SIMULATION OF THE EFFECTS OF TIME AND SIZE AT STOCKING ON PCB ACCUMULATION IN LAKE TROUT

Manipulations of size at stocking and timing of stocking have already been used to improve survival of stocked salmonines in the Great Lakes . It should be possible to stock salmonines into the Great Lakes in a way that reduces the rate of polychlorinated biphenyl (PCB) accumulati on in these fishes. An individual-based model (IBM) was used to invest igate the effects of size at stocking and timing of stocking on PCB ac cumulation by lake trout Salvelinus namaycush in Lake Michigan. The in dividual-based feature of the model allowed lake trout individuals to encounter prey fish individuals and then consume sufficiently small pr ey fish. The IBM accurately accounted for the variation in PCB concent rations observed within the Lake Michigan lake trout population. Resul ts of the IBM simulations revealed that increasing the average size at stocking from 110 to 160 mm total length led to an increase in the av erage PCB concentration in the stocked cohort at age 5, after the fish had spent 4 years in the lake, from 2.33 to 2.65 mg/kg; the percentag e of lake trout in the cohort at the end of the simulated time period with PCB concentration of 2 mg/kg or more increased from 62% to 79%. T hus. PCB contamination was reduced when the simulated size at stocking was smallest. An overall stocking strategy for lake trout into Lake M ichigan should weigh this advantage regarding PCB contamination agains t the poor survival of lake trout that may occur if the trout are stoc ked at too small a size.