Research Document - 2009/069
English sole (Parophrys vetulus) in British Columbia, Canada: Stock Assessment for 2006/07 and Advice to Managers for 2007/08
By Paul J. Starr
Abstract
Information pertaining to English sole (Parophrys vetulus) in British Columbia was reviewed and updated for inclusion in a delay-difference stock assessment model. This model was used to determine the status of two stocks of English sole: 5CD (Hecate Strait) and 3CD5AB (combined west coast Vancouver Island and Queen Charlotte Sound) and to provide quantitative advice on levels of catch and the associated risk relative to selected management performance indicators for each of these stocks.
A range of model uncertainties in both stock assessments were explored through sensitivity runs which varied model assumptions which could not be easily reconciled through inspection of the model fits to the data. Three pairs of alternative model assumptions were investigated: a) estimating M, the rate of instantaneous natural mortality through the use of mean weight data sampled from the fishery or fixing M at the preferred value of 0.20 and dropping the mean weight data; b) varying the age of knife-edged recruitment between age 4 and age 5 (5CD only); c) applying a single CPUE series for the entire model period, effectively assuming that the fishery catchability has been constant for 40 years or splitting the CPUE series between 1995 and 1996 in recognition of the severe management restrictions that were applied at that time.
The 5CD modelling results showed that within the range of the criteria investigated, the effects of fixing or estimating M and the age of knife-edge recruitment were relatively minor, with the management advice almost identical across these options. However, the effect of splitting the CPUE series was important, with the model estimating a drop in catchability in recent years and consequently being more optimistic about stock status. Both of the CPUE hypotheses indicate that 5 year projections of landings at the current 5CD TAC have a greater than 50% probability of remaining above Bmin (99–100%) and Bref (64–98%) reference points at the end of the projection period in 2012. The probability of staying above Bmin and Bref remains above 50% at catch levels higher than the current TAC, with the amount varying depending on the model hypothesis. However, all model runs predict that current stock abundance has a greater than 50% probability of declining by 2012 at landing levels greater than the current TAC.
The 3CD5AB modelling results are less optimistic, with stronger differences between the CPUE hypotheses. The split CPUE hypothesis indicates that the current TAC will remain above the selected Bmin and Bref reference points while the single CPUE hypothesis indicates that landings at this level are too high. There is also some sensitivity to the M estimation method, with the model runs which fix M predicting that the stock will decline at catch levels lower than the model runs which estimate M.
The credibility of both assessments requires the assumption that the fishery dependent CPUE series are tracking the abundance of these stocks. Neither assessment fits the available survey biomass indices very well and the 3CD5AB assessment is based on 5CD biological data because it lacks stock specific biological information.
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