Bifrost serves both as a model for the historic assessment of the capelin SSB and as a tool for evaluating harvest rules using long term stochastic simulation. Bifrost is a multispecies model with cod-capelin interactions as the basic vehicle for driving the population dynamics. Consumption of cod and capelin by harp seals is implemented on an experimental basis. Herring affects capelin recruitment as a covariate in the SSB-recruitment submodel.
Parameter estimation is based om maximum likelihood and uncertainty parameters in the distrubutional functions are estimated along with the parameters governing biological processes in the model. The estimation of parameters in the consumption model rests on terms in the likelihood derived from consumption of capelin per cod in the model and empirical consumption of capelin per cod derived from stomach content data and evacuation rates measured in a laboratory.
The primary data sources are a yearly trawl-acoustic survey in September, the assessment of cod made by the ICES Arctic Fisheries WG, the joint Russian-Norwegian stomach content data base and laboratory measurements of stomach evacuation rates made at the University of Tromsø.