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Fishery Bulletin 93(1), 1995 



annually, but the season when this ring is formed 

 varies between studies and species. Such ambigu- 

 ities cast doubt on the validity of the conclusions and 

 suggest that differences in the estimated growth rate 

 and age of Lutjanus populations may be related to 

 problems of interpretation rather than to biological 

 differences. 



A radiometric method has recently been used suc- 

 cessfully to estimate the age of long-lived fishes 

 (Bennett et al., 1982; Fenton et al., 1991). This 

 method uses the known decay rates of isotopes of 

 Radium-226 ( 226 Ra) and Lead-210 ( 210 Pb) in bony 

 parts to estimate the age of fish. It does not rely on 

 operator interpretation to estimate age and there- 

 fore is particularly useful for ageing long-lived spe- 

 cies where the growth rings are often not clearly de- 

 fined (Casselman, 1974). 



The objectives of the present study were 1) to esti- 

 mate the age and growth of Lutjanus malabaricus, 

 L. erythropterus, and L. sebae from the Gulf of 

 Carpentaria by counting rings in whole and sectioned 

 otoliths; and 2) to use the 210 Pb/ 226 Ra radiometric 

 ageing method to make an independent age estima- 

 tion of the same fish. 



Materials and methods 



Sampling 



Most samples of Lutjanus erythropterus, L. mala- 

 baricus, and L. sebae were collected during a sys- 

 tematic survey of the Gulf of Carpentaria between 

 long. 136° and 142°E in November 1990. Two similar 

 random-sampling surveys were made across the 

 northern Gulf of Carpentaria (north of lat. 14°30'S) 

 in November 1991 and January 1993. Samples 

 of Lutjanus malabaricus were also collected during 

 a survey of eight areas in the Gulf of Carpen-taria by 

 the commercial trawler Clipper Bird in June 1990 

 (Fig. 1). Details of survey design, trawl gears, and 

 trawl durations are given in Blaber et al.(1994). 



Commercial-sized Lutjanus malabaricus (1-3 kg) 

 were obtained from fish retained for sale after the 

 June 1990 survey. During the systematic survey in 

 November 1990, all specimens of the three target 

 species of lutjanids were retained for ageing studies. 

 In November 1991 and January 1993, only fish from 

 length classes underrepresented in previous samples 

 were processed. All fish were measured (standard 

 length [SL] in mm), weighed (±1 g), and sexed, and 

 both sagittae were removed, dried, and stored in la- 

 belled bags for future analysis. 



Radiometry 



Radioanalysis requires about 1 g of sample material; 

 therefore, fish were pooled to obtain the necessary 

 sample weight. For juveniles, up to four otoliths were 

 required to obtain this weight. Otoliths used for 

 radioanalysis were chosen in two ways. First, for each 

 species, otoliths from juvenile, maturing, and ma- 

 ture fish that had the same sectioned-otolith ages, 

 similar otolith weights, and similar sizes, and came 

 from the same region of the Gulf of Carpentaria were 

 pooled for radioanalysis. Second, otoliths of the same 

 whole-otolith age and from fish of similar size were 

 abraded with a mechanical sander to a central core 

 approximating the weight (see Table 2), length, and 

 shape of the otolith of a fish whose whole-otolith age 

 was 3 (otolith length=11.6 ± 0.7 for L. erythropterus; 

 12.7 ± 0.4 for L. malabaricus; and 11.4 ± 0.4 for L. 

 sebae). The exception was sample 2490 for which 

 otoliths were ground to a core age of 2 (weight 0.156 

 ± 0.005 g; otolith length 9.2 ± 0.16 mm; n=92). The 

 otolith nucleus at the center of the cores was located 

 by examining intact otolith morphology and by sec- 

 tioning other samples (Campana et al., 1990). This 

 age is less than the age at sexual maturity for all 

 species. 



