Implications of Bycatch, Discards, and Size Limits on Reference Points in the Pacific Halibut Fishery

Abstract

The current harvest policy for the Pacific halibut fishery uses a 32-inch minimum size limit in the directed commercial fishery, and total annual catches in each of the eight regulatory areas are based on area-specific exploitation rate targets. In nondirected fisheries retention of halibut is prohibited. Post-release survival rates are gear dependent and partially based on observer accounts of halibut release condition. The current assumption is that 84% of the sub-legal halibut discarded from the directed halibut fishery survive each year and this rate is the same for all sizes of fish. This paper examines how sensitive estimates of maximum sustainable yield (MSY) and spawning biomass per recruit– based reference points are to the assumptions of post-release survival and the cumulative effects of size-selective fishing. A joint probability model for surviving the capture process is developed for modeling the instantaneous rates of retention and discarding in directed fisheries, as well as the cumulative effects of size-selective mortality from all sources. Evaluation of the current minimum size limit and discard mortality rates, and alternatives, for MSY-based reference points is based on assumptions about an underlying stock-recruitment relationship. The trade-offs between discard mortality, size limits, bycatch, and fish ing intensity are examined from a long-term equilibrium perspective using isopleths that describe per recruit changes in spawning biomass,