Blar i Brage IMR 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) -
Can the precision of bottom trawl indices be increased by using simultaneously collected acoustic data? The Barents Sea experience
Hjellvik, Vidar; Tjøstheim, Dag; Godø, Olav Rune (Journal article; Peer reviewed, 2007-12)Acoustic data are recorded continuously during the winter survey for demersal fish in the Barents Sea. This paper presents a method for using the information from the acoustic recordings between trawl stations in an attempt ... -
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 acoustic densities: why do we see less in the dark?
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (Journal article; Peer reviewed, 2005)Diurnal fluctuations in total integrated echo abundance and in vertical density profiles were examined using data from the Norwegian combined acoustic and bottom-trawl survey for demersal fish during winter in the ... -
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 bottom trawl survey catches: does it pay to adjust?
Hjellvik, Vidar; Godø, Olav Rune; Tjøstheim, Dag (Journal article; Peer reviewed, 2002)The diurnal bias of bottom trawl survey catches is studied 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 sampling volume of trawl and acoustics: estimating availability probabilities from observations of tracked individual fish
Handegard, Nils Olav; Tjøstheim, Dag (Journal article; Peer reviewed, 2009-03-10)The effective sampling volume of trawl and acoustics is an important parameter in fish abundance estimation surveys. This paper presents a method to compute the probability of a fish being available to the bottom trawl and ... -
Simulations of multi-beam sonar echos from schooling individual fish in a quiet environment
Holmin, Arne Johannes; Handegard, Nils Olav; Korneliussen, Rolf J.; Tjøstheim, Dag (Journal article; Peer reviewed, 2012-12)A model is developed and demonstrated for simulating echosounder and sonar observations of fish schools with specified shapes and composed of individuals having specified target strengths and behaviors. The model emulates ... -
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 ... -
When fish meet a trawling vessel: examining the behaviour of gadoids using a free-floating buoy and acoustic split-beam tracking
Handegard, Nils Olav; Tjøstheim, Dag (Journal article; Peer reviewed, 2005)The reaction of individual gadoids to a bottom-trawling vessel has been observed in situ in the Barents Sea using a free-floating buoy and acoustic target-tracking methods. More than 20 000 tracks were analysed in terms of ...