and burbot liver from subarctic Lake Laberge, Yukon Territory. As a result of the elevated toxaphene concentrations in these fishes, a health advisory was issued by Health Canada in 1991, and the commercial, sport and subsistence fisheries on the lake were closed. Previous studies on Lake Laberge have revealed that the fish community structure is atypical when compared to other regional lakes, with high biomasses of burbot and longnose sucker and low biomasses of lake trout and lake whitefish. Likewise, the lake trout are known to be faster growing, fattier and strictly fish eating, unlike other populations from nearby lakes. It was hypothesized that the high concentrations of organochlorines in Laberge fishes were the result of an unusually long food chain in this lake, a factor that has been shown to affect the pollutant concentrations in fish from temperate lakes.

To characterize food web structure and examine the biomagnification of organochlorines through the food web of Lake Laberge and two reference lakes, Fox and Kusawa, fishes and invertebrates were analysed for stable nitrogen isotope ratios to quantify trophic position, and persistent organochlorines. Stable nitrogen isotope ratios were significant predictors of the organochlorine concentrations through these food chains and in the top predators from these lakes. The slopes of these regressions were greater for the more lipophilic organochlorines when compared to the less lipophilic organochlorines and appear to directly reflect a pollutantıs potential to biomagnify through the food chain. Both within and among species, lipid content was a better predictor of the less lipophilic organochlorines such as hexachlorocyclohexane, while stable nitrogen isotope ratios were better predictors of the more lipophilic organochlorines such as PCBs and toxaphene. Lake trout and burbot from Lake Laberge fed at a significantly higher trophic position, as determined by stomach content analyses and stable nitrogen isotope ratios, than the same species from Fox and Kusawa lakes. The high concentrations of toxaphene in fishes from Laberge have been attributed to an unusually long food chain and not to elevated inputs of this pesticide.