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

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 fishing intensity are examined from a long-term equilibrium perspective using isopleths that describe per recruit changes in spawning biomass, 0042 Martell et al.—Halibut Bycatch, Discards, and Size Limits yield, discard, and mean weight and composition of the landed catch. Determining optimal harvest rates for the directed fishery is strongly linked with amounts of bycatch mortality, size limits, and discard mortality rates. Three alternative bycatch mortality scenarios of 0, 10, and 20 million pounds were explored. Potential losses in landed value to the directed fishery associated with discarding sub-legal fish are on the order of $10 to $24 million per year, while total losses associated with discarding and bycatch mortality are on the order of $70 to $120 million per year.