MEASURING THE EFFECT OF CHANGES IN CATCHABILITY ON THE VARIANCE OF MARINE SURVEY ABUNDANCE INDEXES

The average catch per tow by a research vessel survey is often used as an index of abundance. An estimate of the variance of such indices th at is based only on the between station variability in catch may under estimate the true variance if catchability varies over time. In this p aper, the survey index variance is estimated indirectly by cross-calib rating time series of virtual population analysis estimates and trawl survey indices of abundance. The method is applied to surveys of some fish stocks on Georges Bank, in southern New England, and in the Baren ts Sea. For these surveys, it appears that the true variance of the su rvey indices is approximately twice as large as the usual estimates ba sed on the within survey variance. As an application, a time series te chnique, which requires an estimate of the survey index variance, is u sed to generate a more precise index of abundance. The results indicat e that for the surveys examined the variance of the estimated abundanc e index is 30-40% smaller than the original (average catch per tow) in dex.