Science Advisory Report 2019/044

Widow Rockfish (Sebastes entomelas) stock assessment for British Columbia in 2019

Summary

• A single Widow Rockfish (WWR) stock was identified along the British Columbia (BC) coast based on no observable differences in mean weight, observed length, and growth models among three regional areas (BC North=PMFC 5E, BC Central=PMFC 5ABCD, BC South=PMFC 3CD). Also, there were no observable differences in WWR biology by fishing gear type.
• The WWR stock was assessed using a single fishery, annual two-sex catch-at-age model, implemented in a Bayesian framework to quantify uncertainty of estimated quantities. A composite base case that combined nine models across three fixed values for natural mortality (M) and three accumulator ages was used to evaluate this stock.
• The median (5 and 95 percentiles of the Bayesian results) female spawning biomass at the beginning of 2019 (B₂₀₁₉) is estimated to be 0.37 (0.26, 0.54) of unfished female spawning biomass (B0). Also, B2019 is estimated to be 1.51 (0.92, 2.61) of the equilibrium spawning biomass at maximum sustainable yield, B_MSY.
• At current catch levels, there is an estimated probability of >0.99 that B₂₀₁₉ > 0.4B_MSY and a probability of 0.98 that B₂₀₁₉ > 0.8B_MSY (i.e. of being in the healthy zone). The probability that the exploitation rate in 2018 was below that associated with MSY is 0.82.
• Advice to management is presented in the form of decision tables using the provisional reference points from the Fisheries and Oceans Canada (DFO) Sustainable Fisheries Framework (SFF) Precautionary Approach (DFO 2009). The decision tables provide five-year projections across a range of constant catches (Table 2).
• The appropriateness of the MSY based reference points for long-lived low productivity species is uncertain; consequently advice to management relative to reference points based on 0.2 and 0.4 of B₀ is also presented as alternative options (Appendix F, of the associated Research Document).
• It is recommended that the next assessment occur after 2024, with three new index values available from each of the three biennial synoptic trawl surveys relevant to WWR and five additional years of ageing and catch data. Intermediate progress before the next assessment year can be tracked using the commercial bottom trawl catch per unit effort (CPUE).
• Recommended future work includes the possible use of acoustic surveys to index WWR abundance, an increase in the level of biological sampling of commercial trawl catch of WWR, the use of geo-statistical methods for computing survey biomass indices and exploring improved biological sampling within the suite of groundfish.

This Science Advisory Report is from the June 18-19, 2019 regional peer review on Widow Rockfish (Sebastes entomelas) stock assessment for British Columbia in 2019. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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