Field Estimates of Net Trophic Transfer of PCBs from Prey Fishes to Lake Michigan Salmonids

Abstract

Dietary exposure is the principal source of polychlorinated biphenlys (PCBs) to Great Lakes salmonids. PCB transfer efficiency has typically been determined with laboratory studies using high food PCB concentrations and clean animals, making extrapolations to the field uncertain. Here, we present estimates of net PCB transfer efficiencies for Lake Michigan coho salmon, chinook salmon, and lake trout based on 15 years of PCB concentrations in these fish and their prey. Between 1975 and 1990, large changes in Lake Michigan's pelagic food web led to changes in coho and chinook salmon diets and chinook salmon growth efficiency. Early in this period, PCB transfer efficiencies increased in lake trout but decreased in chinook and coho salmon. These early trends carry relatively high uncertainty due to high variability in prey and predator PCB concentrations and are probably not ecologically significant. After 1983, PCB transfer efficiencies remained relatively constant at ∼0.55 for lake trout and at ∼0.60 for chinook salmon and increased slightly to ∼0.50 for coho salmon. PCB transfer efficiencies appear to be little affected by changes in prey PCB concentration, shifts in prey type, and shifts in predator gross growth efficiency. Use of a constant PCB transfer efficiency in models that consider PCB accumulation by piscivorous fishes is therefore appropriate.