Recent development of methods for analytical fish stock assessment within ICES

Analytical age-structured stock assessments commenced in ICES in the late 1960s and have constituted the basis for ICES advice on fish stock management for the last 20-30 years. In this paper, the age-disaggregated assessment methods that have been standard tools in ICES are described and discussed both in a methodological and a historical perspective. The general theoretical framework for these methods is outlined, including the virtual population analysis (VPA) algorithm, ad hoc tuning of the VPA, separable VPA, and statistical catch-at-age methods exemplified by ICA. Why did the assessment methodology develop as it did in ICES and, in particular, why were age-structured models preferred to production models? Why did VPA-type models dominate for so long? We suggest in this paper that this was caused by a combination of institutional tradition, the availability of data, historical emphasis on year-class variation, and, later, the managers’ preference for advice in absolute terms.