Research Document 2018/038
Stock assessment for Pacific Ocean Perch (Sebastes alutus) in Queen Charlotte Sound, British Columbia in 2017
By Haigh, R., Starr, P.J., Edwards, A.M., King, J.R., and Lecomte, J.-B.
Abstract
Pacific Ocean Perch (Sebastes alutus, POP) is a commercially important species of rockfish that inhabits the marine canyons along the coast of British Columbia. The status of POP in Queen Charlotte Sound, British Columbia, is assessed under the assumption that it is a single stock harvested entirely in Pacific Marine Fisheries Commission (PMFC) major areas 5A, 5B, 5C, and in 5E south of 52°20. This stock has supported a domestic trawl fishery for decades and was heavily fished by foreign fleets from the mid-1960s to mid-1970s. We used an annual catch-at-age model tuned to two fishery-independent trawl survey series, annual estimates of commercial catch since 1940, and age composition data from the two survey series (11 years of data) and the commercial fishery (34 years of data). The model starts from an assumed equilibrium state in 1940, and the survey data cover the period 1967 to 2016 (although not all years are represented). The two-sex model was implemented in a Bayesian framework (using the Markov Chain Monte Carlo search procedure) under a scenario that estimates both sex-specific natural mortality (M) and steepness of the stock-recruit function (h). Seven sensitivity analyses were performed to test the effect of data inputs to the model. A bridging analysis was performed using the 2010 data from the previous assessment to determine the effect of weighting the age frequencies with a new procedure that downweights composition data instead of the 2010 procedure that uses multinomial weighting. The base model run suggests that strong recruitment in the early 1950s sustained the foreign fishery, and that a few strong year classes spawned in the late 1970s and 1980s sustained the domestic fishery into the 1990s. The spawning biomass (mature females only) at the beginning of 2017, B2017, is estimated to be 0.27 (0.18, 0.42) of unfished biomass (median and 5th and 95th quantiles of the Bayesian posterior distribution). B2017 is estimated to be 1.03 (0.54, 1.96) of the spawning biomass at maximum sustainable yield, BMSY. Advice to managers is presented as decision tables that provide probabilities of exceeding limit and upper stock reference points over a five-year projection period across a range of constant catches. The DFO provisional ‘Precautionary Approach compliant’ reference points were used, which specify a ‘limit reference point’ of 0.4BMSY and an ‘upper stock reference point’ of 0.8BMSY. The estimated spawning biomass at the beginning of 2017 has a 0.99 probability of being above the limit reference point, and a 0.74 probability of being above the upper stock reference point. Five-year projections using a constant catch of 2500 t/y (near the recent average five-year catch of 2400 t/y) indicate that, in 2022, the spawning biomass has probabilities of 0.97 of remaining above the limit reference point, and 0.71 of remaining above the upper stock reference point. We developed a Bayesian method to investigate potential ecosystem influences on recruitment and applied it to the estimated recruitment from the 2010 stock assessment using a suite of climatic and environmental indicators. Results show that none of the investigated indicators were able to reliably predict observed recruitment deviations, leading to the conclusion that we are unable at this time to use environmental information to improve model predictions for this stock.
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