Science Advisory Report 2018/020

Walleye Pollock (Theragra chalcogramma) stock assessment for British Columbia in 2017

Summary

• A new stock assessment is presented for two British Columbia (BC) stocks of Walleye Pollock (Theragra chalcogramma), with the BC North stock encompassing the three most northerly Pacific Marine Fisheries Commission (PMFC) major areas (5C, 5D, 5E) and the BC South stock including the remaining four outside PMFC major areas (3C, 3D, 5A, 5B plus minor areas 12 & 20).
• Stock definitions were selected on the basis of a difference in observed mean weights, with the BC North mean weights estimated near 1.0 kg/fish while the BC South mean weights averaged near 0.5 kg/fish.
• A delay-difference production model was used to assess each stock, using data from fishery-independent surveys, a CPUE series derived from bottom trawl commercial catch rates, and an annual mean weight series derived from unsorted commercial catch samples from each area.
• The available age data from BC were inadequate to derive growth models specific to BC; therefore, survey age samples from the Gulf of Alaska were used to specify growth for the BC North stock while a published Walleye Pollock growth model from the Asian Sea of Okhotsk was used for the BC South stock.
• Each assessment explored a range of plausible natural mortality values as well as a range of ages for the knife-edge selectivity assumption because the biomass indices and the mean weight data used in the delay-difference model were not informative for these parameters.
• The stock assessment was conducted in a Bayesian framework. Twelve model runs were made for the BC North stock and 11 for the BC South stock. Model-averaged scenarios were used to represent each stock based on model runs selected with criteria established at the peer review meeting.
• The two model-averaged scenarios were evaluated against reference points based on the biomass trajectory. The average spawning biomass from 1967-2016 (B_avg), was used as a proxy for BMSY, while B_min, the minimum spawning biomass from which the stock subsequently recovered to B_avg, was used as the Limit Reference Point (LRP), in place of a provisional LRP of 0.4B_MSY (DFO 2009). The Upper Stock Reference (USR) was defined as 2 x B_min, in place of a provisional USR of 0.8B_MSY (DFO 2009). An Average Removal Rate, calculated as the estimated average fishing mortality for the period 1967-2016 (u_avg), was used in place of umsy.
• The model-averaged BC North stock was evaluated as being above the USR after 2000. The probabilities that the estimated spawning biomass at the beginning of 2017 (B2017) was greater than the limit reference point (B_min), and greater than the upper stock reference point (2B_min) are 0.99 and 0.62, respectively. The probability that B2017 was greater than B_avg is 0.27. The estimated harvest rate in the final year (u2016) has a probability of 0.74 of being greater than the estimated average harvest rate (u_avg), which means that the current harvest rate is likely above the average removal rate.
• The model-averaged BC South stock was evaluated as being above the USR for most of the period after 2008. The probabilities that the estimated spawning biomass at the beginning of 2017 (B2017) is greater than the LRP and the USR are 1.00 and 0.96, respectively. The probability that B2017 is greater than B_avg is 0.34. The estimated harvest rate in the final year (u2016) has a probability of 0.05 of being greater than the estimated average harvest rate (u_avg), which means that current harvest is likely below the average removal rate.
• For each stock, the assessment provides a decision table which evaluates the probability of the model-averaged case staying above the five reference points across a range of 22 constant catches. The delay-difference model used in this stock assessment is less capable of making reliable multi-year projections than an age-structured model because it has no latent age structure to inform predictions and the stock-recruitment function is poorly determined. Projections longer than one or two years were considered less reliable; only one-year projections are presented in the SAR.

This Science Advisory Report is from the November 14-15, 2017 regional peer review on Walleye Pollock (Theragra chalcogramma) stock assessment for British Columbia in 2017. 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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