Research Document 2017/053
Assessment of Scotian Shelf Snow Crab in 2016
By Hubley, P.B., Zisserson, B.M., Cameron, B.J., and Choi, J.S.
Abstract
Landings in 2016 for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 290 t and 9,606 t, respectively, and they were 142 t in 4X for the 2015/2016 season, representing a decrease of 53% (N-ENS) and 15% (S-ENS) and an increase of 73% (4X) relative to the previous year. Total Allowable Catches in 2016 were 286 t, 9,614 t and 140 t in N-ENS, S-ENS and 4X, respectively. Non-standardized catch rates in 2016 were 110 kg/trap haul in N-ENS, 106 kg/trap haul in S-ENS, and 31 kg/trap haul in 4X in 2015/2016 – which relative to the previous year represents an increase of 7%, no change and an decrease of 9%, respectively. The capture of soft-shelled crab in N-ENS remained below 1% over the past two seasons. In S-ENS, the relative occurrence of soft-shell crab was 4.5% for the 2016 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. Although very limited local recruitment into the fishery is expected for the short term in N-ENS, the persistent gap between immature and mature male crab continues to shrink. The leading edge of this recruitment pulse could result in significant recruitment in 2-3 years. Male crab were observed in all size classes in S-ENS, suggesting stable recruitment into the future. Crab Fishing Area 4X shows little potential for substantial internal recruitment to the fishery for the next 4 to 5 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 appear to be increasing, likely to be further bolstered by strong signals of immature female Snow Crab in both N-ENS and S-ENS. The post-fishery fishable biomass index of Snow Crab in N-ENS was estimated to be 3,750 t (2,799 t in 2015). In S-ENS, the post-fishery fishable biomass index was estimated to be 19.8 × 103 t (25.7 ×103 t in 2015). In 4X, the pre-fishery fishable biomass was 907 t (476 t in 2015/2016). These population characteristics are tempered by a number of uncertainties, including the influence of predation, especially upon immature and soft-shelled Snow Crab by groundfish, as well as large and rapid temperature swings (especially in CFA 4X and parts of CFA 24), as they can have both direct and indirect influences upon Snow Crab, which are cold-water stenotherms. Signs of a potential return of ecological, social, and economic indicators in the direction of a system less dominated by invertebrates, adds further uncertainty to the medium- to long-term sustainability of the population. Fishing mortality in N-ENS was estimated to be 0.07, a sharp decrease from 0.20 in 2015 and 0.28 in 2014. Fishable biomass had been decreasing, mainly through a lack of recruitment, but has recovered somewhat since 2014. In N-ENS, long-term prospects have improved due to the presence of pre-recruits between 30 and 80 mm. The fishable biomass has returned to the “healthy” zone likely supported by a reduction of fishing mortality in last year’s fishery. In N-ENS, a modest increase in TAC is recommended. Fishing mortality in S-ENS was estimated to be 0.39, an increase from 0.26 in 2013. Good recruitment still suggests a positive outlook. The fishable biomass remains in the “healthy” zone, but is declining and very close to the “cautious” zone. In S-ENS a decrease in harvest strategy is recommended. Fishing mortality in 4X for 2015/2016 was estimated to be 0.26. In 4X, the modeled fishable biomass is highly erratic and is currently estimated to be in the “cautious” zone, albeit with considerable uncertainty, and recruitment into next season is uncertain. A continued conservative harvest strategy is recommended pending further analysis prior to the 2015-2016 season.
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