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Monitoring changes in abundance of gadoids with varying availability to surveys

Godø, Olav Rune; Wespestad, Vidar G.
Working paper
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CM_1991_G_76.pdf (4.720Mb)
URI
http://hdl.handle.net/11250/104902
Date
1991
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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 use of scientific survey data for assessing abundance of commercially important

fish stocks has become a managerial necessity in many regions. Errors and bias in estimates

from such surveys may thus have great economic impact. In the Northeast Atlantic

abundance of cod has been estimated from bottom trawl surveys and acoustic surveys since

1981. In the Svalbard area, such survey abundance indices have differed considerably with

the discrepancy varying, from year to year. At low stock abundance and when the population

is comprised of young fish, cod are distributed close to bottom and are thus optimally located

for a bottom trawl survey, but are poorly recorded by the acoustic technique. In contrast,

when abundant year-classes attain an age of 2-3 years and older, they are more pelagically

distributed and clearly available for acoustic observation. Dependency on only one of the

survey techniques could seriously underestimate the stock. A combination of estimates from

the two methods is at present hindered by the lack of an absolute estimate of abundance, and

by uncertainty in the actual volume sampled volume due to movement of fish during survey

sampling. The vertical profile of the acoustic recordings and size composition and density

of the population are factors which will affect fish availability to the two survey techniques

differently. Such variation in availability seems to affect gadoid surveys in general. With

a limited amount of additional effort changes in availability-selectivity can be monitored

during the survey, and observed changes can be incorporated into analyses to compensate for

potential bias due to variation in these parameters.
Publisher
ICES
Series
ICES CM Documents;1991/G:76

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