## Measuring the effect of changes in catchability on the variance of marine survey abundance indices

##### Working paper

##### Permanent lenke

http://hdl.handle.net/11250/105042##### Utgivelsesdato

1992##### Metadata

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##### Originalversjon

This report is not to be cited without prior reference to the authors##### Sammendrag

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 that is based only on the between station variability in
catch may underestimate the true variance if catchability varies
over time. In this paper, the survey index variance is estimated
indirectly by cross calibrating time series of VPA estimates and
trawl survey indices of abundance. The method is applied to several
species on Georges Bank and to haddock in the Barents Sea. For
these surveys, it appears that the true variance of the survey
indices is approximately twice as large as the usual estimates
based on the within survey variance. As an application, a time
series technique, which requires an estimate of the survey index
variance, is used to generate a more precise index of abundance.
The results indicate that for the surveys examined the variance of
the estimated abundance index is 30 - 40% smaller than the original
(average catch per tow) index.