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

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 targ...

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Bibliographic Details
Main Authors: Hjellvik, Vidar, Godø, Olav Rune, Tjøstheim, Dag
Format: Report
Language:English
Published: ICES 2004
Subjects:
acoustics
akustikk
stock assessment
bestandsberegning
VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Fisheries technology: 924
Barents Sea
Online Access:http://hdl.handle.net/11250/106609
Description
Summary: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.

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