Science Advisory Report  2014/033

Assessment of Nova Scotia (4VWX) Snow Crab

Summary

• Landings in 2013 for N-ENS and S-ENS were 783 and 11,309 t, respectively, and they were 118 t in 4X for the 2012/13 season, representing an increase of 17%, a decrease of 3% and a decrease of 66% relative to the previous year, respectively. Total allowable catches (TACs) in 2013 were 783,11,311 and 263 t in N-ENS, S-ENS and 4X, respectively.
• Non-standardised catch rates in 2013 decreased by 10% in N-ENS, increased by 6% in S-ENS and decreased by 57% in 4X, relative to the previous year.
• In N-ENS, the estimated soft shell crab discard (% of total landings) decreased to 3.4% in 2013 as compared to 8.9% in 2012. This is substantially lower than previous levels (111% in 2007), due to an increase in spring fishing and an ageing fishable stock. In S-ENS, 2013 estimated soft shell discards decreased from 2012 from 6.3 to 1.7% of the TAC. The shift towards earlier fishing seasons has improved soft shell crab handling rates in both N- and S-ENS though continued diligence is important to protect incoming recruitment. Soft shell discards in 4X are negligible, in large part due to a fall / winter fishery.
• The post-fishery fishable biomass decreased by 17% in N-ENS and by 1% in S-ENS relative to 2012. In 4X, the preliminary pre-fishery fishable biomass decreased by 56% relative to 2011/2012, but further analysis of the 4X survey data is required, pending the outcome of the 2013/14 fishing season.
• The recruitment index varies by region. In N-ENS there was an increase in recruitment to near the high levels observed in 2008 which may be due to a net immigration of recruits. Consistent levels of recruitment in S-ENS, based on the post-2004 (stable survey design) time series, continue to 2013. Recruitment in 4X is low and variable, although there was an increase in the index between 2012 and 2013.
• Female Snow Crab abundance and associated egg production in all areas continue to decline after reaching highs in 2007/2008. Egg production is now below the long-term mean; however, it is expected to increase in the next 2-4 years due to an increase in immature female crab, particularly in N-ENS.
• Sex ratios (proportion female) are now male-dominated in N-ENS, are stabilizing at low levels in S-ENS and are increasing at more moderate levels in 4X.
• Bycatch levels are extremely low in this fishery. In N-ENS and S-ENS, combined estimates of bycatch were 0.01% of Snow Crab landings. Crab Fishing Area (CFA) 4X shows higher (relative to N-ENS) bycatch rates at 2 % of Snow Crab landings.
• High relative densities of predators (based on Snow Crab trawl survey, e.g. Thorny Skate) were found in some areas with high densities of immature Snow Crab, however, predators remain at levels well below those historically reported. Increased predation may lower future recruitment to the fishable biomass and affect movement patterns of Snow Crab.
• Average bottom temperatures in 2013 were generally cooler than 2012 in all areas; although a warming trend since the early 1990s should be noted. Within the survey sets in each of the Snow Crab areas average bottom temperatures were generally stable with long-term means of 3.4, 3.8 and 6.1°C in N-ENS, S-ENS and 4X, respectively. The current mean temperature in 4X remains above the temperature preference for Snow Crab.
• A reference points-based Precautionary approach (PA) has been implemented in this fishery. The Limit reference point (LRP) is 25% of carrying capacity and the Upper stock reference (USR) is 50% of carrying capacity. The Target Removal reference is 20% of the fishable biomass in each area and the Removal reference is not to exceed F_MSY. Various secondary (population and ecosystem) indicators are taken into consideration for management decisions.
• In N-ENS, fishable biomass (FB) has been stable and in the “healthy” zone (FB > USR) over the past four years. However, in the medium to long-term, there is a need to be mindful of the gap in the size-frequency distribution, suggesting limited local recruitment in the near future. This could limit the scope for flexibility in harvest strategies in this area without immigration from other areas. Combining the desire to protect the immigrant recruits and fish the ageing fishable biomass suggests a status quo to marginal decrease in TAC is recommended.
• In S-ENS, the population is considered to be in the “healthy” zone (FB > USR). As the fishable biomass continues to be near historically high levels, with a stable size distribution of immature male crab suggesting that positive recruitment is expected for at least the next three to four years. There is considerable scope for flexibility in harvest strategies. A status-quo to a marginal increase in TAC is recommended.
• In 4X, fishable biomass is in the “cautious” zone (FB < USR), potentially resulting from environmental influences in the recent years. As recruitment and potential immigration into the 2014/2015 season is fishing season.

This Science Advisory Report is from the 25 February 2014 Assessment of Nova Scotia (4VWX) Snow Crab. Additional publications from this process will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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