Research Document - 2009/086

Integrated assessment of the snow crab resident on the Scotian Shelf in 2008

By J.S. Choi and B.M. Zisserson

Abstract

Landings in 2008 were 238 and 8,253 t for northern and southern areas of Eastern Nova Scotia (ENS), respectively, and 230 t for Crab Fishing Area (CFA) 4X in 2007/2008. The associated Total Allowable Catches (TACs) were 244, 8,316 and of 230 t, respectively. Average, non-standardised catch rates were 33.7, 96.1 and 29.1 kg trap-¹, respectively. These catch rates represent a 43% increase for N-ENS (though still well below the 12 year mean of 53.8 kg trap-¹), a marginal decrease in S-ENS, and a 61% increase in CFA 4X, relative to 2007. Soft-shelled crab incidence in the commercial catch continued to be high in N-ENS at 49%. In contrast, S-ENS maintained a lower rate of incidence at 13%, 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 and S-ENS in 2009. Bycatch of non-target species is estimated to be less than 0.005% and 0.5% of total snow crab landings in ENS and CFA 4X, respectively.

In the short-term, recruitment is expected to continue for the next 4-5 years in all areas, with the exception of CFA 4X. Pre-recruits near the 80 mm carapace width (CW) modal group (instars 11-13) have been found in large numbers. The leading edge of this modal group began recruiting to the fishable biomass in 2007 in S-ENS and in 2008 in N-ENS. Full entry is expected in 2010/2011. In the long-term, as the reproductive potential of the Scotian Shelf population peaked in 2007 and is now on a declining trend, larval production should continue for another 3-4 years. However, potential predators of immature and soft-shelled snow crab have been found concentrated in areas with high densities of immature snow crab. Increasing bottom temperatures on the Scotian Shelf and an associated reduction of potential snow crab habitat may have negative consequences, especially in CFA 4X and parts of S-ENS. In addition to these factors, the signs of a gradual return of ecological, social and economic indicators of system state, in the direction of a low invertebrate dominated system adds, uncertainty to the medium- to long-term sustainability of the snow crab population.

The post-fishery fishable biomass of snow crab was estimated to be 3,200 t (with a 95% confidence range of: 2,500 t to 4,000 t) – an increase of approximately 200% relative to 2007 (1,070 t). This increase was most evident in the northern basin of N-ENS. In S-ENS, the post-fishery fishable biomass declined marginally to 54.3 × 10³ t (with a 95% confidence range of: 41.4 to 71.4 × 10³ t) – a decrease of 0.3% from 54.5 × 10³ t in 2007. The majority of the increases were evident in all core areas of S-ENS. In CFA 4X, the pre-fishery fishable biomass was 360 t (with a 95% confidence range of 200 to 570 t) – an increase of 12.5% from 320 t in 2007.

Relative exploitation rates (by biomass) in N-ENS were 7% in 2008. Projections suggest that an exploitation rate between 10 and 20% may be suitable for long-term sustainability in N-ENS. Good recruitment suggests a positive outlook although soft-shell handling remains an issue. A moderate increase in TAC is recommended.

Relative exploitation rates in S-ENS were 13% in 2008. Maintaining exploitation rates between 10% and 30% may provide the greatest longevity to this fishery. Good recruitment and good control of soft-shell crab capture suggests a positive outlook. A moderate increase in TAC is recommended.

Relative exploitation rates in CFA 4X were 38% in 2008/2009. The region is currently exploiting crab at the highest rates on the Scotian Shelf even though environmental conditions are the least favourable. Exploitation rates between 10% and 30% may provide the greatest longevity to this fishery. A status-quo TAC is recommended until the strength of recovery can be verified.

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