On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates
When assessing Northeast Arctic cod it is assumed that the estimates of numbers at age generated by a converged VPA type analysis are equal, on average, to the true numbers at age. Based on fishery independent survey data, we show that it is likely that the converged VPA estimates are biased, especi...
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ftimr:oai:imr.brage.unit.no:11250/100641 2023-05-15T14:30:24+02:00 On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates Nakken, Odd Pennington, Michael 2001 49547 bytes application/pdf http://hdl.handle.net/11250/100641 eng eng ICES ICES CM documents 2001/Q:15 http://hdl.handle.net/11250/100641 14 s. cod torsk stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920 Working paper 2001 ftimr 2021-09-23T20:16:15Z When assessing Northeast Arctic cod it is assumed that the estimates of numbers at age generated by a converged VPA type analysis are equal, on average, to the true numbers at age. Based on fishery independent survey data, we show that it is likely that the converged VPA estimates are biased, especially so for small cohorts. The probable cause of this bias is either inaccurate catch data including erroneous age readings and/or using the wrong functional form in the VPA tuning procedure. We provide evidence that it is appropriate to assume that the expected survey-based estimates are proportional to actual abundance. Therefore, we suggest that proportionality should be assumed for all ages when calculating survey catchability (i.e. in the VPA tuning procedure) or using the survey to estimate abundance directly. Since the converged VPA estimates of small cohorts seem to have a relatively high bias compared with those for large cohorts, only the converged VPA estimates for large cohorts should be used in the tuning process. This will not eliminate the bias in the VPA estimates caused by the tuning procedure, but it should reduce it. Report Arctic cod Arctic Northeast Arctic cod Institute for Marine Research: Brage IMR Arctic |
institution |
Open Polar |
collection |
Institute for Marine Research: Brage IMR |
op_collection_id |
ftimr |
language |
English |
topic |
cod torsk stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920 |
spellingShingle |
cod torsk stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920 Nakken, Odd Pennington, Michael On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
topic_facet |
cod torsk stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920 |
description |
When assessing Northeast Arctic cod it is assumed that the estimates of numbers at age generated by a converged VPA type analysis are equal, on average, to the true numbers at age. Based on fishery independent survey data, we show that it is likely that the converged VPA estimates are biased, especially so for small cohorts. The probable cause of this bias is either inaccurate catch data including erroneous age readings and/or using the wrong functional form in the VPA tuning procedure. We provide evidence that it is appropriate to assume that the expected survey-based estimates are proportional to actual abundance. Therefore, we suggest that proportionality should be assumed for all ages when calculating survey catchability (i.e. in the VPA tuning procedure) or using the survey to estimate abundance directly. Since the converged VPA estimates of small cohorts seem to have a relatively high bias compared with those for large cohorts, only the converged VPA estimates for large cohorts should be used in the tuning process. This will not eliminate the bias in the VPA estimates caused by the tuning procedure, but it should reduce it. |
format |
Report |
author |
Nakken, Odd Pennington, Michael |
author_facet |
Nakken, Odd Pennington, Michael |
author_sort |
Nakken, Odd |
title |
On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
title_short |
On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
title_full |
On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
title_fullStr |
On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
title_full_unstemmed |
On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates |
title_sort |
on the relation between ‘true’ numbers of northeast arctic cod and vpa- or survey-based abundance estimates |
publisher |
ICES |
publishDate |
2001 |
url |
http://hdl.handle.net/11250/100641 |
geographic |
Arctic |
geographic_facet |
Arctic |
genre |
Arctic cod Arctic Northeast Arctic cod |
genre_facet |
Arctic cod Arctic Northeast Arctic cod |
op_source |
14 s. |
op_relation |
ICES CM documents 2001/Q:15 http://hdl.handle.net/11250/100641 |
_version_ |
1766304244447576064 |