IMPAIRMENT OF LAKE TROUT FORAGING BY CHRONIC EXPOSURE TO CADMIUM - A BLACK-BOX EXPERIMENT

The foraging success of lake trout (Salvelinus namaycush) on fingerlin g rainbow trout (Oncorhynchus mykiss) was evaluated after both species were exposed to 0.5 and 5 mu g Cd l(-1) for 8-9 months (up to 277 day s). Predation of rainbow fingerlings by lake trout was measured by dai ly counts of remaining prey in tanks where groups of exposed and unexp osed prey fish were presented to exposed and unexposed predators. Usin g this measure, exposed prey presented to unexposed predators showed s light, but no significant increases in vulnerability to predation. Sig nificant dose-dependent decreases of foraging rates were observed when exposed lake trout were presented with unexposed prey. Of four lake t rout tissues sampled, posterior kidney had the highest cadmium concent ration reaching 2.5 mu g g(-1) after 9 months exposure to 5.0 mu g Cd l(-1). Thyroid follicle epithelial cell height (a measure of thyroid f unctional state) was significantly decreased at both 6 and 9 months ex posure to 5 mu g Cd l(-1) Results underline a low response threshold f or cadmium-caused behavioral changes, determined in a simple, efficien t experimental design.