Science Advisory Report 2019/008

Assessment of British Columbia Pacific Cod for Areas 3CD, and 5ABCD in 2018

Summary

• Pacific Cod (Gadus macrocephalus) is a commercially important, short-lived species that occurs along the entire coast of British Columbia (BC). It is primarily caught by the groundfish bottom trawl fishery and in small amounts by the hook and line fishery. The majority of catches are taken in Hecate Strait and Queen Charlotte Sound (Area 5ABCD, average 739.6 t/yr, 2013-2017). Although large catches were taken historically off the West Coast of Vancouver Island (Area 3CD), catches in recent years have been lower (average 363 t/yr, 2013-17).
• Four stocks of Pacific Cod are defined for management purposes in BC: Strait of Georgia (4B); West Coast Vancouver Island (3CD); Queen Charlotte Sound (5AB); and Hecate Strait (5CD). Historically each area has been assessed separately.  For the purposes of this assessment, data from Areas 5AB and 5CD were combined into a single stock assessment, due to the lack of biological evidence for separate stocks and improved fits to the combined data compared to data from area 5AB alone. Area 3CD was assessed separately. Area 4B was not assessed at this time as there is no directed commercial fishery there.
• Pacific Cod is difficult to age, making statistical catch-age models inappropriate for this species. Therefore, stocks in Areas 5ABCD and 3CD were assessed using Bayesian delay-difference models. The models were fit to fishery-independent indices of abundance, mean annual weight in the commercial catch, and new standardized commercial catch-per-unit-effort (CPUE) indices that were developed using Tweedie generalized linear mixed effect models (GLMMs). Updated estimates of growth parameters were also incorporated into the models.
• The absolute scale of the estimated biomass in the two assessed areas, determined by estimated catchability (q) in the survey indices, was considered the major axis of uncertainty in this assessment. Several sensitivity analyses were done to evaluate the possible magnitude of this uncertainty. Relatedly, uncertainty in the relative biomass scales informed by catch and CPUE data between the historical (pre-1996) and modern (1995+) eras was also identified as a major axis of uncertainty, especially for Area 5ABCD.
• Due to uncertainty in model parameters, biological reference points based on equilibrium assumptions (e.g., maximum sustainable yield [MSY]) were not used. Instead, following the approach in previous stock assessments for Area 5CD, reference points were based on estimated historical biomass. For both stocks, the recommended upper stock reference (USR) is the average estimated biomass between 1956 and 2004; and the recommended limit reference point (LRP) is an agreed-upon undesirable low biomass state to be avoided (B2000 in Area 5ABCD; B1986 in Area 3CD). The recommended limit removal rate (LRR) is the average estimated fishing mortality between 1956 and 2004.
• For each of the two assessed stock areas, advice is provided as a decision table that summarizes the probability of breaching reference points over a range of fixed catch levels for a one-year projection using a model-averaging approach. The model- averaged decision tables were constructed using unweighted posterior samples from a reference case model and six sensitivity cases for each stock, to encompass the range of parameter uncertainty in the assessments.
• Model-averaged estimated biomass in Area 5ABCD declined after 2011, with a slight increase in the last two years of the time series. The model-averaged median estimate of 2018 biomass was above the median LRP but below the USR. Model-averaged estimated annual recruitments were mostly below average during the last two decades of the time series.
• Model-averaged estimated biomass in Area 3CD declined after 2015. The model-averaged median estimate of 2018 biomass was above the median LRP but below the median USR.  Model-averaged estimated annual recruitments were below average for most years in the last two decades in the time series.
• For Area 5ABCD, model-averaged biomass at the beginning of 2019 (B2019) was projected to be 0.60 (0.39-1.01) of unfished biomass (B0). For Area 3CD, model-averaged B2019 was projected to be 1.13 (0.78-1.73) of B0. Proportions denote median (and 2.5 - 97.5 percentiles).
• It is recommended that the next assessment should occur after a suite of species, including Pacific Cod, is evaluated using a simulation-based management procedure approach for data limited and data moderate groundfish species, to be reviewed through the Canadian Science Advisory Secretariat (CSAS) in 2019/20. It is also recommended that any future updates to the current assessments should be done in years immediately following the biennial groundfish synoptic bottom trawl survey in each area (i.e., when the most recent survey index point is available). 

This Science Advisory Report is from the October 10-11, 2018 regional peer review on the Assessment of British Columbia Pacific Cod for Areas 3CD, and 5ABCD in 2018. 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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