Research Document - 1999/196

Survey Design Considerations for Pacific Cod in Hecate Strait.

By A. Sinclair

Abstract

This research document reviews aspects of the Hecate Strait groundfish assemblage survey design that may be modified to improve Pacific cod estimates while not compromising the main objectives of the survey. This included an analysis of past surveys and commercial fisheries data from the 1996-99 observer program. Consultation were held with Pacific cod fishermen on aspects of survey design and the interpretation of results. There are advantages to conducting bottom trawl surveys at times when the target species are dispersed throughout the survey area, to avoid seasons when the fish are highly aggregated, and to avoid periods when they have migrated out of the survey area. Cod spawning is reported in Hecate Strait between January and March. Commercial catch rates decline in the September - December period indicating that Pacific cod availability in Hecate Strait may be reduced then. It would appear that the month of June, when the survey has been conducted, is a good choice for this survey. Less than half the area of Hecate Strait is covered by the survey. A crab fishery occurs over a large portion of the Strait east of the Queen Charlotte Islands and the bottom conditions there are unfavorable for trawling. It would be illustrative to examine the fish by-catch in the crab traps for Pacific cod. If cod are present in significant numbers, then it may be worthwhile having the crab traps lifted at the time of the survey to allow fishing there. There appears to be a depth dependent pattern in Pacific cod distribution in Hecate Strait that would justify a stratified survey design to reduce variance. Having one station per stratum makes the estimation of sampling variance difficult. It would be useful to consider modifying the survey design where fewer strata and more stations per stratum. The basis for stratification should be further examined, taking into consideration other species for which the survey is now used (e.g. flatfish). Consideration should be given to trade-offs in station allocation and stratification. The current surveys have between 80 - 100 fishing sets. Increasing the number of sets would be expected to reduce the standard deviation of the mean in proportion to the square root of the number of observations. Roughly speaking, doubling the number of sets may result in a 30% reduction in the standard deviation. It is strongly recommended that length frequencies be taken for all Pacific cod catches made on the groundfish assemblage survey. Filling in for missing length frequencies introduces unnecessary uncertainties to the survey results.

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