Research Document 2021/048
Assessment of Scotian Shelf Snow Crab in 2018
By Zisserson, B.M., Cameron, B.J., Glass, A.C., and Choi, J.S.
Abstract
In 2018, landings of Snow Crab (Chionoecetes opilio) for North-Eastern Nova Scotia (N-ENS) and South-Eastern Nova Scotia (S-ENS) were 742 t and 6,064 t, respectively, and were 55 t in 4X for the 2016–2017 season, representing decreases of 9% (N-ENS), 10% (S-ENS), and 31% (4X) relative to the previous year. Total Allowable Catches (TACs) in 2018 were 786 t, 6,057 t, and 110 t in N-ENS, S-ENS, and 4X, respectively. The TAC was not reached in N-ENS or 4X. Non-standardized catch rates in 2018 were 62 kg/trap haul in N-ENS, 116 kg/trap haul in S-ENS, and 12 kg/trap haul in 4X in 2017–2018—which, relative to the previous year, represents a decrease of 31% (N-ENS), an increase of 23% (S-ENS), and a decrease of 52% (4X). The capture of soft-shelled crab in N-ENS increased to 25% from approximately 5% in 2017, almost exclusively during the summer fishery. In S-ENS, the relative occurrence of soft-shell crab was 1.6% for the 2018 season. Soft-shell discard rates in 4X remain very low, due to the timing of the season. Bycatch of non-target species is extremely low (< 0.4%) in all Crab Fishing Areas (CFAs). In both N-ENS and S-ENS, moderate internal recruitment to the fishery is expected for next year (and likely for 3–4 years) based on size-frequency histograms. In CFA 4X, there appears to be limited potential for both internal (low sub-legal numbers of crab) and external (immigration from neighbouring areas) recruitment to the fishery. Since 2016, there has been a substantial recruitment of female crab into the mature segment of the population in all CFAs. The potential population benefits of the maturation/recruitment of both sexes are tempered by a number of uncertainties, including the influence of predation and temperature changes (especially in CFA 4X and parts of CFA 24). Predation from Atlantic Halibut is a potentially large and increasing source of natural mortality for Snow Crab on the Scotian Shelf. The results from the current spatiotemporal models of variability (stmv) modelling approach produce abundance estimates with very little inter-annual variability, particularly in N- and S-ENS. This hyper-stability is incongruous with the temporal abundance trends observed in geometric mean survey catch densities, length frequencies, catch rates, etc. This disparity from other data suggests caution is warranted when using the modelled biomass and fishery mortality estimates. The modelled post-fishery fishable biomass index of Snow Crab in N-ENS was estimated to be 3,203 t, relative to 3,358 in 2017. In S-ENS, the post-fishery fishable biomass index was 33,190 t, relative to 32,040 t in 2017. In 4X, the pre-fishery fishable biomass was 403 t, relative to 354 t in 2017. The N-ENS fishing mortality (F) estimate for 2018 is 0.24, an increase from 0.17 in 2017. The S-ENS F estimate for 2018 is 0.22, a decrease from 0.26 in 2017. The 4X F estimate for 2017–2018 is 0.23 (exploitation rate 0.21), a decrease from 0.26 in 2016–2017. In N-ENS, the fishery removals for the past two seasons appear to have been an overly aggressive harvest strategy, as catch rates have fallen and the 2018 TAC was not reached. A more conservative harvest approach in N-ENS could stabilize catch rates, protect incoming recruitment, and allow commercial biomass to rebuild. Substantial TAC reductions over the past three seasons have helped maintain stable fishery performance in S-ENS in spite of contracting habitat, reduced recruitment, and increased predation. Geometric mean catches from the survey are stable and, based on stock structure, increased recruitment to the fishery is likely to occur for the upcoming season. A moderately more aggressive harvest strategy can be considered. Habitat in CFA 4X is “marginal” relative to the other CFAs. Low internal recruitment and the large inter-annual temperature variations increases the uncertainty associated with this area. The Snow Crab population has not rebounded from an extreme warm-water even in 2012–2013. A zero TAC was set for the 2018–19 fishing season due to low commercial biomass. Survey catches indicate that 4X commercial biomass levels remain very low but have improved.
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