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

Pennington, Michael; Godø, Olav Rune
Working paper
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CM_1992_D_09.pdf (1.503Mb)
URI
http://hdl.handle.net/11250/105042
Date
1992
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  • ICES CM documents authored by IMR scientists (1949-2011) [3138]
Original version
This report is not to be cited without prior reference to the authors  
Abstract
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.
Publisher
ICES
Series
ICES CM Documents;1992/D:9

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