Research Document - 2002/113

West Coast Vancouver Island Pacific Cod Assessment: 2002

By P.J. Starr, A.S. Sinclair, J. Boutillier

Abstract

The 2001 assessment of the west coast of Vancouver Island Pacific cod stock was updated for 2002 by adding an additional year of catch data and new CPUE index, incorporating abundance indices derived from a shrimp trawl survey, and converting model catch data from a calendar year basis to standard DFO fishing years (1 April to 31 March). Catch and effort data from a shrimp trawl survey operated by DFO since 1973 were analysed for consistency of coverage over the history of the survey. Tows were retrospectively assigned to areal strata and an index of abundance was calculated for each year by treating the tows as if they had been randomly sampled, weighting the stratum CPUE by the area of the stratum. The resulting indices were very similar to the indices previously estimated using an interpolation method as well as being similar to the CPUE indices calculated from the commercial fishery since 1988.

Two versions of a delay-difference assessment model were used to predict the response of the stock to varying fishing levels for the 2003/04 fishing year, one which used the shrimp trawl abundance data and the second which left out these data to maintain comparability with the 2001 assessment. The shrimp trawl index for May 2002 was used directly as an abundance index for the 2002/03 fishing year in the assessment model which used the shrimp trawl data and the index was used indirectly to predict the 2002/03 CPUE index for the assessment model which did not use the shrimp trawl data. Both models showed that the stock had increased since the previous assessment and the model catch predictions indicated that higher catches could be taken in 2003/04. The respective assessment models, with and without the shrimp trawl indices, estimated the biomass levels at the beginning of 2002/03 at 104% and 39% of BMSY. Respective catch levels that maintained the same stock size between 2003/04 and 2004/05 were estimated at 690 t and 570 t for each model. The respective catch levels which corresponded to FMSY were estimated at 1,290 t and 510 t for each model. The assessment model which incorporated the shrimp trawl data was preferred as it included all of the available data for this stock. The BMSY and FMSY reference points are not well estimated and future models should provide estimates of parameter uncertainty.

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