Knut Korsbrekke - Academia.edu (original) (raw)
Papers by Knut Korsbrekke
17 s., 2001
Embryological studies on living cod eggs in NUNCtrays showed that spontaneous malformations durin... more Embryological studies on living cod eggs in NUNCtrays showed that spontaneous malformations during embryo development are lethal. They are called natural egg mortality in contrast to external lethal factors, as predation, extreme physical conditions and pollution. The natural mortality in cod eggs, both coastal and Arcto-Norwegian, is significantly reduced from first to second spawning, from laboratory experiments. The same embryological method was used during field studies on natural egg mortality on planktonic cod eggs in the spawning area in Lofoten, Northern Norway, in the March-April years 2000 and 2001. The types malformations which was found in a fild also corresponding to certain morphogenetic movements in embryodevelopment and equel discover in the experimental work. Most part of lehal malfor-spawners was specially high. A comparison between the years 2000 and 2001 demonstrated an increase in natural mortality from 23 to 31 %. In Vesterålen, the area of highest frequency of first time spawners, natural egg mortality increased from 31 to 46 %. Because of two systematic errors the numbers are minimum values.
Journal of Northwest Atlantic Fishery Science, Dec 1, 1999
Data from the Norwegian Barents Sea bottom trawl surveys in February from 1985 through 1996 are u... more Data from the Norwegian Barents Sea bottom trawl surveys in February from 1985 through 1996 are used to analyse variations in haddock (Melanogrammus aeglefinus L.) maturity. Several link functions to estimate maturity ogives are compared. Due to the clustering of samples, both the goodness of fit and various other test statistics are calculated using an effective sampling size lower than the total number of samples. Length and yearclass strength are used as continuous explanatory variables while year, age, sex and area are class variables. A strong year effect was detected which appears to be related to the very large 1990 year-class. The year-class strength explains additional variation. That is: when abundance is high, the proportions mature are reduced for otherwise similar haddock (with respect to length, age, sex and area). Both sex and area effects were significant. Males mature at younger ages and shorter lengths than females and there is a tendency for proportions mature to be lower in the eastern part of the Barents Sea.
Ices Journal of Marine Science, Jun 1, 1999
Diurnal variation in capture efficiency may add to the variability in swept area estimates (or in... more Diurnal variation in capture efficiency may add to the variability in swept area estimates (or indices) of abundance from bottom-trawl surveys. In the present study the relationship between the day/night ratio of swept area estimates and fish length was examined for five species observed in the Barents Sea bottom-trawl survey in winter in the years 1985-1996. Generally, most species showed increased catch rates during daylight at all sizes as compared with darkness. For cod the day/night ratio peaked at a length interval 23-31 cm with a substantial reduction for larger fish. For haddock the ratio was highest at the minimum size group, 12-15 cm, and decreased with increasing size. Some possible behavioural explanations for these findings are discussed. A change in groundgear during the time period analysed had a pronounced effect on the day/night ratios for all species. In addition there seem to be a tendency for the day/night ratios of catch rates for both cod and haddock to increase with stock size. This is a matter to be aware of when survey results are interpreted and used in stock assessments.
A model descnbing gashic evacuation developed by dos Santos (1990) using a modified version of th... more A model descnbing gashic evacuation developed by dos Santos (1990) using a modified version of the power exponential model for stomach evacuation is applied to the Northeast Arctic cod stomachs collected during 24 hour fishery. An approach to estimate the individual daily ration that takes into account the effects of initial meal size and fish weight on gastric evacuation rate in the course of 24 hour veriod is presented. The results indicate that the Northeast Arctic cod (length groups 40-44 cm and 45-49 cm) during intensive feeding on deep sea shrimp Pandalus borealis consumed between 0.50 % and 0.58 % of the body weight.
Gadoid Fisheries Symposium DTU Aqua, DFO, ICES, NAFO. St. Andrews 15.10.13 - 18.10.13When seeking... more Gadoid Fisheries Symposium DTU Aqua, DFO, ICES, NAFO. St. Andrews 15.10.13 - 18.10.13When seeking a location for spawning, fish appear to be under the influence of two groups of factors. Firstly, environmental cues such as temperature and salinity. Secondly, interactions with other individuals, like entrainment of recruit spawners or competition for favourable locations. Many stocks have several spawning grounds, each containing several centres of spawning, with variable proportions between these centres. One such stock is North East Arctic cod, which has undergone a history of low stock size and truncated age structure in recent decades, followed by a rapid recovery since the mid 1990’s. Combining time series of egg distribution from the 1980’s, 1990’s and from 2004 to 2012, we attempt to elucidate possible controls for inter-annual variations between centres of spawning in its primary spawning grounds around the Lofoten islands. We describe how the distribution changes from one egg stage to the next and how the proportion observed in the wide fjord between the island chain and the mainland varies on a sub-decadal time scale. In our models we relate egg distribution to population dependent factors (SSB, spawning stock age structure, ratio of recruit and repeat spawners, spatial structuring) and independent factors (local hydrography, Kola-section temperature) and examine possible controls on these temporal variations. We evaluate the contributions of the different factors to the explained variation and discuss possible causal mechanisms for this specific case and in a broader perspective
14 s., 1999
Acoustic and bottom trawl surveys of the Northeast Arctic cod stock provide fishery independent e... more Acoustic and bottom trawl surveys of the Northeast Arctic cod stock provide fishery independent estimates of trends in abundance and population characteristics. Given the recent uncertainties in the catch based assessment of the stock, survey based abundance estimates of ages 5+ cod, generated by calibrating the survey indices with converged estimates from a VP A, were compared with the estimates from the annual assessments. The results indicate that the survey based estimates were more accurate than the estimates from the annual assessments. Since the survey and the VPA abundance estimates for younger cod were inconsistent, the amount of information contained in the surveys was tested indirectly by determining how well estimates based on previous survey indices could predict the following year's survey value. A time series model of the ages 3+ cod and haddock survey indices predicted the subsequent value of the cod survey index fairly accurately. Furthermore, the Lofoten acoustic survey indicates a much greater decrease in spawning stock size over the last several years than do the VP A estimates. It is concluded that it is likely that these fishery independent surveys are providing a more accurate assessment of the current health of the cod stock than the assessment based on combining the survey data with commercial catch statistics. CORE Metadata, citation and similar papers at core.ac.uk
Presented at ICES/SCOR Symposium on Ecosystem Effects of Fishing in Montpellier, France, 15-19 Ma... more Presented at ICES/SCOR Symposium on Ecosystem Effects of Fishing in Montpellier, France, 15-19 March 1999.
Pathophysiology of Haemostasis and Thrombosis, 1994
An analysis of prothrombin time (PT) standardization methods is presented. The present recommende... more An analysis of prothrombin time (PT) standardization methods is presented. The present recommended ratio method demonstrated complex calibration and wide therapeutic ranges. Standardization by coagulation activities resulted in different therapeutic ranges due to the different sensitivity of assays for the protein induced by vitamin K absence (Pivka) inhibitor. A new method – the modified coagulation activity method – applied a reference assay against which any PT assay can be calibrated by clotting times, and the results were expressed in coagulation activities by the reference assay. This method was preferable due to simple calibration, narrow therapeutic ranges and identical results by any PT assay.
Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-2... more Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-24 June 1997.
Journal of Northwest Atlantic Fishery Science, 2003
The variation in population structure of Greenland halibut (Reinhardtius hippoglossoides Walbaum)... more The variation in population structure of Greenland halibut (Reinhardtius hippoglossoides Walbaum) in the Northeast Arctic is analysed using data from three different surveys using trawl, longline and gillnet gears, in the slope area of the western Barents Sea in the period 19922000. The design of the longline and gillnet survey had limitations in that they were set to simulate the commercial fisheries, but the vessels were forced to cover the most important part of the slope area. Greenland halibut was the dominant species both in numbers and weight and was usually caught in the range of 515 years old, but the catch was dominated by ages 612. The data showed differences in sex composition and age composition both by area and by depth, and catches from trawl showed the most evident pattern. Greenland halibut caught by gillnet were larger and older than fish caught by trawl and the results from longline were in between. In most of the age groups, males were significantly smaller than females and this general trend was shown for all gears. Males also dominated the younger age groups in all gears and Greenland halibut older than 10 years were virtually all females. Fish from the earliest cohorts in the years investigated were generally smaller than fish from later ones.
ICES Journal of Marine Science, 2002
The scope of this study is to identify temporal dynamics in fishing power, by deriving three diff... more The scope of this study is to identify temporal dynamics in fishing power, by deriving three different indices (IFP1, IFP2, IFP3) based on three independent methods. IFP1 is derived from the GLM analysis of the relationship between fishing mortality and fishing effort, assuming that total fishing mortality estimates from XSA (eXtended Survivors Analysis) are accurate. IFP2 is derived from the GLM analysis of the difference between the Log-CPUE of a vessel and the average Log-CPUE of a set of reference vessels, which are chosen with regards to the stability of their Log-CPUE over time. IFP3 is derived from the GLM analysis of the Log-CPUE of a vessel relative to some external survey abundance index. Particular attention is paid to the horsepower and year effects in IFP1, IFP2, and IFP3. This methodology is applied to the Danish, Dutch, English and Norwegian demersal fisheries of the North Sea. The fishing power estimated by all indices increases with horsepower, particularly in relation to target species. Despite less consensus in the estimation of annual variations in fishing power, some important features are highlighted. First, there are cases where fishing power has consistently increased over the period of investigation, possibly through an overall increase in fishing efficiency. Second, there are examples where fishing power has increased relative to one species, and remained constant or even decreased in relation to another one. In the context of mixed-species fisheries, this feature might reveal a shift in fishing tactics.
An acoustic surve y of the spawning stock of North East Arctic C od off Lofoten has been performe... more An acoustic surve y of the spawning stock of North East Arctic C od off Lofoten has been performed annually in March-April since 1982. The estimates for 1985-1997 have been recalculated and mean length and weight at age are shown together with the abundance indices (both numbers and biomass). The abundance indices for each age are compared with VP A results from a reasonably converged part of the series. It is shown that the VPAtuning assumption of constant log catchability at age should not be used for this survey series. As the survey produces abundance indices for the mature part of the cod stock these indices should be corrected with proportions mature before they are used in traditional assessment methods estimating year class strength.
Et kombinert akustikk- og bunntråltokt med sikte på å framskaffe mål for tallrikhet og utbredelse... more Et kombinert akustikk- og bunntråltokt med sikte på å framskaffe mål for tallrikhet og utbredelse hos torsk, hyse, blåkveite og uerartene, samt lengde og vekt ved alder for torsk og hyse, er gjennomført siden 1981. Vinteren 1995 var tallrikheten av 1- gruppe torsk den høyeste i tidsserien og den tilsvarende av hyse en av de høyeste. Antallet 2-4 år gammel torsk var noe lavere enn i fjor, mens de samme aldersgruppene av hyse var sterkt redusert i 1995. Det var mer 5-6 år gammel torsk og hyse enn noen gang tidligere i undersokelsen. Lengde og vekt ved alder var lav for begge artene, men kondisjonsfaktorene var tilnærmet normale og på nivå med fjorårets. Tallrikhetsindeksene økte noe for begge de kommersielle uerartene og for blåkveite, men det ble funnet svært lite blåkveite < 5 år
17 s., 2001
Embryological studies on living cod eggs in NUNCtrays showed that spontaneous malformations durin... more Embryological studies on living cod eggs in NUNCtrays showed that spontaneous malformations during embryo development are lethal. They are called natural egg mortality in contrast to external lethal factors, as predation, extreme physical conditions and pollution. The natural mortality in cod eggs, both coastal and Arcto-Norwegian, is significantly reduced from first to second spawning, from laboratory experiments. The same embryological method was used during field studies on natural egg mortality on planktonic cod eggs in the spawning area in Lofoten, Northern Norway, in the March-April years 2000 and 2001. The types malformations which was found in a fild also corresponding to certain morphogenetic movements in embryodevelopment and equel discover in the experimental work. Most part of lehal malfor-spawners was specially high. A comparison between the years 2000 and 2001 demonstrated an increase in natural mortality from 23 to 31 %. In Vesterålen, the area of highest frequency of first time spawners, natural egg mortality increased from 31 to 46 %. Because of two systematic errors the numbers are minimum values.
Journal of Northwest Atlantic Fishery Science, Dec 1, 1999
Data from the Norwegian Barents Sea bottom trawl surveys in February from 1985 through 1996 are u... more Data from the Norwegian Barents Sea bottom trawl surveys in February from 1985 through 1996 are used to analyse variations in haddock (Melanogrammus aeglefinus L.) maturity. Several link functions to estimate maturity ogives are compared. Due to the clustering of samples, both the goodness of fit and various other test statistics are calculated using an effective sampling size lower than the total number of samples. Length and yearclass strength are used as continuous explanatory variables while year, age, sex and area are class variables. A strong year effect was detected which appears to be related to the very large 1990 year-class. The year-class strength explains additional variation. That is: when abundance is high, the proportions mature are reduced for otherwise similar haddock (with respect to length, age, sex and area). Both sex and area effects were significant. Males mature at younger ages and shorter lengths than females and there is a tendency for proportions mature to be lower in the eastern part of the Barents Sea.
Ices Journal of Marine Science, Jun 1, 1999
Diurnal variation in capture efficiency may add to the variability in swept area estimates (or in... more Diurnal variation in capture efficiency may add to the variability in swept area estimates (or indices) of abundance from bottom-trawl surveys. In the present study the relationship between the day/night ratio of swept area estimates and fish length was examined for five species observed in the Barents Sea bottom-trawl survey in winter in the years 1985-1996. Generally, most species showed increased catch rates during daylight at all sizes as compared with darkness. For cod the day/night ratio peaked at a length interval 23-31 cm with a substantial reduction for larger fish. For haddock the ratio was highest at the minimum size group, 12-15 cm, and decreased with increasing size. Some possible behavioural explanations for these findings are discussed. A change in groundgear during the time period analysed had a pronounced effect on the day/night ratios for all species. In addition there seem to be a tendency for the day/night ratios of catch rates for both cod and haddock to increase with stock size. This is a matter to be aware of when survey results are interpreted and used in stock assessments.
A model descnbing gashic evacuation developed by dos Santos (1990) using a modified version of th... more A model descnbing gashic evacuation developed by dos Santos (1990) using a modified version of the power exponential model for stomach evacuation is applied to the Northeast Arctic cod stomachs collected during 24 hour fishery. An approach to estimate the individual daily ration that takes into account the effects of initial meal size and fish weight on gastric evacuation rate in the course of 24 hour veriod is presented. The results indicate that the Northeast Arctic cod (length groups 40-44 cm and 45-49 cm) during intensive feeding on deep sea shrimp Pandalus borealis consumed between 0.50 % and 0.58 % of the body weight.
Gadoid Fisheries Symposium DTU Aqua, DFO, ICES, NAFO. St. Andrews 15.10.13 - 18.10.13When seeking... more Gadoid Fisheries Symposium DTU Aqua, DFO, ICES, NAFO. St. Andrews 15.10.13 - 18.10.13When seeking a location for spawning, fish appear to be under the influence of two groups of factors. Firstly, environmental cues such as temperature and salinity. Secondly, interactions with other individuals, like entrainment of recruit spawners or competition for favourable locations. Many stocks have several spawning grounds, each containing several centres of spawning, with variable proportions between these centres. One such stock is North East Arctic cod, which has undergone a history of low stock size and truncated age structure in recent decades, followed by a rapid recovery since the mid 1990’s. Combining time series of egg distribution from the 1980’s, 1990’s and from 2004 to 2012, we attempt to elucidate possible controls for inter-annual variations between centres of spawning in its primary spawning grounds around the Lofoten islands. We describe how the distribution changes from one egg stage to the next and how the proportion observed in the wide fjord between the island chain and the mainland varies on a sub-decadal time scale. In our models we relate egg distribution to population dependent factors (SSB, spawning stock age structure, ratio of recruit and repeat spawners, spatial structuring) and independent factors (local hydrography, Kola-section temperature) and examine possible controls on these temporal variations. We evaluate the contributions of the different factors to the explained variation and discuss possible causal mechanisms for this specific case and in a broader perspective
14 s., 1999
Acoustic and bottom trawl surveys of the Northeast Arctic cod stock provide fishery independent e... more Acoustic and bottom trawl surveys of the Northeast Arctic cod stock provide fishery independent estimates of trends in abundance and population characteristics. Given the recent uncertainties in the catch based assessment of the stock, survey based abundance estimates of ages 5+ cod, generated by calibrating the survey indices with converged estimates from a VP A, were compared with the estimates from the annual assessments. The results indicate that the survey based estimates were more accurate than the estimates from the annual assessments. Since the survey and the VPA abundance estimates for younger cod were inconsistent, the amount of information contained in the surveys was tested indirectly by determining how well estimates based on previous survey indices could predict the following year's survey value. A time series model of the ages 3+ cod and haddock survey indices predicted the subsequent value of the cod survey index fairly accurately. Furthermore, the Lofoten acoustic survey indicates a much greater decrease in spawning stock size over the last several years than do the VP A estimates. It is concluded that it is likely that these fishery independent surveys are providing a more accurate assessment of the current health of the cod stock than the assessment based on combining the survey data with commercial catch statistics. CORE Metadata, citation and similar papers at core.ac.uk
Presented at ICES/SCOR Symposium on Ecosystem Effects of Fishing in Montpellier, France, 15-19 Ma... more Presented at ICES/SCOR Symposium on Ecosystem Effects of Fishing in Montpellier, France, 15-19 March 1999.
Pathophysiology of Haemostasis and Thrombosis, 1994
An analysis of prothrombin time (PT) standardization methods is presented. The present recommende... more An analysis of prothrombin time (PT) standardization methods is presented. The present recommended ratio method demonstrated complex calibration and wide therapeutic ranges. Standardization by coagulation activities resulted in different therapeutic ranges due to the different sensitivity of assays for the protein induced by vitamin K absence (Pivka) inhibitor. A new method – the modified coagulation activity method – applied a reference assay against which any PT assay can be calibrated by clotting times, and the results were expressed in coagulation activities by the reference assay. This method was preferable due to simple calibration, narrow therapeutic ranges and identical results by any PT assay.
Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-2... more Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-24 June 1997.
Journal of Northwest Atlantic Fishery Science, 2003
The variation in population structure of Greenland halibut (Reinhardtius hippoglossoides Walbaum)... more The variation in population structure of Greenland halibut (Reinhardtius hippoglossoides Walbaum) in the Northeast Arctic is analysed using data from three different surveys using trawl, longline and gillnet gears, in the slope area of the western Barents Sea in the period 19922000. The design of the longline and gillnet survey had limitations in that they were set to simulate the commercial fisheries, but the vessels were forced to cover the most important part of the slope area. Greenland halibut was the dominant species both in numbers and weight and was usually caught in the range of 515 years old, but the catch was dominated by ages 612. The data showed differences in sex composition and age composition both by area and by depth, and catches from trawl showed the most evident pattern. Greenland halibut caught by gillnet were larger and older than fish caught by trawl and the results from longline were in between. In most of the age groups, males were significantly smaller than females and this general trend was shown for all gears. Males also dominated the younger age groups in all gears and Greenland halibut older than 10 years were virtually all females. Fish from the earliest cohorts in the years investigated were generally smaller than fish from later ones.
ICES Journal of Marine Science, 2002
The scope of this study is to identify temporal dynamics in fishing power, by deriving three diff... more The scope of this study is to identify temporal dynamics in fishing power, by deriving three different indices (IFP1, IFP2, IFP3) based on three independent methods. IFP1 is derived from the GLM analysis of the relationship between fishing mortality and fishing effort, assuming that total fishing mortality estimates from XSA (eXtended Survivors Analysis) are accurate. IFP2 is derived from the GLM analysis of the difference between the Log-CPUE of a vessel and the average Log-CPUE of a set of reference vessels, which are chosen with regards to the stability of their Log-CPUE over time. IFP3 is derived from the GLM analysis of the Log-CPUE of a vessel relative to some external survey abundance index. Particular attention is paid to the horsepower and year effects in IFP1, IFP2, and IFP3. This methodology is applied to the Danish, Dutch, English and Norwegian demersal fisheries of the North Sea. The fishing power estimated by all indices increases with horsepower, particularly in relation to target species. Despite less consensus in the estimation of annual variations in fishing power, some important features are highlighted. First, there are cases where fishing power has consistently increased over the period of investigation, possibly through an overall increase in fishing efficiency. Second, there are examples where fishing power has increased relative to one species, and remained constant or even decreased in relation to another one. In the context of mixed-species fisheries, this feature might reveal a shift in fishing tactics.
An acoustic surve y of the spawning stock of North East Arctic C od off Lofoten has been performe... more An acoustic surve y of the spawning stock of North East Arctic C od off Lofoten has been performed annually in March-April since 1982. The estimates for 1985-1997 have been recalculated and mean length and weight at age are shown together with the abundance indices (both numbers and biomass). The abundance indices for each age are compared with VP A results from a reasonably converged part of the series. It is shown that the VPAtuning assumption of constant log catchability at age should not be used for this survey series. As the survey produces abundance indices for the mature part of the cod stock these indices should be corrected with proportions mature before they are used in traditional assessment methods estimating year class strength.
Et kombinert akustikk- og bunntråltokt med sikte på å framskaffe mål for tallrikhet og utbredelse... more Et kombinert akustikk- og bunntråltokt med sikte på å framskaffe mål for tallrikhet og utbredelse hos torsk, hyse, blåkveite og uerartene, samt lengde og vekt ved alder for torsk og hyse, er gjennomført siden 1981. Vinteren 1995 var tallrikheten av 1- gruppe torsk den høyeste i tidsserien og den tilsvarende av hyse en av de høyeste. Antallet 2-4 år gammel torsk var noe lavere enn i fjor, mens de samme aldersgruppene av hyse var sterkt redusert i 1995. Det var mer 5-6 år gammel torsk og hyse enn noen gang tidligere i undersokelsen. Lengde og vekt ved alder var lav for begge artene, men kondisjonsfaktorene var tilnærmet normale og på nivå med fjorårets. Tallrikhetsindeksene økte noe for begge de kommersielle uerartene og for blåkveite, men det ble funnet svært lite blåkveite < 5 år