Aging methods for yellowtail kingfish, Seriola lalandi, and results from age- and size-based growth models

Dorsal spines, otoliths, scales, and vertebrae collected from yellowtail ki ngfish (Seriola lalandi) in NSW, Australia, were assessed for usefulness in estimating age. Legibility of growth zones, the time scale at which zones form, and precision of age estimates were evaluated for fish sizes from 323 to 1090 mm FL. All calcified structures contained growth zones, but dorsal spines were unsuitable for age determination because it was likely that ea rly growth zones were lost. From marginal increment analysis, it appeared t hat one zone was laid down per year far otoliths and possibly scales, but a clear pattern was not found for vertebrae. Although exact agreement betwee n repeated age readings was relatively low (50-66%), agreement within one z one was higher (92-96%) and scales provided the most precise readings. Prec ision decreased with increasing age of fish. Growth curves derived from oto liths and scales were similar for all ages except fish from the first age c lass; those derived from otoliths and vertebrae were similar for all fish w ith less than eight growth zones. Although statistical differences were fou nd between the growth curves of scales and vertebrae for some age classes, with the exception of the first age class these differences were not biolog ically important. Growth rates estimated from length-frequency (age-based) and mark-recapture (length-based) data compared favorably with those estima ted from calcified aging structures. Otoliths, scales, and vertebrae all sh owed promise as structures for aging kingfish, but further work is needed t o determine the position of the first zone and to validate estimates for al l age classes. Until such work is completed, we recommend that scales and e ither otoliths or vertebrae be used for aging kingfish.