Abstract. — A radiometric age- 

 ing method was used to resolve con- 

 flicting results from ageing tropi- 

 cal lutjanids based on annual ring 

 counts in whole and sectioned 

 otoliths. The number of rings de- 

 tected in sectioned otoliths of Lut- 

 janus erythropterus, L. malabar- 

 icus, and L. sebae from unexploited 

 populations in the Gulf of 

 Carpentaria, Australia, were 1.6 to 

 2.4 times the number found in 

 whole otoliths. To obtain an inde- 

 pendent estimate of age, we mea- 

 sured 210 Pb/ 226 Ra radioactive dise- 

 quilibria of both whole and cored 

 otoliths. As all species had high lev- 

 els of 226 Ra, they could be aged with 

 relative accuracy by this method. 

 Samples of whole otoliths and cores 

 with a similar ring count had simi- 

 lar radiometric ages. In samples 

 whose sectioned and whole-otolith 

 ages differed by more than 4 years, 

 the whole otolith ring count agreed 

 better with the radiometric age (for 

 an uptake activity ratio i?=0.0). 

 This result stands in marked con- 

 trast to the radiometric age valida- 

 tion of section counts for slow-grow- 

 ing, long-lived fish inhabiting tem- 

 perate to subtemperate waters. In 

 this region, all species lived less 

 than 10 years and grew to a maxi- 

 mum size of up to 600 mm SL. They 

 reached a similar length in one 

 year, but L. erythropterus grew 

 faster than the other two species 

 thereafter. The sexes had the same 

 growth rates. Our results were 

 similar to those found for these spe- 

 cies elsewhere and suggest that in 

 tropical fishes, such as lutjanids, 

 rings observed in sectioned otoliths 

 and other hard parts may not be 

 formed annually. Where possible, 

 ages derived from counts in these 

 structures should be verified by 

 independent methods. 



Ageing of three species of 

 tropical snapper (Lutjanidae) from 

 the Gulf of Carpentaria, Australia, 

 using radiometry and 

 otolith ring counts 



David A. Milton 



CSIRO Division of Fisheries 



Marine Laboratories. RO. Box 1 20 Cleveland. Queensland 4 1 63, Australia 



Steven A. Short 



Environmental Radiochemistry Laboratory 

 ANSTO. Private Mail Bag 1 

 Menai. NSW. 2234. Australia 



Present address: Kmgett Mitchell and Assoc. 



RO. Box 33-849. Auckland. New Zealand 



Michael F. O'Neill 

 Stephen J. M. Blaber 



CSIRO Division of Fisheries 



Marine Laboratories. RO. Box 1 20 Cleveland. Queensland 4 1 63. Australia 



Manuscript accepted 18 August 1994. 

 Fishery Bulletin 93:103-115 (1995). 



Tropical fishes can be difficult to age 

 because many species do not deposit 

 annual rings in their hard parts 

 (Longhurst and Pauly, 1987). 

 Lutjanids, which are highly valued 

 commercial fishes in the tropical 

 Indo-Pacific region, often have ring 

 patterns in their hard parts that are 

 difficult to interprete (e.g. Davis and 

 West, 1992). The age and growth of 

 many lutjanid species have been 

 well studied and the results of these 

 studies have formed the basis of 

 age-structured stock assessments 

 upon which the management of 

 these fisheries is based (e.g. Sains- 

 bury, 1988). 



In the western Pacific, Lutjanus 

 malabaricus has been the most 

 widely studied lutjanid, as it is the 

 main catch of trawl and line fisher- 

 ies in northern Australia, adjacent 

 Indonesian waters, and in the South 

 China Sea. The reported maximum 



age (up to 10 yr ) and growth param- 

 eters differ both between regions 

 (Lai and Lui, 1974, 1979) and 

 within one area (northern Austra- 

 lia: Lai and Lui, 1979; Chen et al., 

 1984; Edwards, 1985; McPherson 

 and Squire, 1992). These studies 

 estimated age from growth rings in 

 vertebrae (Lai and Lui, 1979; Chen 

 et al., 1984; Edwards, 1985) or in 

 whole otoliths (McPherson and 

 Squire, 1992). The latter method 

 may underestimate the age of 

 longer-lived species because of the 

 difficulty of distinguishing all the 

 growth rings (Casselman, 1974). 



The timing of formation of annual 

 growth rings in Lutjanus from 

 northern Australia has not been 

 fully verified. Several authors (Lai 

 and Lui, 1974, 1979; Chen et al., 

 1984; Yeh et al., 1986; Davis and 

 West, 1992) have concluded that the 

 outer ring is probably deposited 



103 



