Science Advisory Report 2017/033

Assessment of Nova Scotia (4VWX) Snow Crab

Summary

• Landings in 2016 for N-ENS and S-ENS were 290 t and 9,606 t, respectively, and 142 t in 4X for the 2015/2016 season.  This represents a decrease of 53% in N-ENS, a decrease of 15% in S-ENS, and an increase of 73% in 4X relative to the previous year. Total allowable catches in 2016 were 286 t, 9,614 t and 150 t in N-ENS, S-ENS and 4X (2015/2016 season).
• 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.  Relative to the previous year, this represents an increase of 7%, no change, and a decrease of 9% in N-ENS, S-ENS, and 4X.
• Commercial catches of soft shelled (newly moulted) crab were below 5% in N-ENS and S-ENS for the 2016 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 has been less reliable in 2014 and 2015 based on difficulties in assessment methodology. More robust assessment methodologies have been developed and adopted to help stabilize the biomass estimation process. Current and past biomass estimates have been determined through current methodologies to allow for direct comparison.
• The modelled post-fishery fishable biomass of Snow Crab in N-ENS was estimated to be 3,750 t, relative to 2,799 t in 2015. In S-ENS, the modelled post-fishery fishable biomass was 19,835 t, relative to 25,672 t in 2015. In 4X, the modelled pre-fishery fishable biomass was 907 t, relative to 476 t in the 2015/2016 fishing season.
• The leading edge of a recruitment pulse created substantial increases in the abundance of mature female crab in N-ENS in 2016. S-ENS also showed the first increase in mature female levels since the early to mid-2000s. Large-scale maturation of female crab is expected in N-ENS and S-ENS for the next 2-3 years. Low to moderate future recruitment to the mature segment of the female population in 4X is expected in the next 1-3 years.
• Limited local recruitment into the fishery is expected for the short term in N-ENS, though the leading edge of a recruitment pulse could result in significant recruitment in 2-3 years.  Male crab were observed in all size classes in S-ENS, suggesting continuing recruitment into the future. CFA 4X shows little potential for substantial internal recruitment to the fishery for the next four to five years.
• The relative proportion of CC4 (older since terminal moult) crab has increased in N-ENS in both trawl survey (48%) and commercial catches (14%) in 2016, which is indicative of an aging population. Commercial catches have shown increases in CC4 crab in S-ENS and 4X.
• Bycatch of non-target species is extremely low (<0.1%) in N-ENS and S-ENS. Area 4X bycatch levels decreased to <1%, likely due to a constriction of fishery footprint.
• 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 (<3%). Increasing population trends in these and other predators of Snow Crab could lower future recruitment to the fishable biomass.
• Average bottom temperatures in the 2016 Snow Crab survey were warmer in all areas, which continues a general warming trend observed since the early 1990s. Temperatures are more stable in N-ENS than S-ENS. Area 4X exhibits the most 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 FMSY. Various secondary (population and ecosystem) indicators are taken into consideration for management decisions.
• In N-ENS, current assessment methodologies indicate that the TAC reductions taken in the past two seasons have helped stabilize the fishable component of the population and the stock is in the Healthy Zone. Continuing low recruitment to the fishable biomass promotes a cautious harvest strategy. Based on current fishable biomass estimates an increased TAC is recommended.
• In S-ENS, current assessment methodologies indicate that the fishable biomass has been declining since 2013. Without TAC decreases relative to fishable biomass reductions, the exploitation rate (fishing mortality) has been steadily increasing since 2013. The S-ENS population is considered to be in the Healthy Zone but close to the transitional area. As recruitment is expected for at least the next three to four years, there remains scope for flexibility. A decrease in TAC is strongly recommended.
• As 4X is the southern-most area of Snow Crab distribution, existing in more marginal environments relative to the “prime” areas of S- and N-ENS, an explicitly precautionary approach towards this fishery is essential. Current assessment methodologies indicate that the stock has increased in 2016 but remains in the Cautious Zone.  The erratic temperature fields and constriction of Snow Crab habitat in 4X support the continuation of a very cautious approach in harvesting strategy. In addition, recruitment into next season is uncertain leading to the recommendation of a status quo to a marginal increase in TAC.

This Science Advisory Report is from the February 23, 2017, Stock 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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