Biomass estimation of rockfish stocks off the west coast of the Queen Charlotte Islands during 1978 and 1979 / by B.M. Leaman and D.A. Nagtegaal.: Fs97-4/1652E-PDF

"Biomass assessment cruises of rockfish stocks off the west coast of the Queen Charlotte Islands were conducted during 1978 and 1979. Biomass of commercially important rockfishes was estimated using a swept-area trawl survey. A stratified-systematic sampling design was used in both years although the location of the sampling stations within the strata was not always fixed, but was determined by the availability of trawlable bottom. The 1979 survey was further modified by increasing sampling density in areas of greater fish abundance, as determined in 1978, and treating these areas separately in the estimation procedure. Results from the two years are not directly comparable because of these differences in survey methodology. The 1978 cruise resulted in an estimated rockfish biomass of approximately 6400 t ± 50% if the trawlable area detected on the survey is considered solely, and of approximately 10,500 t ± 50% if all areas are considered to be equivalent to the sampled area. In contrast, the 1979 survey resulted in an estimated biomass of approximately 18,300 t. If 1979 data were not segregated by locality the biomass estimates analogous to those derived in 1978 would be approximately 9700 t ± 37% and 15,900 t ± 37%, respectively, although there is greater information content in the 1979 estimates. The 1979 biomass estimate based on locality segregation of data has an unrealistically narrow confidence interval (± 10%). This is due in part to some large hauls from individual localities which contributed significantly to the total biomass, yet had low variance. The ± 40% confidence interval of the other elements in the data set is considered more realistic. The biases in the survey methodology used are briefly reviewed. Biological sampling of the species encountered revealed a majority of older (≥20 y) fish, particularly for Pacific ocean perch (Sebastes alutus), and consequently lower mortality rates than previously assumed. The possible origins of this stock and the implications of its age composition to long-term management are discussed"--Abstract, page iii.

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