Ad hoc multispecies VPA tuning applied for the Baltic and North Sea fish stocks

Abstract

Ad hoc multispecies tuning is a method for estimation of terminal fishing mortalities in multispecies virtual population analysis (VPA) using the existing single-species tuning packages. The method works by an iterative exchange of natural mortalities and terminal fishing mortalities between multispecies VPA and single species tuning modules, until equilibrium is obtained. Multispecies tuned assessment is compared with single species assessments using constant at-age natural mortality for the Baltic and the North Sea fish stocks. Correlation between estimated stock size and survey abundance indices is similar for tuned multi- or single species assessment. The tuned multispecies assessment gives, however, higher correlation coefficients for the North Sea whiting (Merlangius merlangus) and the Baltic Sea sprat (Sprattus sprattus), whilst the traditional single species assessment performed better for North Sea haddock (Melanogrammus aeglefinus) and herring (Clupea harengus). Tuned multispecies assessment performs best when there is a clear temporal trend in the estimated natural mortality whereas the correlation between estimated stock size and survey indices was not improved for species with highly variable natural mortalities. 2001 International Council for the Exploration of the Sea