Research Document - 2008/012
Integrated assessment of the snow crab resident on the Scotian Shelf in 2007
By Choi, J.S., B.M. Zisserson and P. Kuhn
Abstract
Landings in 2007 were 233 and 4,942 t for N- and S-ENS, respectively and 317 t for CFA 4X in 2006/2007. The associated TACs were 244, 4950 and of 337.6 t, respectively. Average, non-standardized catch rates were 23.6, 100.1 and 27.7 kg trap-1, respectively. These catch rates were the lowest in the historical record for N-ENS, an increase in S-ENS, and a marginal decline in CFA 4X, relative to 2006. Soft-shelled crab incidence in the commercial catch of legal sized crab was very high in N-ENS at 41%. In contrast, S-ENS maintained a lower rate of incidence at 9% even with the influx of recruitment to the area. CFA 4X had virtually no soft-shell incidence due to their offset season. Soft-shell incidence will continue to be an issue in N-ENS in 2008. By-catch of non-target species is estimated to be less than 0.02% and 0.33% of total snow crab landings in ENS and CFA 4X, respectively.
In the short-term, recruitment is expected to continue to increase for the next 4 years. Pre-recruits near a 68 mm CW modal group (instars 10/11) have been found in large numbers. The leading edge of this modal group have begun to recruit to the fishable biomass in 2007 and full entry is expected in 2011. In the long-term, the reproductive potential of the Scotian Shelf population has increased with the substantial increase in berried female abundance in all areas. Larval production should continue for another 4 years. However, potential predators of (immature and soft shelled) snow crab have been found concentrated in areas with high densities of immature snow crab. This adds uncertainty to the potential strength of future fishable biomass recruitment. Increasing bottom temperatures on the Scotian Shelf resulting in shrinking of potential habitat is also an uncertainty that may have particularly negative consequences upon the snow crab resident in CFA 4X and parts of S-ENS.
The post-fishery fishable biomass of snow crab was estimated to be 970 t (with a 95% confidence range of: 730 t to 1,230 t) – an increase of approximately 28% relative to 2006 (750 t). This increase was most evident in the northern basin of N-ENS. In S-ENS, the post-fishery fishable biomass increased rapidly in 2007 to 41.59 x 103 t (with a 95% confidence range of: 37.26 to 46.32 x 103 t) – an increase of 47% from 28.22 x 103 t in 2006. The majority of the increases were evident in all core areas of S-ENS, including the Chedabucto area. In CFA 4X, fishable biomass was estimated to be 1,030 t (with a 95% confidence range of: 470 to 1,710 t). This represents no significant change relative to 2006, however the uncertainty about the 2007 estimate was double that of 2006, mostly due to the large temperatures differences between the two years for the 4X region and the lack of sampling stations in these potential habitats.
Relative exploitation rates (by biomass) in N-ENS were 19% in 2007. Projections suggest that an exploitation rate between 10 and 20% may be suitable for long-term sustainability in N-ENS, especially as the area is not able to manage soft-shell capture. A reduction in TAC is recommended.
Relative exploitation rates in S-ENS were 10% in 2007. Maintaining exploitation rates between 10% and 30% would provide the greatest longevity to this fishery. Good recruitment and good control of soft-shell crab capture suggests a positive outlook. An increase in TAC is recommended.
Relative exploitation rates in CFA 4X for 2006/7 was 23%. The region has been exploiting crab at rates between 18 and 36%, which is closer to the range observed in N-ENS, where environmental conditions are thought to be more favourable to crab. Caution is warranted for 2007/8 due to low fishable biomass, high exploitation rates and variable environmental conditions. However the presence of immigration and low soft-shell capture suggests a slightly more positive outlook. A status-quo to reduction in TAC is recommended.
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