Impact Assessments of Fishing Using Size-Based Models

Implementation of the ecosystem approach to fisheries management requires impact assessments of how fishing on one part of the ecosystem affects the rest of the ecosystem.  This should be done with models of various complexity.  I will introduce the size- and trait-based approach to designing models of intermediate complexity of a fish community.  In the models, individual fish are characterised by size, and their interactions by the simple rule: big fish eat smaller fish.  As an example of an impact assessment I simulate the consequences of optimising ecosystem protein yield or economic rent while avoiding collapse of individual stocks.  Finally I will show how a failure to resolve the stock structure of the individual components of the system will lead to models that are either too unstable or that have too strong indirect interactions in the community.  The modelling framework has been implemented in three web-applications, available from http://ken.haste.dk.