On the performance of fish stock parameters derived from VIT pseudo-cohort analysis

Abstract

The comparison of VIT model results with those of the virtual population analysis assessment of the cod stock in the Skagerrak, North Sea and Eastern Channel established by the International Council for the Exploration of the Sea (ICES ) reveal that overall trends in fishing mortality and stock size are well captured over a period of three decades (1963-1992). However, the implications of the equilibrium assumption (steady state) made by the VIT model are enormous, as it forces strong inter-annual variations in the estimated fishing mortality, especially in years when the age composition shows strong year class effects. Estimated VIT output parameters should be carefully interpreted when the assessed stock suffers from reduction in abundance and high exploitation, as this condition is likely to deviate from steady state. The VIT estimates of the virgin stock biomass appear very variable, extremely high and fairly dependent on annual stock size estimates. Their use in fisheries management as a realistic biological stock biomass reference is not recommended. The VIT estimates of F0.1 and Fmax are found to be in line with the ICES estimates. Therefore, model results should allow qualitative conclusions regarding the status of marine living resources in relation to a target reference of exploitation rate. Quantitative conclusions regarding the assessment of the exploitation status are only recommended when the model is applied to short time series of consecutive annual data and the resulting variation in the estimated stock parameters appears reasonably low. As such, the model results may form the basis for scientifically sound fisheries management advice covering the actual state of the stock as well as medium-term forecasts of catch and biomass under different options.