Effect of nonlinear predation rates on rebuilding the Georges Bank haddock (Melanogrammus aeglefinus) stock

Abstract

The collapse of several northwest Atlantic groundfish stocks, including Georges Bank haddock (Melanogrammus aeglefinus), necessitated new fishing regulations and generated interest in the role of predation on stock productivity. The sharp break between prolonged periods of high (pre-1965) and low (post-1965) haddock abundance suggests differing levels 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 Schaefer (1957. Inter-Am. Trop. Tuna Comm. Bull. 2: 245-285) model without a predation term were fit to haddock data to evaluate various rebuilding strategies with two performance measures: the sums of discounted yield and discounted revenue. Steele and Henderson's model provided plausible parameter estimates for the entire data set (1931-1993) whereas Schaefer's model provided plausible estimates only for years of low productivity (1976-1993). The presence of multiple equilibria in Steele and Henderson's model resulted in minor shifts of F, potentially producing large shifts in projected future biomass. For either model, levels of F that maximize either yield or revenue were lower than the recently adopted target level of F0.1 = 0.24. Recent low production provides impetus for managers to consider a variety of plausible stock production models, and the uncertainty of production dynamics, in choosing rebuilding strategies.