Validation of annual growth increments 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 increments in Pristipomoides filamentosus


Pilling, Graham M., Richard S. Millner, Michael W. Easey, Christopher C. Mees, Shyama Rathacharen, and Roland Azemia
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The lutjanids Pristipomoides filamentosus and Aprion virescens and the lethrinid Lethrinus mahsena are commercially important demersal bank and deep slope reef fish from the central Indian Ocean. To obtain von Bertalanffy growth parameter estimates for management purposes, length-based methods are commonly applied by the fisheries 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 feasibility of age-based methods (where age is determined from annual increments in otoliths) was investigated. Successful validation of annual or daily increments has been reported in two of these species (P. filamentosus 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 fulfilled 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 individuals. However, edge analysis offered evidence that the increments present in the otoliths were annuli. The combined marginal increment and edge analysis proved inconclusive for P. filamentosus; therefore the increments present in the otoliths of this species could not be validated. Conclusions are drawn regarding the justification of assuming periodicity of increments on the basis of validation achieved in other locations.