Science Advisory Report 2009/053

Assessment of Nova Scotia (4VWX) Snow Crab

Summary

• Landings in 2008 for N-ENS and S-ENS were 238 t and 8,253 t, respectively, and they were 230 t in CFA 4X for the 2008/9 season, all within their respective TACs of 244, 8,316, and 230 t. These changes in landings represent increases of 2%, 67% and 5% in N-ENS, S-ENS and 4X, respectively, relative to the previous fishing year.
• Season extensions were granted in all CFAs (except in CFA 4X) against the advice of DFO Science and the consensus decision of Industry Advisory Panels. This was a conservation concern due to elevated soft-shell capture towards the latter part of the season.
• Average, non-standardized catch rates in 2008 were 33.7 kg/trap haul and 96.1 kg/trap haul in N- and S-ENS, and 29.1 kg/trap haul in 4X in 2008/2009 – representing an increase of 43%, a decrease of 4% and an increase of 61%, respectively, relative to the previous year. Much of the increase in N-ENS was due to higher catch rates in the test spring fishery.
• The soft-shelled crab discard represented up to 119 t (49% of landings) and 1,088 t (13% of landings) being subjected to potential handling mortality in N- and S-ENS, respectively. 4X discard rates of soft crab are very low. Almost no soft crab were observed in the spring fishery in N-ENS.
• The post-fishery fishable biomass of snow crab was estimated to be 3,200 t in N-ENS (with a 95% confidence range of: 2,500 to 4,000 t), representing a 200% increase from the previous year. In S-ENS, the post-fishery fishable biomass was 54.3 × 103 t (with a 95% confidence range of: 41.4 to 71.4 × 103 t), representing a 0.3% decrease. In CFA 4X, the pre-fishery fishable biomass was 360 t (with a 95% confidence range of 200 to 350 t), representing a 12.5% increase. Re-scaled fishable biomass estimates were 1,070, 54.5 × 103 and 320 t for N-ENS, S-ENS and CFA 4X, respectively, in 2007.
• The main pulse of male recruitment continues to grow and is currently centered over an 80 mm CW modal group (instars 11-13). The leading edge of recruitment entered in 2007 and full entry is expected by 2010/2011. Recruitment beyond 2014 is uncertain but positive signs were evident.
• The return of immature crab to the water by the fishery is an important conservation measure that will enhance the mid-term (2-3 year) sustainability of this fishery.
• The reproductive potential of the Scotian Shelf population remains strong with berried female abundance in all areas. Strong larval production should continue for another 2-3 years based on current berried female levels.
• The numerical abundance estimates of old males (CC5) are currently below the detection limit on the Scotian Shelf surveys and low as well (approximately 1% or less) in the at-sea observed data.
• A general increase in viable snow crab habitat has been observed since the mid-1990s in N-ENS and S-ENS, while there has been a decline in CFA 4X since the late 1990s. The temperatures within viable snow crab habitat has been stable although stronger inter-annual variability has been evident in CFA 4X, especially since the mid-1990s.
• Potential predators of immature and soft-shelled snow crab continue to be found in areas with high densities of immature snow crab. This adds uncertainty to the potential strength of future recruitment to the fishable biomass.
• By-catch levels are very low in this fishery less than 0.005% in ENS and less than 0.5% in CFA 4X of annual landings, mostly of other crustacean species.
• Relative exploitation rate (by biomass) in N-ENS was 7% in 2008, relative to approximately 18% in 2007. A moderate increase in TAC is recommended contingent upon management measures to lower the handling of soft-shell crab.
• Relative exploitation rate (by biomass) in S-ENS was 13% in 2008, relative to approximately 8% in 2007. The snow crab in S-ENS can be considered to be in a healthy state. A moderate increase in TAC is recommended contingent upon the better adherence of the fleet to the soft-shell protocol and a fixed season duration policy. Shifting the season earlier in the year may help reduce the handling of soft shelled snow crab.
• Relative exploitation rate (by biomass) in CFA 4X was 38% in 2008/2009 relative to 50% in the previous fishing year. A status-quo TAC is recommended until the strength of recovery can be verified.

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