Size Structured Multispecies Models for an Evaluation of Indirect Effects of Fisheries

Physiologically structured models of interacting fish populations, often called size-spectrum models, are a more recent approach to long-term impact assessment of fishing on fish stocks. The models are based on sub-models of individual physiology and resolve the entire life stages of fishes with different life-histories. Size spectrum models have evolved from models representing a community to trait-based models and into models of specific ecosystems. Here we show how a general trait-based multispecies model can be calibrated to large marine ecosystems using few parameters. The calibrated models are used to predict the community size-structure under a given ecosystem specific fishing selectivity. This allows us to see how fishing changes predation mortality and recruitment for interacting species, and thus how indirect effects of fishing shapes the community. Size spectrum models can be used to investigate long-term fishing strategies, such as the consequence of high fishing mortality on forage fish and how that influences distributions of large predatory fish. Finally we identify areas where size structured models provide improvement over existing ecosystem models such as Ecopath w. Ecosim and ATLANTIS, and how to move forward in implementing size-based approaches in ecosystem based fisheries management.