© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
On estimating the age composition of the commercial catch of Northeast Arctic cod from a sample of clusters
Institute of Marine Research PO Box 1870 Nordnes, N-5817 Bergen, Norway
*Correspondence to S. Aanes; tel: +47 55 238627; fax: +47 55 238687. e-mail: sondre.aanes{at}imr.no; michael.pennington{at}imr.no.
Assessment of Northeast Arctic cod is based on estimates of the commercial catch in numbers at age. The age structure of the catch is estimated by sampling fish from commercial fishing trips. Although it is commonly assumed that a 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 be more similar in age than those in the total catch. For Northeast Arctic cod, the intracluster correlation for age is positive, and therefore the effective sample size is much smaller than the number of fish aged. Given the number of fish aged, the precision of the estimated age distribution is rather low, and the number of fish aged from each trip could be reduced from approximately 85 to 20 without a significant loss in precision.
Keywords: age distribution, cluster sampling, effective sample size, intracluster correlation
Received 9 October 2002; accepted 18 December 2002.