Research Document 2018/051
Assessment of Scotian Shelf Snow Crab (Chionoecetes opilio) in 2017
By Zisserson, B.M., Cameron, B.J., Glass, A.C., and Choi, J.S.
Abstract
Landings of Snow Crab (Chionoecetes opilio) in 2017 for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 813 t and 6,719 t, respectively, and they were 80 t in 4X for the 2016/2017 season, representing an increase of 280% (N-ENS) and decreases of 30% (S-ENS) and 47% (4X) relative to the previous year. Total Allowable Catches (TACs) in 2017 were 825 t, 6,730 t and 80 t in N-ENS, S-ENS and 4X, respectively. Non-standardized catch rates in 2017 were 90 kg/trap haul in N-ENS, 94 kg/trap haul in S-ENS, and 25 kg/trap haul in 4X in 2016/2017, which, relative to the previous year, represents decreases of 18%,11% and 19%, respectively. The capture of soft-shelled crab in N-ENS increased to 5% from approximately 1% in 2016. In S-ENS, the relative occurrence of soft-shell crab was 1.5% for the 2017 season. Soft-shell discard rates in 4X remain very low, due to it being a fall and winter fishery. Soft-shell incidence and associated potential handling mortality is forever an issue requiring diligent and adaptive management action. Bycatch of non-target species is extremely low (<0.1%) in N-ENS and S-ENS. Crab Fishing Area (CFA) 4X bycatch levels decreased to <1%, likely due to a constriction of fishery footprint. In both N-ENS and S-ENS, moderate internal recruitment to the fishery is expected for the next year and beyond, based on size-frequency histograms. Crab Fishing Area 4X shows little potential for substantial internal recruitment to the fishery for the next four to five years. Movement will likely be an important source of 4X crab for the next several years. The low abundance of both the mature and immature crab in the adjacent portion of CFA 24 and strong constriction of ideal Snow Crab habitat fields in 4X create future uncertainties. The mature component of female Snow Crab in all areas increased in 2017, due to large scale maturation of a pulse of immature female crab. These population characteristics are tempered by a number of uncertainties, including the influence of predation and rapid temperature swings (especially in CFA 4X and parts of CFA 24). Both can have direct and indirect influences upon Snow Crab, which are cold-water stenotherms. The introduction of the lbm biomass modelling approach for 2016 provided somewhat erratic inter-annual abundance estimates. The current stmv approach has simplified the model inputs and added local temporal smoothing of abundance estimates to provide more reasonable estimates of fishable biomass. The post-fishery modelled fishable biomass of Snow Crab in N-ENS was estimated to be 3,140 t (2,794 in 2016). In S-ENS, the post-fishery modelled fishable biomass index was 37,640 t (40,100 t in 2016). In 4X, the pre-fishery fishable biomass was 120 t (relative to 149 t in 2016/2017). The N-ENS fishing mortality (F) in 2017 has been estimated to have been 0.18 (exploitation rate 0.16), a decrease from 0.33 in 2016. Fishable biomass increased in 2017 and though in the "Healthy" zone, remains low. Recruitment has been low for a number of years but has begun increasing and has the potential to continue for the next 2-5 years, contingent on survival of recruiting size classes. In N-ENS, a moderate decrease in TAC is recommended. The S-ENS fishing mortality (F) in 2017 has been estimated to have been 0.25 (exploitation rate 0.22), a moderate increase from 0.23 in 2016. Good recruitment and a relatively stable size structure suggest a positive outlook. Though the stock is in the "Healthy" zone, modelled biomass estimates are low. In S-ENS, a moderate decrease in TAC is recommended. Fishing mortality in 4X for 2016/2017 was estimated to be 0.36 (0.22 in 2015/2016). Increasing temperature and associated constriction of available viable Snow Crab habitat threatens the existence of the 4X Snow Crab stock. In 4X, the modeled fishable biomass remains low and the stock is currently estimated to be in the "Critical" zone.
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