Milton etal.: Ageing of Lutjanus erythropterus. L. malabancus. and L sebae 



13 



1985; Grant, 1985) or 16 to 22 kg (Grant, 1985; Allen 

 and Swainston, 1988), which is much greater than we 

 recorded (5 kg). This species may, therefore, live more 

 than 10 years. Indeed, large L. sebae (over 800 mm) 

 from the Great Barrier Reef are known to live on deep 

 coral reefs at depths greater than 60 m 1 ; the deepest 

 part of the Gulf of Carpentaria is only 55 m. This sug- 

 gests that fish may move from this region as they grow. 



Our radiometric ageing results have several im- 

 portant implications beyond the verification of the 

 age structure of each species. First, they demon- 

 strated that for species that have a high otolith 226 Ra 

 specific activity, 210 Pb/ 226 Ra activity ratios can be 

 used to age fish as young as 3 years with accuracy. 

 Previously these radioisotopes have only been used 

 to age long-lived species (>10 yr; Bennett et al., 1982; 

 Campana et al., 1990; Fenton et al., 1991). Other 

 radioisotope pairs ( 228 Th: 228 Ra) have been used to age 

 short-lived tropical species (Campana et al., 1993), 

 but these are only useful for fish up to 5 years old 

 because of the short half-life of Th-228. 



Second, for relatively short-lived species, radiomet- 

 ric ageing of whole otoliths and cores using a single- 

 phase linear model of otolith mass growth rate gave 

 similar results. Campana et al. (1990) and Smith et 



1 Williams, D. Australian Institute of Marine Science, PMB No. 

 3, Townsville 4810, Queensland, Australia. Personal commun., 

 1993. 



al. (1991) argued that new material accreting to the 

 outer surface of the otolith may not accrete 226 Ra in 

 similar specific activities to the juvenile (t=0). This 

 would invalidate the use of a simple otolith mass 

 growth model to interpret the radiometric data for 

 otoliths of postjuvenile fish. However, even with a 

 single-phase linear mass growth model (a two-phase 

 model would have reduced the age estimates), we 

 were able to verify that the ring counts in whole 

 otoliths were a more accurate measure of the true 

 age than counts from sectioned otoliths (in accord 

 with core radiometric ages). However, we agree with 

 Smith et al. (1991) that otoliths should be cored for 

 radiometric ageing, if possible, which would avoid 

 the use of an otolith mass growth model. 



The third point that arises from our analyses re- 

 lates to the ratio of allogenic to radiogenic lead in 

 Lutjanus otoliths. We set the uptake activity ratio 

 value at zero (R=0.0) because higher values would 

 have lowered the age estimates (e.g. Smith et al., 

 1991). However, from the stable lead/barium ratios 

 and the high age estimates of two of the L. sebae 

 samples (2068 and 2069) it appears that, at least for 

 this species, the juveniles may be taking up more 

 allogenic 210 Pb than the adults (Fenton and Short, 

 1992). There was no systematic increase in the Pb/ 

 Ba mass ratios of L. malabaricus and there is insuf- 

 ficient data for L. erythropterus to be conclusive (Fig. 



