Assessing The Precision Of Commercial Catch At Age Estimates

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The assessment of Northeast Arctic cod is based on estimates of the commercial catch numbers at age. The age structure of the catch is estimated by sampling individuals from commercial fishing trips. Though it is commonly assumed that the sample of individuals is a random sample from the population, fish sampled from the same trip (i.e., from a ‘cluster’ of fish) tend to have more similar ages than those in the total catch. For Northeast Arctic cod, the intra-cluster correlation for age is positive and thus the effective sample size is much smaller than the number of fish aged. It is shown, given the number of fish aged, that the precision of the estimated age distribution is rather low, and that the number of fish aged from each trip could be reduced from approximately 80 to 20 without a significant loss in precision.