1 14 



Fishery Bulletin 93(1), 1995 



3). However, for lutjanids it appears that a Pb/Ba 

 mass ratio <0.2 probably indicates that the assump- 

 tion of a low initial activity ratio (R) is valid, whereas 

 the three samples where the Pb/Ba mass ratio is 0.3- 

 0.6 indicate that the assumption of low R may be 

 invalid. The Pb/Ba ratios are, therefore, a useful test 

 of the validity of the low R assumption. 



Finally, this appears to be the first instance where 

 radiometric methods are more consistent with whole- 

 otolith ages rather than sectioned-otolith ages. All 

 previous radiometric studies offish from temperate 

 and subtemperate waters have verified section counts 

 (Bennett et al., 1982; Campana et al., 1990; Fenton 

 et al., 1990, 1991; Smith et al., 1991). The metabolic 

 effects of the annual cycle of inorganic and organic 

 deposition in otoliths may be more pronounced in 

 these environments resulting in clear annuli in 

 otoliths offish from more temperate regions. 



Conclusions 



This study has shown that radiometry using 210 Pb/ 

 226 Ra activity ratios in both whole and cored otoliths 

 can accurately estimate the ages of fish as young as 

 3 years. Stable leadibarium mass ratios were used 

 to identify samples that may invalidate the assump- 

 tion of constant uptake of allogenic lead (i?=0). For 

 the lutjanids examined, ring counts in sectioned 

 otoliths were shown to overestimate fish ages. Meth- 

 ods such as marginal increment analysis do not verify 

 that the ageing method used is accurate unless the 

 pattern is demonstrated to be consistent for all age 

 classes. This indicates that tropical fish should be 

 aged by two independent methods where possible to 

 help minimize possible ageing errors. 



Acknowledgments 



We thank John Salini, David Brewer, and Ted 

 Wassenberg for coordinating otolith collection and 

 Robert Chisari for meticulously performing the ra- 

 diochemical alpha source preparations. Gwen Fenton 

 and Chris O'Brien made constructive comments on 

 an earlier draft of the manuscript. This project was 

 partly funded by the Australian Fishing Industry 

 Research and Development Council (FRDC grants 

 88/90 and 29/91). 



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