Cappo et al,; A new approach to validation of periodicity and timing of opaque zone formation 



487 



concluded from disequilibria in ^"^Pb to --'^Ra ratios of 

 pooled otoliths that shorter lifespans obtained from whole 

 otolith readings isensu McPherson and Squire, 19921 were 

 more accurate than the higher estimates from section 

 counts. This independent method of age determination had 

 previously helped differentiate between very different age 

 interpretations otdeep-water Hoplostrthiis and Scbastes that 

 were thought to attain high longevities. However, Campana 

 and Jones ( 1998 ) noted that radiochemical dating was too 

 imprecise for detailed or individual age determinations, and 

 West and Gauldie (1994) also concluded that the method 

 held promise but was inadequate to validate fish ages. 



Regional differences in the ease of interpretation of oto- 

 lith macrostructure have been reported in tropical species 

 separated by only four degrees of latitude (Fowler, 1995) 

 and may partly explain the differences between our results. 

 The lack of interpretable macrostructure in an otolith 

 section from L. erythropteriis from the Gulf of Carpentaria 

 is evident when Figure 5b in Milton et al. (1995) is com- 

 pared with the well-defined incremental structure for the 

 same species in our Figure 2A. Alternatively, there may 

 be real latitudinal differences in the demography of these 

 species. Comparisons of the relationships between otolith 

 weight and age estimates from the Gulf of Carpentaria 

 and the central GBR, and further validation with the 

 "radiocarbon bomb chronometer" iKalish, 1995; Campana 

 and Jones, 1998), OTC marking, or other independent 

 techniques, may account more precisely for the differences 

 between the validation studies. 



Models with ratios of increment measurements around 

 OTC marks in order to estimate time intei-vals could provide 

 a statistical protocol to accompany the emerging definitions 

 of various levels of validation. Francis ( 1995) proposed that 

 the highest degree of validation requires demonstration 

 that inner and outer zones are formed annually and that 

 quantitative estimates of accuracy are provided for the 

 process of converting a count of zones to an age estimate. 

 This method requires knowledge of hatching dates and the 

 age at completion of the first annulus, as well as decisions 

 about the nominal calendar dates when opaque zones can be 

 distinguished on otolith margins. The approach presented 

 in our study could allow precise estimates of the timing of 

 some of these latter events if improved theoretical models 

 of growth of opaque zones are developed. We conclude that, 

 for at least the three "reds" species and L. johnii, we have 

 attained Francis's ( 1995) second level of validation — namely, 

 that the opaque zones were formed annually and completed 

 from spring through early summer, but there remains a 

 need to produce quantitative estimates of the accuracy of 

 this aging method. 



Acknowledgments 



This work was supported by grants to D. McB. Williams and 

 G. Russ from the Australian Research Council. Fieldwork 

 was greatly assisted by the fishing skills of S. Boyle, J. 

 Balling, A. Mead, G. Muller and D. Moore. The sea cages 

 were designed and built by P. Speare, and we are grateful to 

 the officers of Argus Security Pty Ltd for feeding the captive 



fish. At the Central Ageing Facility we wish to thank K. 

 Hall for sample registration and database entry and C. 

 Green for advice on interpretation and photomicrography 

 of otoliths. M. Fimeri offered support with image analysis 

 and M. Eden and T. Simmonds drafted the figures. We 

 are particularly grateful for the improvement of the 

 manuscript by three anonymous referees, and for critique 

 of our first working approximation by A. J. Fowler and R. 

 W. Gauldie. 



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