Reproductive buffers on exploitation in male-only fisheries: Tanner Crab (Chionoecetes bairdi) management strategy evaluation case study

Abstract

Eastern Bering Sea Tanner crab (Chionoecetes bairdi), one of Alaska's more cyclical fisheries, has one of the most complex harvest strategies of any Bering Sea-Aleutian Islands crab stock. This stock experiences major fluctuations in annual abundance and, although a male-only fishery, the state harvest control rule (HCR) used to set the allowable catch depends on the abundance of females. Failure to meet minimum thresholds on female abundance has led to fishery closures during five of the last ten fishing seasons. The economic performance of the fishery, linked to its frequent closures required by current management regulations, led to an industry-initiated effort to modify the state HCR to better account for stock status and reduce large inter-annual changes in catch, as well as to evaluate the utility of including mature female biomass in the HCR. A workshop involving stakeholders, university affiliates, and managers concluded that management strategy evaluation was the most appropriate way to test candidate HCRs. Fifteen HCRs with various levels of incorporation of female stock levels (from primary to ignored) were tested using 100-year forecasts to evaluate how including females in harvest policies affected stock sustainability and productivity based on conservation and economic criteria. Several male-only HCRs performed similarly to HCRs that accounted for females, including those using mature female biomass (MFB) to determine the maximum exploitation rate on mature male biomass (MMB) as part of the HCR rather than as a threshold to open the fishery. HCRs including both sexes appropriately balanced conservation and economic considerations while acknowledging the uncertainty around reproductive dynamics given the lack of an identified stock-recruitment relationship. The harvest policy selected by the Alaska Board of Fisheries included: 1) a threshold for opening the fishery of 25% of average MMB during 1982–2018, 2) an exploitation rate for males based on the ratio of MMB to its long-term average, and 3) a maximum exploitation rate determined by the ratio of MFB to average MFB during 1982-2018. This HCR also included a maximum exploitation of 50% of exploitable, industry-preferred size legal male abundance.