ON THE QUALITY OF NORTH-SEA COD STOCK FORECASTS

Sensitivity analysis is performed on the procedure for forecasting yield and biomass of North Sea cod (Gadus morhua L.) currently used by the International Council for the Exploration of the Sea (ICES). The yield forecast is most sensitive to estimates of recruitment during the forecast period, while spawning-stock biomass is most sensitive to mortality rates. Confidence intervals for forecast yield, spawning-stock biomass, and total stock biomass are calculated. These confidence intervals are based on variance estimates calculated by fitting a separable fishing mortality model to catch-at-age data. The calculated confidence intervals indicate that a useful improvement in the precision of the forecast can be achieved by using as much of the most recent recruitment data as possible to reduce the variance of the estimates of recruitment. In the case of North Sea cod this means delaying the assessment on which the forecast is based until late in the year. The analysis suggests that an autumn assessment would provide an adequate forecast of yield up to the third calendar year from the assessment and a forecast of spawning stock up to four years ahead. © 1991 International Council for the Exploration of the Sea.