Blar i ICES CM documents authored by IMR scientists (1949-2011) på forfatter "Tjøstheim, Dag"
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Can the precision of bottom trawl indices be increased by using simultaneously collected acoustic data? The Barents Sea experience
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (ICES CM documents, Conference object, 2006) -
Diurnal variation in acoustic densities: why do we se less in the dark?
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (ICES CM Documents;2004/R:33, Working paper, 2004)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 ... -
Diurnal variation in bottom trawl survey catches: does it pay to adjust?
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (ICES CM Documents;2001/Q:11, Working paper, 2001)We have studied the diurnal bias of bottom trawl survey catches with the purpose of adjusting for it and thereby improving the accuracy of abundance estimates. The correction term is estimated with uncertainty, and thus ... -
Diurnal variation in frequency response of gadoids in the Barents Sea
Godø, Olav Rune; Hjellvik, Vidar; Tjøstheim, Dag (ICES CM documents, Working paper, 2006)Earlier studies have demonstrated substantial diurnal variation in acoustic abundance of fish. Both the level and the uncertainty of abundance estimates from surveys carried out around the clock are affected. It has been ... -
Minimalistic surveys clear the way for priorities in the ecosystem approach
Godø, Olav Rune; Hjellvik, Vidar; Tjøstheim, Dag; Korsbrekke, Knut (ICES CM documents;2009/I:20, Working paper, 2009)Standardized surveys are an integral part of most fish stock assessments. Ship time is expensive and new demands set by the ecosystem approach challenge present survey and sampling strategies with respect to coverage of ... -
Modelling Diurnal Variation in Bottom Trawl Catches and Potential Application in Surveys
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (ICES CM documents, Working paper, 1999)Diurnal variability in bottom trawl survey catches may influence the quality of the survey results when not taken properly into account. This is especia]!.y the case if diurnal effects vary substantiaIly from year to ... -
The effective swept volume of a bottom trawl
Handegard, Nils Olav; Tjøstheim, Dag (ICES CM Documents;2004/R:7, Working paper, 2004)The reaction of individual fish towards an approaching trawling vessel has previously been measured using target-tracking methods. In this paper these measured velocities are fitted to a deterministic mean behaviour ... -
The measurement error of marine survey catches: the bottom trawl case
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (ICES CM Documents;2001/P:6, Working paper, 2001)We have formulated a model for analyzing the measurement error of marine survey abundance estimates using data from parallel surveys (trawl haul or acoustic measurement). The measurement error is defined as the component ...