Research Document - 2013/072
Assessment of Scotian Shelf Snow Crab in 2012
By J.S. Choi, B.M. Zisserson, and B.J. Cameron
Abstract
Landings in 2012 for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 603 and 11,707 t, respectively, and 345 t in 4X for the 2011/2012 season, representing an increase of 13% (N-ENS), a decrease of 4% (S-ENS) and no change relative to the previous year. Total Allowable Catches TAC's) in 2012 were 603 t, 11,733 t and 346 t in N-ENS, S-ENS and 4X. Average, non-standardized catch rates were 117, 98, and 29 kg trap-1, respectively. These catch rates represent an increase of 6%, and decreases of 8% and 24%, respectively, relative to the previous year. The capture of soft-shelled crab in N-ENS increased to 9% of landings from 1.7% in 2011, though they are still lower than the soft crab catches in 2005-2008. This increase is likely associated with a return to more summer fishing activities (less spring fishing) in 2012. In S-ENS, the relative occurrence of soft-shell crab was 6.3%, unchanged since 2011 and lower than the soft crab catches in 2010 and 2009 (8% and 16%, respectively). This decline is also attributable to earlier fishing activities. Soft-shell discard rates in 4X remain very low, due to it being a fall and winter fishery. Soft-shell incidence and associated potential handling mortality continues to be an issue requiring diligent and adaptive management action. Bycatch of non-target species is extremely low, 0.0% and 0.2% of total snow crab landings in Eastern Nova Scotia (ENS) and 4X, respectively, over the past three years. Very limited recruitment into the fishery is expected for the short to medium term in N-ENS due to a lack of male crab between 20 and 85 mm carapace width. Male crab were observed in all size classes in S-ENS, suggesting more stable recruitment into the future. 4X shows little potential for substantial internal recruitment to the fishery in the next three to four years. Movement has likely been an important source of 4X crab in the past. The low abundance of both the mature and immature crab in the adjoining portion of Area 24 and erratic temperature fields in 4X create future uncertainties. The leading edge of the current recruitment pulse began entering the fishable biomass in 2007 in S-ENS; 2008 in N-ENS; and 2009 in 4X. Female snow crab in all areas appear to be completely recruited as of the 2012 survey with a distinct lack of immature snow crab in all areas. Current egg and larval production is expected to be low in both S-ENS and 4X and nearly non-existent in N-ENS. The post-fishery fishable biomass index of snow crab in N-ENS was estimated to be 3,130 t (3,430 t in 2011). In S-ENS, the post-fishery fishable biomass index was estimated to be 34.1 × 103 t (35.4 × 103 t in 2011). In 4X, the pre-fishery fishable biomass was 1.7 t (2.4 t in 2011/2012). These generally positive population characteristics are tempered by a number of uncertainties, including the influence of predation, especially upon immature and soft-shelled snow crab by groundfish, as well as large and rapid temperature swings (especially in 4X and parts of Area 24), as they can have both direct and indirect influences upon snow crab, which are cold-water stenotherms. Signs of a potential return of ecological, social, and economic indicators in the direction of a system less dominated by invertebrates adds further uncertainty to the medium- to long-term sustainability of the population. Fishing mortality in N-ENS was estimated to be 0.11, relatively unchanged since 2009. High biomass and significantly reduced soft-shell handling results in a positive short-term outlook. Medium to long-term prospects are poor due to an absence of pre-recruits. The fishable biomass is in the “healthy” zone. A status quo TAC is recommended. Fishing mortality in S-ENS was estimated to be 0.21, relatively unchanged since 2011. Good recruitment suggests a positive outlook; however, the capture of soft-shell crab and illegal landings remain important issues for this fleet. The fishable biomass is in the “healthy” zone. A status quo to a marginal decrease in harvest strategy is recommended. Fishing mortality in 4X for 2010/2011 was estimated to be 0.23. The fishable biomass is in the “healthy” zone. However, as recruitment into the 2011/2012 season is uncertain and biomass trends are declining, a decreased harvest strategy (rate) is recommended.
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