Research Document 2021/014
Redstripe Rockfish (Sebastes proriger) stock assessment for British Columbia in 2018
By Starr, P.J. and Haigh, R.
Abstract
Redstripe Rockfish (Sebastes proriger, RSR) is a commercially important species of rockfish that inhabits the marine canyons along the coast of British Columbia (BC). The BC stock has supported a domestic trawl fishery for decades and was heavily fished by foreign fleets from the mid-1960s to mid-1970s. The status of RSR in BC is assessed as two stocks harvested in Pacific Marine Fisheries Commission (PMFC) major areas 5DE (BCN) and 3CD5ABC (BCS). The separation into two stocks was based on higher mean weights in the BCN population, a consistent observation that was confirmed across years, across research surveys and within the commercial fisheries. Additional checks comparing survey mean lengths by year and growth parameter estimates confirmed that BCN RSR are larger than BCS RSR, regardless of sex.
We use an annual catch-at-age model tuned to fishery-independent trawl survey series (two in BCN, four in BCS), bottom trawl CPUE series, annual estimates of commercial catch since 1940, and age composition data from survey series (BCN: 5 years of data from 2 surveys; BCS: 14 years from 3 surveys) and the commercial fishery (BCN: 12 years of data; BCS: 24 years). The model starts from an assumed equilibrium state in 1940, and the survey data cover the period 1967 to 2018 (although not all years are represented). The two-sex models were implemented in a Bayesian framework (using the Markov Chain Monte Carlo procedure) under a scenario that estimates both natural mortality (M) and steepness of the stock-recruit function (h). Sensitivity analyses were performed (four in BCN, five in BCS) to test the effect of alternative model assumptions.
The base model runs for BCN and BCS suggest that low exploitation in the early years, including that by foreign fleets, coupled with several strong recruitment events (in 1982 and 1996 for BCN and in 1974 and 2001 for BCS) have sustained the population to the present.
The spawning biomass (mature females only) at the beginning of 2018 for BCN and BCS is estimated to be 0.91 (0.69, 1.13) and 0.62 (0.47, 0.81) of unfished biomass (median and 5th and 95th quantiles of the Bayesian posterior distribution), respectively. For BCN and BCS, this biomass is estimated to be 3.16 (2.02, 4.00) and 2.43 (1.51, 3.77) of the spawning biomass at maximum sustainable yield, BMSY, respectively.
Advice to managers is presented as decision tables that provide probabilities of exceeding limit and upper stock reference points for five-year projections across a range of constant catches. The DFO provisional ‘Precautionary Approach compliant’ reference points were used, which specify a ‘limit reference point’ (LRP) of 0.4BMSY and an ‘upper stock reference point’ (USR) of 0.8BMSY. The estimated spawning biomass at the beginning of 2018 has a probability of 1 of being above the LRP, and a probability of 1 of being above the USR for both stocks. Five-year projections using a constant catch of 100 t/y in BCN and 700 t/y in BCS indicate that, in 2023, the spawning biomass has probabilities of 1 (BCN) and 1 (BCS) of remaining above the LRP, and 1 (BCN) and 1 (BCS) of remaining above the USR. The uMSY reference point, however, suggests that catches in excess of 500 t in BCN and 1300 t in BCS will breach the Sustainable Fisheries Framework guidelines on fishing mortality, assuming that the manager wishes to be 95% certain that the harvest rate in 2023 will be less than uMSY.
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