Pacific halibut (Hippoglossus stenolepis) are an ecologically, commercially, and culturally important Alaskan groundfish species. Commercial harvest of halibut dates back to the late 19th century and has been managed by the International Pacific Halibut Commission (IPHC) since 1921. IPHC surveys have revealed declining trends in survey biomass in multiple regions and region‐specific declines in mean size‐at‐age (size‐at‐age) over the past two decades (>50% in some areas). Changes in size‐at‐age can arise from a variety of physical, ecological, sampling, and fishery effects, including size‐dependent fishery or predation mortality, alteration in growth from variability in prey quality or quantity, and changes in temperature‐dependent metabolic demands. Here, we develop and apply a bioenergetics model for halibut using survey‐based diet and temperature data for Alaska to evaluate potential environmental drivers of size‐at‐age. In general, juvenile (<40 cm fork length) foraging rates were highest in the Gulf of Alaska concomitant with higher potential growth and elevated basal metabolic demands during warm summer conditions. In contrast, adult (40–120 cm FL) potential growth was highest in the Eastern Bering Sea, potentially reflecting lower metabolic costs and higher rates of prey consumption in that region. We additionally find evidence for interannual variation in potential growth, with a higher frequency of reduced growth potential in the last decade, particularly in the Eastern Bering Sea in 2015 and 2016 for both juvenile and adult halibut. These results suggest the potential for patterns in size‐at‐age to arise from trophic and environmental constraints that collectively limit growth in some regions and years.

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 recruitbased 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.

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