Research Document - 2013/093
Pacific Ocean Perch (Sebastes alutus) stock assessment for the west coast of Vancouver Island, British Columbia
By Andrew M. Edwards, Rowan Haigh and Paul J. Starr
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 off the west coast of Vancouver Island, British Columbia, is assessed here under the assumption that it is a single stock harvested entirely in Pacific Marine Fisheries Commission (PMFC) major areas 3C and 3D. This is the first time that a population model has been used to assess this stock.
We used an annual two-sex catch-at-age model tuned to: three fishery-independent trawl survey series, annual estimates of commercial catch since 1940, and age composition data from the commercial fishery (15 years of data) and from one of the survey series (four years of data). The model starts from an assumed unfished equilibrium state in 1940, and the survey data cover the period 1967 to 2012 (although not all years are represented). The model was implemented in a Bayesian framework (using the Markov Chain Monte Carlo procedure) to quantify uncertainty of estimated quantities.
Estimated exploitation rates were calculated as the ratio of total catch to the vulnerable biomass in the middle of each year. Rates peaked in the mid-1960s due to large catches by foreign fleets, and peaked again (though not as high) in the early 1990s. Exploitation rates have remained low since the mid-1990s, with the exploitation rate for 2012 estimated as 0.035 (0.018-0.077), denoting median and 5th and 95th quantiles of the Bayesian posterior distribution.
The spawning biomass (mature females only) at the beginning of 2013 is estimated to be 0.41 (0.19-0.68) of unfished spawning biomass. It is estimated to be 1.53 (0.55-3.32) of biomass at maximum sustainable yield BMSY, where BMSY is the equilibrium spawning biomass that would support the maximum sustainable yield (MSY).
Advice to managers is presented as a set of decision tables that provide probabilities of exceeding limit and upper stock reference points for ten-year projections across a range of constant catch scenarios. The primary reference points used are a limit reference point of 0.4BMSY and an upper stock reference point of 0.8BMSY, which are the Fisheries and Oceans Canada Precautionary Approach provisional reference points. Decision tables are also presented with respect to alternative reference points based on proportions of unfished equilibrium biomass, on current biomass and on the exploitation rate at MSY.
The estimated spawning biomass at the beginning of 2013 has a 0.99 probability of being
above the limit reference point of 0.4BMSY, and a 0.87 probability of being above the upper stock reference point of 0.8BMSY.
The estimated median MSY (tonnes) is 1,048 (700-1,509), compared to the recent mean catch (from 2007-2011) of 547 t. The probability that the exploitation rate in 2012 is below that associated with MSY is 0.89.
Ten-year projections, for constant catches of 600 t, indicate essentially no change in the aforementioned probabilities of the spawning biomass being above the reference points, and indicate a projected increase in the median spawning biomass.
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