Science Advisory Report  2015/034

Assessment of Nova Scotia (4VWX) Snow Crab

Summary

• Landings in 2014 for N-ENS and S-ENS were 778 and 11,267 t, respectively, and they were 79 t in 4X for the 2013/2014 season, representing similar landings to the previous year in S-ENS and N-ENS and 33% decrease in 4X relative to the previous year. Total Allowable Catches (TACs) in 2014 were 783 t, 11,311 t and 80 t in N-ENS, S-ENS and 4X, respectively.
• Non-standardized catch rates in 2014 decreased by 2% in N-ENS and increased by 8% in S-ENS and 25% in 4X, relative to the previous year.
• In N-ENS, the estimated soft shell crab discard rate (% of total landings) was 3.2% in 2014, consistent with 3.4% in 2013. This is substantially lower than previous levels (111% in 2007), due to an increase in spring fishing and an ageing fishable stock. In S-ENS, 2014 estimated soft shell discards were 1.9% of landings, relative to 1.7% in 2013. The shift towards earlier fishing seasons has improved soft shell crab handling rates in both N-ENS and S-ENS though continued diligence is important to protect incoming recruitment. Soft shell discards in 4X are negligible, in large part due to a fall / winter fishery.
• The post-fishery fishable biomass decreased by 40% (2,075 t relative to 3,451 t in 2013) in N-ENS and by 13% (40,119 t relative to 46,203 t in 2013) in S-ENS.  In 4X, the biomass estimate increased by 388% (2059 t relative to 531 t in 2013), based on only a subset of stations completed in 2014. The results in 4X should be considered preliminary, pending further analysis of survey and commercial fishing data. In all areas, results were more uncertain in 2014 compared to previous years due to changes in survey timing, survey duration and vessel. An additional uncertainty exists in N-ENS, surrounding migration into the area which is not detected during trawl surveys, which may affect the biomass available during the fishery.
• Based on size-frequency histograms of the male snow crab population, very limited internal recruitment to the fishery is expected in N-ENS and 4X. S-ENS internal recruitment is expected to remain at moderate levels. Migration of crab from outside a given area can serve as recruitment to its fishery though is unreliable based on its apparent episodic nature.
• Mature female Snow Crab abundance has continued to decline in N-ENS and S- ENS and assumed associated egg production continues to decline after reaching highs in 2007/2008. Sex ratios (proportion female) continue to be male-dominated in N-ENS (though slightly higher than 2013), are stabilizing at low levels in S-ENS and declined from more moderate levels in 4X. There was a strong pulse of immature crab observed in the 2014 survey in all areas, which may provide increased production in the future. Newly mature female crab are expected in the next 1-3 years in N-ENS and S-ENS. In 4X there is more uncertainty surrounding short term expectations of egg production.
• Estimated fishing mortality rates for 2014 were 0.30, 0.18 and 0.08 for N-ENS, S-ENS and 4X, respectively.
• Average bottom temperatures in 2014 were slightly warmer in N-ENS and S-ENS compared to 2013, which continues a general warming trend observed since the early-1990s. The bottom temperatures decreased in 4X in each of the past two years after the extreme warm water event of 2012 though still remain above the long-term mean for the area. Changing temperatures will have an impact on available Snow Crab habitat into the future.
• Atlantic Halibut, Atlantic Wolfish and skate species appear to be the predominant predators of snow crab on the Scotian Shelf though snow crab does not appear to be an important part of their diet (2.2%). Population trends from these species are uncertain though increased predation is likely to lower future recruitment to the fishable biomass and affect movement patterns of Snow Crab.
• Bycatch levels are extremely low in this fishery. In N-ENS and S-ENS, combined estimates of bycatch were 0.01% of Snow Crab landings. 4X shows higher (relative to ENS) bycatch rates at 2.2 % of Snow Crab landings.
• The potential impacts of seismic activity on Snow Crab productivity are unknown but anecdotal evidence suggests it may be detrimental to recruitment and migrations. Unreported or misreported landings may further increase uncertainty in the stock assessment results. Anecdotal evidence from commercial fishers suggest unreported landings may be occurring in S-ENS.
• A reference points-based Precautionary approach (PA) has been implemented in this fishery. The Limit reference point (LRP) is 25% of carrying capacity and the Upper stock reference (USR) is 50% of carrying capacity. The Target Removal reference is 20% of the fishable biomass (FB) in each area and the Removal reference is not to exceed F_MSY. Various secondary (population and ecosystem) indicators are taken into consideration for management decisions.
• In N-ENS, fishable biomass has declined over the past three years, although the stock remains in the “healthy” zone (FB > USR), it is getting close to the “cautious” zone. A gap in the length frequency histogram suggests limited local recruitment in the medium to long-term. Based on the biomass trends obtained during the survey a decreased TAC is recommended.
• S-ENS fishable biomass is considered to be in the “healthy” zone (FB > USR). Despite the decrease in fishable biomass it continues to be high relative to historic levels and with recruitment expected for at least the next three to four years, there remains scope for flexibility. A status-quo to marginal decrease in TAC is recommended.
• In 4X, fishable biomass has moved from the “cautious” zone (LSR < FB < USR) into the “healthy” zone (USR < FB); however there is uncertainty in this biomass estimate as the westernmost portion of the survey stations in 4X were not completed. In addition, recruitment into next season is uncertain leading to the recommendation of a continued conservative harvest strategy pending further analysis prior to the 2015-2016 season.

This Science Advisory Report is from the February 24, 2015, Assessment of Nova Scotia (4VWX) Snow Crab. Additional publications from this process will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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