Research Document - 2016/042

Stock assessment for Silvergray Rockfish (Sebastes brevispinis) along the Pacific coast of Canada

By Paul J. Starr, Rowan Haigh and Chris Grandin

Abstract

Silvergray Rockfish (SGR) along the Pacific coast of Canada has been assessed at the request of the DFO Groundfish Management Unit (GMU) with respect to its status relative to provisional reference points established by the DFO Sustainable Fisheries Framework (SFF) (DFO 2009). These reference points are the upper stock reference point (USR) of 0.8B_MSY and the Limit Reference Point (LRP) of 0.4B_MSY. Current status and 10-year projection probabilities for the population are given with respect to the above DFO SFF reference points as well as the reference points of 0.2B₀ and 0.4B₀, the probability of an increase in population size and the probability of exceeding u_MSY, the equilibrium exploitation rate at MSY.

This stock was assessed as a single coastwide stock after consultation with an assigned Technical Working Group (TWG), which considered the similarity in biology and abundance trends among sub-areas as well as the nearly continuous distribution of catch and CPUE along the coast. The TWG agreed to assess SGR as a single coastwide stock, with the provision that the current management approach of distributing the catches into management units be maintained as a precautionary measure and that 3CD (west coast of Vancouver Island) would have the lowest TAC among the management units.

An annual catch-at-age model tuned to six fishery-independent survey series, annual estimates of commercial catch since 1940, nine years of age composition data from four survey series, and 25 years of age composition data from the commercial fishery provided the foundation of this advice. The model was started from an equilibrium state in 1940, and the survey data spanned the period 1967 to 2013. The two-sex model was implemented in a Bayesian framework (using the Markov Chain Monte Carlo procedure) with 26 estimated parameters in addition to the recruitment deviations. These parameters included natural mortality (M) for each sex and “steepness” (h) to determine the Beverton-Holt stock-recruitment function, which were both constrained by informative priors, as were the 15 selectivity function parameters.

Both model scenarios imply a slow-growing, low productivity stock that has undergone periods of high recruitment in the 1980s and recently in the early 2000s. The base case run estimated B₂₀₁₄/B₀=0.56 (5–95% range: 0.41–0.70) and B/₂₀₁₄/B_MSY=2.04 (5–95% range: 1.22–3.00), indicating that the stock is in the “healthy zone” as defined by the DFO Sustainable Fisheries Framework (SFF). The 2013 exploitation rate u₂₀₁₃ (ratio of total commercial catch to vulnerable biomass) is estimated to be 0.044 (5–95% range: 0.030-0.068) compared to u_MSY=0.145 (5–95% range: 0.064-0.030).

Ten year projections, assuming random recruitment, provide probabilities of future population status with respect to the above reference points over a range of constant catch scenarios, applied without feedback controls.

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