A SUITE OF EXTENSIONS TO A NONEQUILIBRIUM SURPLUS-PRODUCTION MODEL

Surplus-production models, because of their simplicity and relatively undemanding data needs, are attractive tools for many stock assessment s. This paper reviews the logistic production model, starting with the basic differential equation and continuing with a description of the model development without the equilibrium assumption. It then describe s several extensions, including ''tuning'' the model to a biomass inde x; partitioning fishing mortality by gear, time, or area; and making p rojections. Computation of confidence intervals on quantities of inter est (e.g. maximum sustainable yield (MSY), effort at MSY, level of sto ck biomass relative to the optimum level) can be done through boot-str apping, and the bootstrap can also be used to construct nonparametric tests of hypotheses about changes in catchability. To fit the model, a n algorithm that uses a forward solution of the population equations c an be implemented on a small computer. An example of the utility of su rplus-production models (illustrating several of these extensions) is given. The example is loosely based on swordfish (Xiphias gladius) in the North Atlantic Ocean, but is not intended to describe the actual s tatus of that stock.