SUSTAINABILITY OF HATCHERY-DEPENDENT SALMONINE FISHERIES IN LAKE-ONTARIO - THE CONFLICT BETWEEN PREDATOR DEMAND AND PREY SUPPLY

The offshore fish community of Lake Ontario is presently dominated by intensively managed, nonnative species: alewife Alosa pseudoharengus a nd rainbow smelt Osmerus mordax at the planktivore level and stocked s almonines at the piscivore level. Salmonine stocking rates per unit ar ea of Lake Ontario are the highest in the Great Lakes, and fishery man agers are concerned about the sustainability of the fishery under pres ent stocking policies, particularly with the recent collapse of the La ke Michigan fishery for chinook salmon Oncorhynchus tshawytscha. In th is paper, we describe and present the results of a simulation model th at integrates predator demand estimates derived from bioenergetics, pr ey and predator population dynamics, and a predation model based on th e multiple-species functional response. Model reconstructions of histo rical alewife biomass trends and salmonine diets corresponded reasonab ly well with existing data for the period 1978-1992. The simulations s uggest that current predator demand does not exceed the threshold beyo nd which alewife biomass cannot be sustained, but they indicate that t he sustainability of the prey fish community is extremely sensitive to fluctuations in overwinter survival of alewife; an additional mortali ty 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 spatia l and temporal interactions among predators and prey, and incorporatio n of the effects of recently observed declines in system productivity at lower trophic levels would significantly increase confidence in the model's projections.