838 



Fishery Bulletin 92(4). 1994 



opment. Our current data are much too sparse for 

 any statistically powerful test of year-class differ- 

 ences, but preliminary results suggest only small 

 differences among years for most sites. This is con- 

 sistent with the similar classifications of adults and 

 juveniles for both the NSW/Victorian and Tasmanian 

 regional groupings; although the adult and juvenile 

 samples differ in mean birth date by five years ( adults 

 from the 1980-84 year classes, juveniles from the 

 1987 and 1988 year classes), the samples overlap 

 broadly in the concentrations of the regionally diag- 

 nostic elements. 



We tentatively conclude that the regionally diag- 

 nostic 'base' concentrations of most measured ele- 

 ments probably have a genetic basis. This conclu- 

 sion conflicts with both genetic analyses of the spe- 

 cies in Australia (Richardson, 1982; Elliott and Ward, 

 1994; Grewe et al., 1994), which indicate no regional 

 differences and with our preliminary, conservative 

 estimate of larval mixing among regional groupings. 

 At this point, the data are not adequate to resolve 

 this contradiction. Its resolution, however, critically 

 affects the way compositional data obtained from 

 electron-probe microanalysis are used for stock de- 

 lineation. If the regional differences are primarily 

 genetically determined, then year-class effects are 

 likely to be relatively unimportant. This simplifies 

 analysis but also implies that the usefulness of the 

 approach depends on the extent and pattern of ge- 

 netic differentiation among populations. However, if 

 regional differences in 'base' concentrations are pri- 

 marily determined by environmental effects, perhaps 

 via a 'locked phenotype' mechanism of some kind, 

 then variability among year classes could be a criti- 

 cal covariate in an analysis of population structure. 

 In that case, electron-probe microanalysis is likely 

 to be useful wherever significant environmental dif- 

 ferences between spawning grounds are known or 

 suspected. 



Acknowledgments 



We thank N. Barrett, C. Bobbi, T Carter, A. Gronell, 

 A. Jordan, S. Sie, and G. Suter for assistance in labo- 

 ratory and field studies, J. Kalish, N. Manning, and 

 T Rees for advice on specimen preparation, K. 

 Haskard and M. Cameron for advice on statistical 

 analyses, V. Mawson, D. Mills, D. Secor, R. Ward, 

 and two anonymous referees for comments on the 

 manuscript, and C. McRae and R Rummel for assis- 

 tance in microprobe analysis and discussion of re- 

 sults. This study was funded in part by grants 1987/ 

 15 and 1989/30 from the Australian Fishing Indus- 

 try Research and Development Committee. 



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