Effect of non-additivity in mortality rates on predictions of potential yield of forage fishes

Ecosystem models typically assume additive effects of predation and other natural mortality rates in prediction of net production for forage fishes, resulting in prediction of substantial increase in forage fish production when predator abundances are reduced. But if vulnerability to predation is determined by stress factors like hunger and parasite loads that would result in higher mortality rates of vulnerable forage fish individuals even if predators were removed, there may be little decrease in forage fish natural mortality rates and hence little or no increase in net production rates. We propose a vulnerability exchange model to account for such effects, and this model predicts that under such conditions there may be little or no increase in forage fish productivity when predator abundances are reduced. We present a case study of harbour seal predation on Chinook and coho salmon, which underlines that uncertainty about additive mortality is important for evaluating alternative hypotheses of direct predation and indirect environmental impacts. The new model formulation will be implemented as an option in the Ecopath with Ecosim modelling approach and software.