Blar i Brage IMR på forfatter "Hjellvik, Vidar"
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A vessel specific bias in echo sounder recordings during trawling
Handegard, Nils Olav; Hjellvik, Vidar (ICES CM Documents;2004/R:35, Working paper, 2004) -
Acoustic data collected during and between bottom trawl stations: consistency and common trends
Bez, Nicholas; Reid, David; Neville, Suzanna; Vérin, Yves; Hjellvik, Vidar; Gerritsen, Hans D. (Journal article; Peer reviewed, 2007)Acoustic data are often collected during bottom trawl surveys. Their use can potentially improve the precision and accuracy of fish abundance estimates if acoustic data collected between trawl stations are consistent with ... -
Assessing trawl-survey estimates of frequency distributions
Pennington, Michael; Burmeister, Liza-Mare; Hjellvik, Vidar (ICES CM Documents;2000/K:23, Working paper, 2000)Marine trawl surveys catch a cluster of fish at each station and fish caught together tend to have more similar characteristics, such as length, age, stomach contents etc., than those in the entire population. When this ... -
Can subjective evaluation of echograms improve correlation between bottom trawl and acoustic densities?
Godø, Olav Rune; Hjellvik, Vidar; Greig, Tony; Beare, Douglas (ICES CM documents, Working paper, 2004)Direct comparison of acoustic densities and trawl catches involves fundamental difficulties related to species and size selectivity of the trawl and efficiency of the acoustic instrumentation. In some surveys, therefore, ... -
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) -
Comparison of Northeast Atlantic mackerel (Scomber scombrus) distribution patterns in the Norwegian Sea using lidar, sonar, and trawl
Tenningen, Eirik; Godø, Olav Rune; Iversen, Svein A.; Slotte, Aril; Hjellvik, Vidar; Torkelsen, Terje (ICES CM documents, Working paper, 2003)In July 2002 the Institute of Marine Research (IMR), Bergen mapped the distribution and density of mackerel in the Norwegian Sea using lidar, sonar and trawl. A major objective was to test the efficiency of the lidar as ... -
Correcting for avoidance in acoustic abundance estimates for herring using a generalized linear model
Hjellvik, Vidar; Handegard, Nils Olav; Ona, Egil (ICES CM documents, Working paper, 2006)When a research vessel passes over a herring school or layer, the herring may avoid the vessel by swimming downwards and horizontally. The fish may also change its orientation, which may alter its mean target strength. ... -
Differences between near bottom biomass spatial structure observed in the Irish Sea, the North Sea and the Barents Sea in recent years
Mireille, Bouleau; Nicolas, Bez; Hjellvik, Vidar; Godø, Olav Rune; Reid, David; Beare, Douglas; Greig, Tony; Armstrong, Mike; Gerritsen, Hans D.; Mackinson, Steven; Neville, Suzanna; Kooij, Jeroen van der; Vérin, Yves; Massé, Jacques (ICES CM documents, Working paper, 2003)Based on acoustic data and trawl catches collected during bottom trawl surveys, we compare the spatial structure of the biomass available in the first meters above the bottom in three different regions, the Barents Sea ... -
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 ... -
How many trawl hauls, and how many fish from each haul are needed to obtain a given precision in the age structure estimate for Norwegian spring spawning herring? A bootstrap approach
Hjellvik, Vidar; Ona, Egil (ICES CM documents, Working paper, 2006)The Norwegian spring spawning herring is assessed each year from acoustic surveys. The age structure in the stock is estimated from trawl samples. Typically, about 10 trawl samples are taken per survey, and 100 fish from ... -
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 ... -
Positive relationships between bottom trawl and acoustic data
Beare, Douglas; Reid, David; Greig, Tony; Bez, Nicholas; Hjellvik, Vidar; Godø, Olav Rune; Bouleau, Mireille; Kooij, Jeroen van der; Neville, Suzanna; Mackinson, Steven (ICES CM documents, Working paper, 2004)Demersal fish stock biomass is usually estimated using commercial landings data, ‘tuned’ by various types of survey data, usually trawl. When collecting fish data by trawl it is only possible to take relatively small numbers ... -
Silent research vessels are not quiet
Ona, Egil; Godø, Olav Rune; Handegard, Nils Olav; Hjellvik, Vidar; Patel, Ruben; Pedersen, Geir (Journal article; Peer reviewed, 2007)Behavior of herring Clupea harengus stimulated by two ocean-going research vessels; respectively designed with and without regard to radiated-noise-standards. Both vessels generate a reaction pattern, but, contrary to ...