Science Advisory Report  2015/027

Assessment of Pacific Cod (Gadus macrocephalus) for Queen Charlotte Sound (Area 5AB) in 2013

Summary

• Pacific Cod (Gadus macrocephalus) is a commercially important species of cod that occurs along the entire coast of British Columbia (BC). It is primarily caught by the groundfish trawl fishery and occasionally by the hook and line fishery. The majority of catches are taken in Hecate Strait (Area 5CD, ~700 t in 2013) and Queen Charlotte Sound (Area 5AB, ~180 t in 2013), although large catches were taken historically off the west coast of Vancouver Island (Area 3CD).
• This advisory report refers to the Queen Charlotte Sound (5AB) stock after revisions to the 2013 assessment.
• Pacific Cod stocks in BC are difficult to assess, primarily due to the relatively short time series of fishery-independent indices of abundance, changes in fishery selectivity over time, and historical changes in management and fishery operations. Historical management changes include the transition to quotas (between the early 1990s and 1997), the introduction of 100% at-sea observer coverage (February 1996), and several voluntary and regulation mesh-size changes. There are very few survey age composition data for this difficult-to-age species, and no age composition data from the commercial fishery.
• The status of the Pacific Cod populations in 5AB was assessed using a Bayesian delay-difference model fit to fishery-independent survey data, commercial catch-per-unit-effort data, commercial catch data, and estimates of annual mean weights from the trawl fishery.
• Despite large uncertainty, biomass in 5AB is estimated to have been stable since 2001 on, but is estimated to be below the historical average.
• Advice to managers is provided in a decision table that summarizes the probability of breaching reference points at a range of fixed catches for a one-year projection. Due to model sensitivity to a number of model assumptions, the table uses a model-averaging approach intended to integrate results across a range of alternative model assumptions.
• Estimates of fishery reference points based on maximum sustainable yield (MSY) were very sensitive to model assumptions, and differed substantially among model sensitivity cases. Unfished biomass (B₀) was also sensitive to model assumptions. The use of reference points based on MSY and B₀ are consequently not supported for this stock.
• Alternative reference points based on estimates of average historical biomass, similar to those adopted for Pacific Cod in Area 5CD, were explored. However, the history-based reference points were problematic for Area 5AB because the historical period chosen (1956-2004) occurred during a time when fleet behaviour differed significantly from recent years. Currently, many vessels in Area 5AB actively avoid Pacific Cod due to low quotas. This was not the case in the historical period, when vessels actively targeted Pacific Cod. Using a more recent period to define the history-based reference points was also problematic, as the resulting estimates of average biomass were very low and were considered to be inconsistent with a precautionary decision-making framework.  The historical biomass-based reference points were therefore rejected for use in Area 5AB.
• Estimated fishing mortality rates were fairly constant over the whole time series and were reasonably robust across a range of model assumptions. Therefore, a provisional, model-averaged reference point based on the average estimated fishing mortality for the period 1956-2004 (F_avg(1956-2004)) was adopted.
• A suggested harvest strategy in the short-term is therefore to continue harvesting at the current levels (200 t), which currently results in approximately F = 0.3, until a limit condition triggers a catch reduction strategy. A suitable trigger for precautionary action could occur when any new index point in the QCS Synoptic survey (after 2013) falls below 50% of the mean survey indices prior to the new index point. Additionally, the stock assessment should be updated on an annual or biennial basis to evaluate the probability that the F-based reference point has been breached.

This Science Advisory Report is from the December 8^th, 2014 Pacific Cod (Gadus macrocephalus) Assessment for Queen Charlotte Sound (5AB), British Columbia in 2013. 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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