Science Advisory Report 2018/046

Assessment of Nova Scotia (4VWX) Snow Crab

Summary

• Landings in 2017 for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 819 tonnes (t) and 6,723 t, respectively, and they were 80 t in 4X for the 2016/2017 season, representing an increase of 180% (N-ENS) and decreases of 30% (S-ENS) and 44% (4X) relative to the previous year. Total Allowable Catches (TACs) in 2017 were 825 t and 6,730 t in N-ENS and S-ENS, respectively, and 80 t for 2016/17 in 4X.
• 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.
• Commercial catches of soft-shelled (newly moulted) crab were below 6% in N-ENS and S-ENS for the 2017 season. 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.
• Fishable biomass estimation was less reliable in 2014 and 2015 due to difficulties in assessment methodology. A new biomass estimation methodology was introduced in 2016 and further refined in 2017, which relates habitat and abundance with environmental and ecosystem variables while also accounting for spatial and temporal variation. Current and past biomass estimates have been determined through current methodologies to allow for direct comparison.
• Changes in the 2017 assessment further simplified model inputs (removing some ecosystem parameters) and added localized temporal smoothing. As a result, fishable biomass estimates are less variable from year to year.
• The modelled post-fishery fishable biomass index of Snow Crab in N-ENS was estimated to be 3,140 t in 2017, relative to 2,794 t in 2016. In S-ENS, the modelled post-fishery fishable biomass index was 37,640 t in 2017, relative to 40,100 t in 2018. In 4X, the modelled pre-fishery fishable biomass was 120 t for 2017/18, relative to 149 t in 2016/2017.
• In N-ENS and S-ENS, maturation of a recruitment pulse of female crab began in 2016 and continued in 2017, creating substantial increases in the abundance of mature female crab and the proportion of mature female to male crab. Area 4X also saw substantial female maturation in 2017 though at density levels lower than other areas. The majority of female crab in all areas are now mature.
• Egg production is expected to increase due to increased number of mature females and the larger egg clutch size in multiparous females.
• Moderate internal recruitment to the fishery is expected for the next year in N-ENS and S-ENS and is possible for the next 4-5 years, based on population size structure. Emigration, increased mortality, or sublegal sized terminal moult can lower expected recruitment. Area 4X internal recruitment is expected to be very minimal.
• Bycatch of non-target species is extremely low (<0.2%) in all Snow Crab fishing areas.
• Based on stomach sampling, Atlantic Halibut, Atlantic Wolffish, Thorny Skate, and other skate species appear to be the predominant predators of Snow Crab, though it does not appear to represent more than 3% of their diet on the Scotian Shelf.
• Average bottom temperatures in the 2017 Snow Crab survey were cooler in all areas than in 2016, which varies from the general warming trend observed since the early 1990s. Temperatures are more stable in N-ENS than S-ENS. Area 4X exhibits more erratic annual mean bottom temperatures.
• 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 in each area and the Removal reference is not to exceed fishing mortality at Maximum Sustainable Yield (F_MSY). Various secondary (population and ecosystem) indicators are taken into consideration for management decisions.
• The N-ENS population is considered to be in the “Healthy” zone. Current fishable biomass estimates are below the long-term mean. Recruitment is expected to continue in coming years. A moderate TAC reduction is recommended.
• The S-ENS population is considered to be in the “Healthy” zone. Fishable biomass estimates have continued to decline in spite of TAC reductions. Current fishable biomass estimates are below the long-term mean. Recruitment is expected for at least the next three to four years. A moderate TAC reduction is recommended.
• In 4X, low recruitment, high inter-annual temperature fluctuations and overall warm water temperatures create uncertainties about this population. The current assessment methodology indicates that the stock is in the “Critical” zone.

This Science Advisory Report is from the February 23, 2018, Assessment of Scotian Shelf 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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