Comparing model predictions for ecosystem-based management

Ecosystem modeling is becoming an integral part of fisheries management, but there is a need to identify differences between predictions derived from models employed for scientific and management purposes. Here, we compared two models: a biomass-based food-web model (Ecopath with Ecosim (EwE)) and a size-structured fish community model. The models were compared with respect to predicted ecological consequences of fishing to identify commonalities and differences in model predictions for the California Current fish community. We compared the models regarding direct and indirect responses to fishing on one or more species. The size-based model predicted a higher fishing mortality needed to reach maximum sustainable yield than EwE for most species. The size-based model also predicted stronger top-down effects of predator removals than EwE.
In contrast, EwE predicted stronger bottom-up effects of forage fisheries removal. In both cases, the differences are due to the
presumed degree of trophic overlap between juveniles of large-bodied fish and adult stages of forage fish. These differences
highlight how each model’s emphasis on distinct details of ecological processes affects its predictions, underscoring the
importance of incorporating knowledge of model assumptions and limitation, possibly through using model ensembles, when
providing model-based scientific advice to policy makers.