AGING OF 3 SPECIES OF TROPICAL SNAPPER (LUTJANIDAE) FROM THE GULF OF CARPENTARIA, AUSTRALIA, USING RADIOMETRY AND OTOLITH RING COUNTS

A radiometric ageing method was used to resolve conflicting results fr om ageing tropical lutjanids based on annual ring counts in whole and sectioned otoliths. The number of rings detected in sectioned otoliths of Lutjanus erythropterus, L. malabaricus, and L. sebae from unexploi ted populations in the Gulf of Carpentaria, Australia, were 1.6 to 2.4 times the number found in whole otoliths. To obtain an independent es timate of age, we measured Pb-210/Ra-226 radioactive disequilibria of both whole and cored otoliths. As all species had high levels of Ra-22 6, they could be aged with relative accuracy by this method. Samples o f whole otoliths and cores with a similar ring count had similar radio metric ages. In samples whose sectioned and whole-otolith ages differe d by more than 4 years, the whole otolith ring count agreed better wit h the radiometric age (for an uptake activity ratio R=0.0). This resul t stands in marked contrast to the radiometric age validation of secti on counts for slow-growing, long-lived fish inhabiting temperate to su btemperate waters. In this region, all species lived less than 10 year s and grew to a maximum size of up to 600 mm SL. They reached a simila r length in one year, but L. erythropterus grew faster than the other two species thereafter. The sexes had the same growth rates. Our resul ts were similar to those found for these species elsewhere and suggest that in tropical fishes, such as lutjanids, rings observed in section ed otoliths and other hard parts may not be formed annually. Where pos sible, ages derived from counts in these structures should be verified by independent methods.