Research Document - 2011/005
Assessment of Scotian Shelf Snow Crab in 2009
By J.S. Choi and B.M. Zisserson
Abstract
Landings in 2009 were 579 and 10,648 t for northern and southern areas of Eastern Nova Scotia (ENS), respectively and 229 t for Crab Fishing Area (CFA) 4X in 2008/2009. The associated TACs were 576, 10,800 and of 230 t, respectively. Average, non-standardized catch rates were 75.7, 89.6 and 28.4 kg trap-1, respectively. These catch rates represent a 125% increase for N-ENS (above the 13 year mean of 55.4 kg trap-1), a marginal decrease in S-ENS, and a 57% increase in CFA 4X, relative to 2008.
The capture of soft-shelled crab in N-ENS declined from 49% in 2008 to 6% in 2009, largely due to 86% of the total N-ENS landings being captured in the spring, when soft-shell catchability is lower. In contrast, in S-ENS, the capture of soft-shelled crab increased from 13% in 2008 to 19% in 2009. CFA 4X had virtually no soft-shell incidence due to their winter season. Soft-shell incidence and associated potential handling mortality will continue to be an issue in 2010 due to continued recruitment in S-ENS. By-catch of non-target species is low, being estimated to be less than 0.013% and 0.783% of total snow crab landings in ENS and CFA 4X, respectively, over the past four years.
Recruitment into the fishery is expected to continue for the next 4-5 years in all areas. High densities of adolescents between 70 to 80 mm CW (carapace width) have been observed in ENS. In CFA 4X, high densities of crab between 20 to 40 mm CW were observed. 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 CFA 4X. Full entry is expected in 2011 to 2014, depending upon area. The reproductive potential of the Scotian Shelf population peaked in 2007/2008 and is now on a declining trend. Larval production should continue for another 2-3 years.
The post-fishery fishable biomass of snow crab was estimated to be 1,342 t (with a 95% confidence range of: 946 t to 2,059 t), relative to 4,836 t in 2008 in N-ENS. In S-ENS, the post-fishery fishable biomass increased to 66.2 × 103 t (with a 95% confidence range of: 55.7 to 77.2 × 103 t), relative to 45.8 × 103 t in 2008. In CFA 4X, the pre-fishery fishable biomass was 1,730 t (with a 95% confidence range of 580 to 5,070 t), relative to 1,180 t in 2008/2009. Large uncertainties are associated with the abundance estimates in S-ENS and CFA 4X due to the extreme temperature conditions and associated large-scaled shifts in spatial distributions observed in 2009.
These positive population characteristics are tempered by a number of additional uncertainties: The influence of predation, especially upon immature and soft shelled snow crab by groundfish. Large and rapid temperature swings as they can have both direct and indirect influences upon snow crab. For example, a strong warming event can have direct deleterious effects as snow crab are cold-water stenotherms. Even the very significant cooling trend observed throughout the Scotian Shelf in 2009 with an associated habitat expansion was observed can have negative indirect consequences such as the introduction or proliferation of invasive species or disease (e.g, >Hematodinium sp.) or the reduction of gross primary production. In addition to these factors, the signs of an initial return of ecological, social and economic indicators of system state in the direction of a low invertebrate dominated system adds, further uncertainty to the medium to long-term sustainability of the Scotian Shelf snow crab population.
Relative exploitation rates (by biomass) in N-ENS were 29% in 2009 (5% in 2008). Projections suggest that an exploitation rate between 10 and 20% may be suitable for long-term sustainability in N-ENS. Good recruitment and a significantly reduced soft-shell handling result in a positive outlook. However, the rapid and unexpected decline in fishable biomass in 2009 tempers this positive outlook. Until a strong and persistent increase in fishable biomass is observed, a decrease in TAC is recommended.
Relative exploitation rates in S-ENS were 14% in 2009 (15% in 2008). Maintaining exploitation rates between 10% and 30% may provide the greatest longevity to this fishery. Good recruitment suggests a positive outlook; however, the capture of soft shell crab remains an important issue for this fleet. An increase in TAC is recommended.
Relative exploitation rates in CFA 4X for 2009/2010 were 12%; for 2008/2009, they were 16%. Exploitation rates between 10% and 30% may provide the greatest longevity to this fishery. A moderate increase in TAC is recommended.
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