Research Document 2020/070
Assessment of Pacific Cod (Gadus macrocephalus) for Hecate Strait and Queen Charlotte Sound (Area 5ABCD), and West Coast Vancouver Island (Area 3CD) in 2018
By Forrest, R.E., Anderson, S.C., Grandin, C.J., and Starr, P.J.
Abstract
The status of two stocks of Pacific Cod (Gadus macrocephalus) in Hecate Strait/Queen Charlotte Sound (Area 5ABCD) and West Coast Vancouver Island (Area 3CD) was assessed using Bayesian delay-difference models. The models were fit to fishery-independent indices of abundance and new standardized commercial catch-per-unit-effort (CPUE) indices that were developed using Tweedie generalized linear mixed effect models (GLMMs). New analyses of growth and maturity were also done and incorporated into the models.
Model estimates of biomass and stock status in both management areas were sensitive to prior assumptions about natural mortality and survey scaling parameters, variance in the mean weight data, and the goodness of fit to the indices of abundance, particularly the commercial CPUE data. Harvest advice was produced in the form of decision tables that summarized the probability of breaching reference points in 2019 over a range of fixed 2018 catch levels. Due to model sensitivity to a number of assumptions, these stocks were assessed using a model-averaging approach which combined the posterior distributions from seven alternative model confgurations for each stock.The resulting distributions were used to assess the historical biomass trajectories, current stock status and decision tables based on catch projections appropriate to each stock.
Reference points based on historical reconstruction of long-term average biomass and fishing mortality were accepted in 2013 for the Area 5CD Pacific Cod stock. “Historical” reference points were recommended because uncertainty in estimates of productivity parameters implied large uncertainty in reference points based on maximum sustainable yield (MSY). On the basis of the previous acceptance of historical reference points for Area 5CD Pacific Cod, the current assessment applies the same approach for the Areas 5ABCD and 3CD stocks. For both stocks, an upper stock reference point (USR) is defned as estimated average biomass during the period 1956-2004. A limit reference point (LRP) is defined as the lowest estimated biomass agreed to be an undesirable state to avoid (occurred in year 2000 in Area 5ABCD, and in 1986 in Area 3CD). For both stocks, a limit removal rate (LRR) is defned as estimated average fishing mortality during the period 1956–2004.
Biomass in Area 5ABCD is estimated to have been on a declining trajectory since 2011, following declining trends in abundance indices, despite low estimated fishing mortality rates over the same period. Median posterior estimates of biomass are estimated to be between the median LRP and median USR for Area 5ABCD. Recruitment is estimated to have been below average for the past two decades.
Biomass in Area 3CD is estimated to have been on a declining trajectory since 2015 after following an increasing trend from a historical low level of biomass between 1998 and 2014. These trends are consistent with the available biomass indices, including a recent downturn in the WCVI synoptic survey and the CPUE series. Median posterior estimates of biomass are estimated to be above the median LRP but below the USR for Area 3CD. Recruitment is estimated to have been below average for most years in the past two decades, with above average peaks in 2009, 2013 and 2014.
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