Publication Abstract

The importance of predator-prey functional response in models exploring marine mammal - fishery interactions

J. L. Blanchard, S. Mackinson, J. K. Pinnegar and R. Scott

Whether marine mammals compete with fisheries is a contentious political and scientific issue. Ecosim has been proposed as a useful tool for exploring interactions between cetacean predators, their prey and fisheries. However, the utility of Ecosim for addressing such questions remains to be tested. One particular issue that remains to be resolved concerns the way that predators respond to changes in prey availability (functional response). There is evidence that the type of functional response used can greatly affect model outcomes. Most existing multispecies models assume a Type II (hyperbolic) relationship but such an assumption could greatly overestimate the effect that generalist predators might have on depleted prey stocks.

Since most marine mammals and boreal fish species are considered to be generalist feeders, a Type III (sigmoidal) functional response is probably more appropriate. Sigmoidal functional responses are rarely used as they are often difficult to implement in complex multispecies models. There are many different mechanisms through which a sigmoidal functional response can be incorporated in models. For example, changes in predator search tactics and in prey behaviour can both result in similar emergent response curves. In Ecosim, the time spent foraging by the prey (and hence its vulnerability to predation) is manipulated, such that as prey density declines a sigmoidal decrease in predator consumption on that prey becomes apparent.

In the present study we carried out simulations using a mass-balance ecosystem model of the Barents Sea. We examined the effect of increased whaling activities on other system components assuming different functional responses. We also determined the effect of increased fishing pressure on marine mammal populations within the model.

Reference:

J. L. Blanchard*, S. Mackinson*, J. K. Pinnegar*, R. Scott*  (2003) The importance of predator-prey functional response in models exploring marine mammal - fishery interactions. Marine Mammal Science, 19: 661-681.