Fishery Bulletin 93(1), 1995 



Figure 4 



Plot of the mean length-at-age (±) range and the 

 growth curves of the three species oiLutjanus based 

 on ring counts in whole and sectioned otoliths. 



than were other species (P<0.05). However, at six 

 years of age (L 3 ) they were about the same size as L. 

 erythropterus but were larger than L. malabaricus 

 (P<0.05). 



Relationship of ring counts in whole and 

 sectioned otoliths with radiometric ages 



There was a significant linear relationship between 

 both whole and sectioned otolith ring counts and ra- 

 diometric age (Fig. 7; P<0.001 in both cases). The 



slopes of the lines of best fit differed (/}=1.04 ± 0.11; 

 r 2 =0.84 for whole otolith ring counts and /?=1.83 ± 

 0.06; r 2 =0.87 for sectioned otolith ring counts). Be- 

 cause the initial activity ratios of the L. sebae samples 

 (R) were obviously greater than 0.0 in at least the 

 whole otolith samples, these were not included in the 

 analyses. 



There was no significant difference between whole 

 otolith ring counts and radiometric ages for all spe- 

 cies combined (T=53.5; P>0.30, re=15) or for L. 

 malabaricus (T=17.5; P>0.15, rc=10). However, for all 

 species combined we found that the sectioned ring 

 counts were significantly greater than the radiomet- 

 ric age of the same fish (T=6.5; P<0.001, ra=15). The 

 sectioned ring counts of L. malabaricus were also 

 greater than the radiometric ages (T=2; P<0.005, 

 n=10). 



Discussion 



This is the first study to use 210 Pb/ 226 Ra activity ra- 

 tios to verify the age of relatively short-lived tropi- 

 cal fishes. Previous studies that have used these ra- 

 tios to estimate age have focussed on species that 

 live to at least 70 years (Bennett et al., 1982; 

 Campana et al., 1990; Fenton et al., 1991). In the 

 Lutjanidae, natural levels of 226 Ra in the otoliths 

 were high, which helped to minimize the variances 

 in the 210 Pb/" 226 Ra activity ratio and hence the errors 

 in the age estimates. Radiometry provided strong 

 evidence that the rings counted in whole otoliths were 

 the best estimate of the true age of the three lutjanids 

 studied. 



The radiometric methods we used tend to overes- 

 timate age because the assumptions concerning the 

 otolith mass growth model and rate of incorporation 

 of allogenic 210 Pb were conservative. The only con- 

 ceivable mechanism that would lead to underesti- 

 mation of ages radiometrically would be a signifi- 

 cant loss of radon ( 222 Rn) from otoliths during growth 

 (West and Gauldie, in press). 



Radon is the daughter of 226 Ra and the only gas- 

 eous precursor of 210 Pb in the decay chain. Its mean 

 lifetime is only 4.8 x 10 5 seconds, and its effective 

 (physical) diffusivity in otoliths would be about 0.5 x 

 10" 12 m 2 -s _1 . Radon diffusion out of otoliths would be 

 further retarded by adsorption to organic matter 

 (Wong et al., 1992). Simple calculations based on the 

 known microstructure of otoliths (Campana and 

 Neilson, 1985) and on the existing data on radon 

 emanation (Morawska and Phillips, 1993) show that 

 significant loss of radon from otoliths is extremely 

 unlikely, as previously suggested from empirical stud- 

 ies (Fenton and Short, 1992). 



