Age validation and analysis of ageing error from marked and recaptured sablefish, Anoplopoma fimbria


Heifetz, Jonathan, Delsa Anderl, Nancy E. Maloney, and Thomas L. Rutecki
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Sablefish, Anoplopoma fimbria, are especially difficult to age. Ages are typically determined by counting growth zones that are assumed to be annuli on the burnt cross section of an otolith. We evaluated the accuracy of ageing methods by comparing the ages obtained from two experienced age readers with those ages that were known. A mark-recapture experiment on sablefish provided a relatively large sample (n=49) of 2–9 year old sablefish. This sample of known-age fish provided a unique opportunity to evaluate the accuracy of ageing methods for young sablefish. Our study generally confirmed the criteria used to age young sablefish. According to the assignment of ages from two experienced age readers, about two-thirds of the fish were misaged. Of these, most fish (81% and 71% for the two readers) were misaged by only one year. After reexamination of the otoliths, most of the discrepancies between reader age and known age could be resolved. Ageing-error matrices that define the probability of assigning an age to a fish of a given true age were estimated from among-reader variability and from comparison of known ages to reader ages. Estimates of ageing errors, based on comparison of known ages to reader ages, were considerably higher than estimates obtained from among-reader variability. We recommend that ageing-error corrections applied in stock assessment models be based on the ageing-error matrix derived from known and reader ages.