Determination of biological reference points for Bristol Bay red king crab

Abstract

The US Magnuson–Stevens Fishery Conservation and Management Act mandates precautionary management to attain near maximum sustainable yield (MSY)-producing stock and exploitation levels on a long-term basis. Various biological reference points (BRPs) have been suggested as surrogates for MSY-producing instantaneous fishing mortality ( F MSY) and total stock biomass (SSB( F MSY)), which are largely derived from finfish biological and fisheries characteristics, for poorly understood stocks. BRP estimation for some crab fisheries has been complicated by the fact that only males are harvested and that recruitment, in many instances, is largely driven by environmental factors rather than density-dependent responses. For managing Bering Sea and Aleutian Islands crab stocks, instantaneous natural mortality ( M) is used as a proxy for F MSY, and mean annual total mature stock biomass during 1983–1997 is used for SSB( F MSY). In this paper, harvest rate- and biomass-based BRPs were determined using crab-specific parameters for Bristol Bay red king crab ( Paralithodes camtschaticus). Effective spawning stock biomass-per-recruit for combined sexes was estimated by length-based analysis and used in conjunction with Beverton–Holt and Ricker stock–recruit (S–R) models for various parametric values to explore the trends in MSY-producing legal male harvest rate at the time of the fishery ( E( F MSY)), as a proportion of stock harvested; log 10 of ( F MSY/ M) ratio; and total mature stock biomass ratio SSB( F MSY)/SSB(0), where SSB(0) was the virgin total mature stock biomass. The F MSY exceeded M for most cases. For a plausible S–R shape parameter ( τ) range=0.3–0.5, M=0.3, handling mortality proportion=0.2, and instantaneous bycatch mortality=0.01; the SSB( F MSY) ranged from 58 to 74% of SSB(0) for the mating ratio 1:1, and 40–55% of SSB(0) for the mating ratio 1:3. The E( F MSY) ranged 16–35% for the mating ratio 1:1, and 40–79% for the mating ratio 1:3. The E( F MSY) was more sensitive to mating ratio and handling mortality than bycatch mortality. Considering the uncertainty in stock parameters and recruitment, a precautionary legal male target harvest rate below 18% was suggested to rebuild the stock equal to or above 66% of the pristine total mature biomass.