Research Document - 2012/024
Assessment of Scotian Shelf Snow Crab in 2011
By J.S. Choi, B.M. Zisserson, and B.J. Cameron
Abstract
Landings in 2011 were 536 t in North-Eastern Nova Scotia (N-ENS) and 12,135 t for South-Eastern Nova Scotia (S-ENS), and they were 345 t for Crab Fishing Area (CFA) 4X in 2010/2011. The associated total allowable catches (TACs) were 534 t, 12,120 t, and 346 t, respectively. Average, non-standardized catch rates were 110.1, 106.4, and 38.3 kg trap-1, respectively. These catch rates represent a doubling for N-ENS, a 3.8% increase in S-ENS, and a 6.3% increase in 4X, relative to 2010. The capture of soft-shelled crab in N-ENS declined from 3.5% in 2010 to 1.7% in 2011. This continues an improving trend in soft crab rate since 2007 in N-ENS when 111% of the landings were estimated to have been discarded as soft crab. After a successful trial in 2008, the majority of landings (>85%) from N-ENS have been caught during the spring season. In S-ENS, soft-shell handling decreased from 7% of landings in 2010 to 5.5% in 2011, a decline in part, attributable to earlier fishing seasons. Soft-shell discard rates in CFA 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 action. Bycatch of non-target species is extremely low, 0.01% and 0.9% of total snow crab landings in ENS and 4X, respectively, over the past four years. Recruitment into the fishery is expected to continue for the next two to four years in N-ENS. Male crab were observed in all size classes in S-ENS, suggesting stable recruitment into the future. In 4X, a lack of crab between 50 and 90 mm suggests potential reductions of recruitment in the two to four year range. This may be offset by movement of crab into the area from S-ENS. 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. Potential larval production peaked in 2007/2008 and is now on a slight declining trend; it should continue for another one to two years in N-ENS and longer in S-ENS and 4X. The post-fishery fishable biomass of snow crab in N-ENS was estimated to be 3,010 t (3,170 t in 2010). In S-ENS, the post-fishery fishable biomass of snow crab was estimated to be 45.8 × 103 t (47.3 × 103 t in 2010). In 4X, the pre-fishery fishable biomass was 540 t (540 t in 2010/2011). 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, 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 0.15 (harvest rate of 14%), relatively unchanged since 2009. Good recruitment and significantly reduced soft-shell handling results in a positive outlook. A strong and persistent increase in fishable biomass was observed. The fishable biomass is in the “healthy” zone, where harvest rates between 10% and 20% are part of the strategy for sustainability in this fishery. A status quo to a marginal increase in harvest strategy (rate) is recommended. Fishing mortality in S-ENS was estimated to be 0.22 (harvest rate of 20.5%), relatively unchanged since 2010. Good recruitment suggests a positive outlook; however, the capture of soft-shell crab remains an important issue for this fleet. The fishable biomass is in the “healthy” zone, where harvest rates between 10% and 30% are part of the strategy for sustainability in this fishery. A status quo to a marginal increase in harvest strategy (rate) is recommended. Fishing mortality in 4X for 2010/2011 was estimated to be 0.44 (harvest rate of 35.6%). The fishable biomass is in the “healthy” zone, where harvest rates between 10% and 30% are part of the strategy for sustainability in this fishery. As recruitment into the 2011/2012 season is uncertain, a decreased harvest strategy (rate) is recommended.
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