On the relation between ‘true’ numbers of Northeast Arctic cod and VPA- or survey-based abundance estimates
Working paper
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http://hdl.handle.net/11250/100641Utgivelsesdato
2001Metadata
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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.
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ICES CM documents2001/Q:15