CCAMLR is developing a management procedure for the krill fishery, including the evaluation of candidate management procedures in a simulation framework, using plausible models of the Antarctic marine ecosystem. This paper develops a framework for erecting such models. It discusses the general model requirements and presents a conceptual framework for allowing flexibility in dealing with biological, spatial and temporal scales. The body of the paper is concerned with conceptualising an element of the ecosystem to account for the varying qualities of data and knowledge and for exploring the consequences of uncertainty.