Harvest policy considerations for re-evaluating the minimum size limit in the PaciÞ c halibut commercial Þ shery

The PaciÞ c halibut harvest strategy uses a minimum size limit of 32 inches (81.3 cm) in its commercial Þ shery. The stock assessment estimates increasing trends in total halibut numbers and total biomass in spite of a decade long of declining trend in exploitable biomass. The differing estimated trends have been interpreted as resulting from decreasing availability of larger halibut to the commercial gear due to a decade’s long decline in size-at-age, and increased estimates of recent recruitment. The estimated coastwide accumulation of halibut below the size limit has prompted requests to consider lowering or eliminating the current minimum size limit. This report evaluates the potential effects of modifying the current minimum size limit (MSL), along with the effect of different assumptions and methods on evaluating results. Three main methods were used. First was a comparison between female maturity at age with both potential future and observed historical changes in selectivity and commercial landings. Second was a coastwide yield per recruit and spawning biomass per recruit analysis. Third was a spatially structured and migratory yield per recruit and spawning biomass per recruit analysis. A fourth analysis was conducted by gradually decreasing the MSL, using simulated changes in selectivity shapes under the assumption of gradual changes in weights at age of the resulting catch. Reducing or eliminating the MSL is expected to increase the capture of immature females, as supported by comparing expected changes in selectivity at age and cumulative distributions of observed catch at age for different historical periods with different MSL. A reduction or elimination of MSL does increase the proportion females at older ages relative to status quo, with as much as 80% improvement, however the actual proportions in the population are still quite low (12% to 22% at age 25). Eliminating the MSL and assuming that retention will be similar to that of the survey, results in decreased coastwide yield per recruit and decreased spawning biomass per recruit when accounting for smaller weights-atage expected from changes in commercial selectivity. When assuming that elimination of the MSL results in commercial landings similar to the IPHC survey, the proportion of females in the catch and in the population only marginally changes from status quo conditions. A larger decrease in the proportion of females in the catch and increase in the proportion in the population could be achieved by a major change in commercial selectivity towards smaller sizes and younger ages. However, this would result in the capture of immature females as much as four years before they mature, would require precise control of a low harvest rate (around 0.1) and would achieve lower yield per recruit than the status quo. The migratory analysis produced similar results to the coastwide yield per recruit and spawning biomass per recruit and it had low sensitivity to different assumptions on migration. Minor reductions in the MSL along with harvest rate reductions are expected to produce at most 3% increases in yield per recruit but greater reductions in the MSL including its elimination are expected to reduce both yield per recruit and spawning biomass per recruit. The proportion of females in the population between the extremes of current MSL and a MSL of 65cm would change from 44% to 45% for ages 6 and older, and from 12% to 22% at age 25. Overall, decreasing the MSL is expected to reduce yield per recruit and spawning per recruit