A Statistical Procedure for Validating Age Estimates from Otolith Bands Using Mark-recapture

age-structured stock assessment models. Historically, age estimates for fish have been obtained from hard structures, such as scales or otoliths, by enumerating seasonally deposited opaque or translucent bands in the calcified tissue (Beamish and McFarlane 1987). Such ageing methods are based on the assumption that the periodicity of band deposition is known or can be validated. To date, the only direct method of validation has been the use of chemical dyes such as tetracycline, which mark the hard structure. This fluorescent mark then serves as a reference point from which the periodicity of the seasonal bands can be deduced once sufficient somatic growth has accrued. Other indirect techniques for the validation of hard-structure banding include marginal increment analysis, backcalculation methods, and the analysis of modal progression of length frequency data in conjunction with age data (Hecht and Smale 1986). In this study, a growth model is developed that utilizes mark-recapture data to validate the periodicity of opaque band deposition in the otoliths of Pomatomus saltatrix captured off KwaZulu-Natal, South Africa. The method is based on the assumption that, if a growth curve (derived from size-at-age data and assuming an arbitrary periodicity of the banding in the hard structure) can reasonably predict the observed growth increment of tagged individuals between the times of marking and recapture, then the growth parameters and the periodicity assumed for the seasonal bands must be valid. MATERIAL AND METHODS