Harvest control rules for stocks displaying dynamic production regimes

Several cod and haddock stocks in Atlantic Canada have undergone substantial changes in productivity. Regimes are defined as periods of relatively stable production separated by rapid transitions to a different level. These regimes are deconstructed into their dominant determinants of growth, survivorship, or recruitment. Using southern Gulf of St Lawrence cod and eastern Scotian Shelf haddock as case studies, regimes are identified based on production. Haddock display an almost threefold difference between productive and nonproductive regimes, owing mainly to a change in reproductive output. Cod have a more complicated history, displaying three regimes, and a variation in production by a factor of approximately eight. The production data in each regime are identified and smoothed to show the underlying dependence on biomass. Finally, simple three-phase harvest control rules (having critical, cautious, and healthy zones) for distinct regimes are presented to illustrate the potential importance of regimes in resource management.