Bioeconomic analysis of a minimum size limit for Gulf of Alaska sablefish using a yield per recruit model

To evaluate the potential of increasing yield in the Gulf of alaska sablefish ( Anoplopoma fimbria) fishery with a minimum size limit, a modified yield per recruit analysis was used to explore trends in yield, equilibrium biomass, reproductive potential, and economic value. The Gulf of Alaska sablefish catch quotas are apportioned to the fixed- and trawl-gear fisheries in the approximate amounts of 86.1% and 13.9%, respectively. The traditional yield per recruit model was modified to incorporate age-specific selectivity rates for each gear type and to compute yield from the combination of the two fisheries. The model also incorporated a discard mortality on undersized fish. With no discard mortality, yield per recruit increases with increasing size limits. However, the increases are significant only at high fishing mortality rates (greater than 0.20). With discard mortality, yields decrease with increasing size limits. Equilibrium biomass and the egg production index increase with increasing size limits, both with and without discard mortality, but the increases are negligible with discard mortality. The effects of a minimum size limit on gross and net value are similar to those on yield, with and without discard mortality, except that the fishing mortality rates that maximize net value are substantially less than those that maximize yield and gross value. As the fishing mortality rate for Gulf of Alaska sablefish is low (0.13), it was concluded that minimum size limits would be ineffective and could even be a detriment because of discard mortality.