Perspectives on the relationship between otolith growth and the conversion of isotope activity ratios to fish ages

Ageing of fish otoliths using radiometric methods is now becoming widely accepted. Using this methodology, one has the choice of extracting otolith cores or modeling otolith mass growth. Modeling otolith growth forgoes the difficult work of extracting cores, but one is left with the nearly equally daunting task of selecting and validating an otolith growth model and deriving the corresponding decay equations. We note that the mathematical aspects of this problem appear to have been satisfactorily resolved. However, problems of interpretation remain, and it is not clear whether practitioners fully appreciate the sensitivity of their results to growth model selection and interpretation. We note that the two-stage linear growth model appears to be generally misapplied in the literature. Also, we present evidence that the process of using otolith growth curves estimated from annular ring counts to validate the method of counting annular rings can constitute circular reasoning. Although tedious work, extracting otolith cores seems the most valid way to avoid some serious problems in radiometric age determination work.