EFFECT OF NONLINEAR PREDATION RATES ON REBUILDING THE HADDOCK,GEORGES,BANK (MELANOGRAMMUS-AEGLEFINUS) STOCK

The collapse of several northwest Atlantic groundfish stocks, includin g Georges Bank haddock (Melanogrammus aeglefinus), necessitated new fi shing regulations and generated interest in the role of predation on s tock productivity. The sharp break between prolonged periods of high ( pre-1965) and low (post-1965) haddock abundance suggests differing lev els of stock productivity, consistent with a surplus production model incorporating a nonlinear predation rate (Steele and Henderson's (1984 . Science (Washington, D.C.), 224: 985-987) model). This model and a S chaefer (1957. Inter-Am. Trop. Tuna Comm. Bull. 2: 245-285) model with out a predation term were fit to haddock data to evaluate various rebu ilding strategies with two performance measures: the sums of discounte d yield and discounted revenue. Steele and Henderson's model provided plausible parameter estimates for the entire data set (1931-1993) wher eas Schaefer's model provided plausible estimates only for years of lo w productivity (1976-1993). The presence of multiple equilibria in Ste ele and Henderson's model resulted in minor shifts of F, potentially p roducing large shifts in projected future biomass. For either model, l evels of F that maximize either yield or revenue were lower than the r ecently adopted target level of F-0.1=0.24. Recent low production prov ides impetus for managers to consider a variety of plausible stock pro duction models, and the uncertainty of production dynamics, in choosin g rebuilding strategies.