Validation of annual growth rings in the otoliths of the lethrinid Lethrinus mahsena and the lutjanid Aprion virescens from sites in the tropical Indian Ocean, with notes on the nature of growth rings in Pristipomoides filamentosus.

Abstract

The lutjanids Pristipo- moides fi lamentosus and Aprion vires- cens and the lethrinid Lethrinus ma h - sena are commercially important de- mersal bank and deep slope reef fi sh from the central Indian Ocean. To obtain von Bertalanffy growth parameter esti- mates for management purposes, length- based methods are commonly applied by the fi sheries institutions of the region. Because the relatively long-lived, slow- growing nature of these species results in a lack of distinct modal progression in length-frequency data, such estimates are unreliable. In an attempt to obtain more reliable growth estimates, the fea- sibility of age-based methods (where age is determined from annual incre- ments in otoliths) was investigated. Successful validation of annual or daily increments has been reported in two of these species (P. fi lamentosus and A. virescens), but not for the target areas of our study: the banks of the Seychelles and Mauritius. A range of methods was used in an attempt to ensure that the otoliths ful- fi lled the criteria for use in aging. Two methods are described in this paper: back-calculation and a combination of marginal increment and edge analysis. The results of validation are presented, along with a description of the problems encountered. Marginal increment and edge analysis both indicated that the increments present in the otoliths of L. mahsena are annuli. For A. virescens, no pattern was present in the marginal increment analysis of older individu- als. However, edge analysis offered evi- dence that the increments present in the otoliths were annuli. The combined marginal increment and edge analysis proved inconclusive for P. fi lamentosus; therefore the increments present in the otoliths of this species could not be vali- dated. Conclusions are drawn regarding the justifi cation of assuming periodic- ity of increments on the basis of valida- tion achieved in other locations.