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Diurnal variation in acoustic densities: why do we se less in the dark?

Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag
Working paper
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CM_2004_R_33.pdf (241.3Kb)
URI
http://hdl.handle.net/11250/106609
Date
2004
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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
Introduction: In the Barents Sea, the echo abundance of demersal fish is about 50% higher at day than at night. Three possible reasons are:

• Fish hide in the bottom dead zone at night.

• Fish target strength changes due to pressure-related swim-bladder changes during vertical migration.

• Fish target strength varies due to higher variation in the tilt-angle distribution at night.

It is very often claimed that the dead zone is most important. We argue that variation in tilt-angle may also be important.
Publisher
ICES
Series
ICES CM Documents;2004/R:33

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