The offshore fish community of Lake Ontario is presently dominated by intensively managed, nonnative species: Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax at the planktivore level and stocked salmonines at the piscivore level. Salmonine stocking rates per unit area of Lake Ontario are the highest in the Great Lakes, and fishery managers are concerned about the sustainability of the fishery under present stocking policies, particularly with the recent collapse of the Lake Michigan fishery for chinook salmon Oncorhynchus tshawytscha. In this paper, we describe and present the results of a simulation model that integrates predator demand estimates derived from bioenergetics, prey and predator population dynamics, and a predation model based on the multiple-species functional response, Model reconstructions of historical alewife biomass trends and salmonine diets corresponded reasonably well with existing data for the period 1978–1992. The simulations suggest that current predator demand does not exceed the threshold beyond which alewife biomass cannot be sustained, but they indicate that the sustainability of the prey fish community is extremely sensitive to fluctuations in overwinter survival of alewife; an additional mortality of 25% in a single winter would be sufficient to cause the collapse of the alewife population. The model includes a number of assumptions and simplifications with a limited empirical basis; better estimates of salmonine survival rates, an evaluation of the importance of spatial and temporal interactions among predators and prey, and incorporation of the effects of recently observed declines in system productivity at lower trophic levels would significantly increase confidence in the model's projections.