More Chickens, Fewer Hogs: Implications of Size-Selective Fishing, Bycatch, and Discards on Declines in Size-at-Age of Pacific Halibut

Recent declines in size-at-age of Pacific halibut have led to large reductions in harvest and triggered an investigation into the potential role of size-selective fishing on size-at-age and biological reference points in the halibut fishery. Current harvest policy uses a 32-inch minimum size limit in the directed commercial halibut fishery and assumes that 84% of discarded sub-legal halibut survive annually regardless of size.  Retention of halibut in non-directed fisheries (e.g. trawl fishery bycatch) is prohibited; however, post-release survival rates are gear dependent and partially based on onboard observations of halibut release condition.  We examined the sensitivity of maximum sustainable yield and spawning biomass-per-recruit-based reference points to assumptions about post-release survival and the cumulative effects of size-selective fishing.  Trade-offs between discard mortality, size limits, bycatch, and fishing intensity were examined from a long-term equilibrium perspective using isopleths that describe per-recruit changes in spawning biomass, yield, discards, and mean weight-at-age of the landed catch.  Resultant potential losses in landed value to the directed fishery associated with discarding sub-legal fish are approximately $10 to $24 million per year, while total losses associated with bycatch mortality are about $70 to $120 million per year.