Science Advisory Report 2019/053

Assessment of Scotian Shelf Snow Crab

Summary

• In 2018, Snow Crab landings for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 742 t and 6,064 t, respectively, and were 55 t in 4X for the 2017/2018 season, representing decreases of 9% (N-ENS), 10% (S-ENS) and 31% (4X) relative to the previous year. Total Allowable Catches (TACs) in 2018 were 786 t, 6,057 t, and 110 t in N-ENS, S-ENS, and 4X, respectively.
• Non-standardized catch rates in 2018 were 62 kg/trap haul in N-ENS, 116 kg/trap haul in S‑ENS, and 12 kg/trap haul in 4X in 2017/2018, which, relative to the previous year, represents a decrease of 31% (N-ENS), an increase of 23% (S-ENS), and a decrease of 52% (4X).
• Commercial catches of soft-shelled (newly moulted) Snow Crab were 25% in N-ENS and <2% in S-ENS for the 2018 season. This is an increase from 5% in N-ENS and consistent in S-ENS from the previous season. Soft-shelled crab catches for N-ENS were almost exclusively from the summer fishery. Soft-shelled crab discards in 4X are negligible, in large part due to a fall/winter fishery.
• A novel modelling approach has been developed for Scotian Shelf Snow Crab, incorporating environmental and ecosystem variables while accounting for spatial and temporal variability. The resulting abundance estimates currently show unrealistically low inter-annual variability.
• Abundance estimates and landings are used to fit a logistic population model that provides  the modelled fishable biomass and reference points.
• The modelled post-fishery fishable biomass index of Snow Crab in N-ENS was estimated to be 3,203 t in 2018, relative to 3,358 t in 2017. In S-ENS, the modelled post-fishery fishable biomass index was 33,190 t in 2018, relative to 32,040 t in 2017. In 4X, the modelled fishable biomass was 403 t in 2018, relative to 354 t in 2017.
• Hyper-stable inter-annual fishable biomass estimates (incongruous with other data sources such as length frequencies, catch rates, and survey catch densities) are minimally informative to harvest strategies.
• In all crab fishing areas, maturation of a recruitment pulse of female crab since 2016 has created substantial increases in the abundance of mature female crab and the proportion of mature female to male crab. The majority of female crab in all areas are now mature. Egg production is currently high.
• Moderate internal recruitment to the fishery is expected for the next year in N-ENS and S‑ENS and is possible for the next 3–4 years, based on population size structure. Emigration, increased mortality, or sublegal sized terminal moult can lower expected recruitment. Internal recruitment in 4X is expected to be minimal.
• Bycatch of non-target species is extremely low (<0.4%) 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 on the Scotian Shelf. Continuing increases in Halibut biomass lowers both the abundance and reproductive potential of Snow Crab on the Scotian Shelf.
• Average bottom temperatures in the 2018 Snow Crab survey were near the long-year median in all areas. A general warming trend has been observed since the early 1990s on the Scotian Shelf. Temperatures are more stable in N-ENS than S-ENS. Annual mean bottom temperatures in 4X are more erratic.
• A reference points-based Precautionary Approach has been implemented in this fishery. The Limit Reference Point is 25% of carrying capacity and the Upper Stock Reference 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 . Various secondary (population and ecosystem) indicators are taken into consideration for management decisions.
• In N-ENS, the harvest strategy for the past two seasons appears to have been overly aggressive, with declining catch rates and increasing soft-shelled crab catches. A more conservative harvest approach, that lowers exploitation in N-ENS, could stabilize catch rates, protect incoming recruitment essential to the fishery, and allow commercial biomass to rebuild.
• In S-ENS, substantial TAC reductions over the past three seasons have helped maintain stable fishery performance. Geometric mean catches from the survey are stable and, based on stock structure, increased recruitment to the fishery is likely to occur for the upcoming season. A moderately more aggressive harvest strategy would be appropriate under these conditions.
• In 4X, low recruitment, high inter-annual temperature fluctuations, and overall warm water temperatures create uncertainties about this population. A zero TAC was set for the 2018/2019 fishing season due to low commercial biomass. Catches from the Snow Crab survey indicate that 4X commercial biomass levels remain low but have improved.

This Science Advisory Report is from the February 27, 2019, 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.

Science Advisory Report 2019/053 (PDF, 2,208 KB)

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