Publication Abstract

A study of market sampling data, precision of age compositions and the effect of mis-ageing: ICES division VIIa cod

D.L. Maxwell, B.D. Rackham, S. Warnes, M. Armstrong, A.W. Newton and R. Officer

Catch numbers-at-age for Irish Sea (VIIa) cod are routinely produced from market sampling data. Details of the level and coverage of the market sampling are presented and discussed. Analytical estimates of coefficient of variance (CV) for catch numbers-at-age are calculated as a guide to precision. Bootstrap methodology is also used to produce estimates of catch numbers-at-age and CV. This allows the between-age correlation structure and the suitability of the delta-method approximation underlying the analytical estimates to be assessed. The currently used method of estimating CV assumes variability due to mis-ageing fish in the age-length key is negligible. The effects of mis-ageing are illustrated. A range of mis-ageing scenarios is applied to the data and the effect on catch numbers-at-age and CV is measured.

Reference:

D.L. Maxwell, B.D. Rackham, S. Warnes, M. Armstrong, A.W. Newton and R. Officer, 2001. A study of market sampling data, precision of age compositions and the effect of mis-ageing: ICES division VIIa cod. ICES CM 2001/P:11. 8pp.