EFFECT OF CHANGES IN RECRUITMENT LEVELS ON MULTISPECIES LONG-TERM PREDICTIONS

A multispecies model (MSFOR) is used to predict the relative change in equilibrium yield and spawning stock biomass (SSB) of commercially important fish stocks in the North Sea resulting from a reduction in the fishing mortality generated by the roundfish fishery. Because predation mortality is a function of the abundance of prey and predators the results will depend on recruitment. Assuming recruitment to be independent of stock sizes the effect of changes in recruitment is studied by repeating the prediction, at all possible combinations of +/-50% changes in predator and prey recruitment levels, All of the predictions result in a relative increase in the SSB of Atlantic cod (Gadus morhua) and saithe (Pollachius virens) and in a relative decrease in the SSB of Atlantic herring (Clupea harengus) and Norway pout (Trisopterus esmarkii). In these cases the sign of the relative change is robust to recruitment changes. However, tor haddock (Melanogrammus aeglefinus), sprat (Sprattus sprattus), and sandeel (Ammodytes marinus) the relative change in SSB is found to be either positive or negative depending on the level ot recruitment. The predictions for haddock are highly sensitive to change in the level of saithe recruitment.