A new approach to validation of periodicity and timing of opaque zone formation in the otoliths of eleven species of Lutjanus from the central Great Barrier Reef


Cappo, Mike, Peter Eden, Stephen J. Newman, and Simon Robertson
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A model was proposed for validation studies of the periodicity and timing of growth checks on fish otolith sections, based on measurements of otolith radii around tetracycline (OTC) marks. Continuous variables were obtained by expressing measurements of the marginal increment and distance between the OTC mark and the subsequent opaque zone as “fractions” of the width of a completed increment cycle within an otolith. The sum of these fractions and the counts of whole cycles completed outside the OTC mark, divided by the known time at liberty, produced estimates of the periodicity of opaque zone completion. Given this rate of completion and known dates of OTC marking and sacrifice, the marginal increment was used to estimate a date on which the outermost opaque zone was completed. The model was applied to 82 marked fish of 11 Lutjanus species recovered after 6–22 months at liberty, and an hypothesis of annual periodicity, about a mean of 0.96 ±0.32 cycles/yr, was retained for the pooled species along the best, ventral reading axis. Model estimates for L. erythropterus, L. johnii, L. malabaricus, and L. sebae were in the range of 0.78 ±0.22 cycles/yr to 1.03 ±0.29 cycles/yr along this axis. Median ages of these fish were 3+ for L. erythropterus and L. sebae and 5+ for L. johnii. A two-fold difference in somatic and otolith growth detected between field-tagged and captive fish did not affect periodicity. Extension of the model suggested false annuli were induced by changes in salinity or adverse weather. On average, annuli were completed within 1–3 months after the minima in water temperature, in the austral spring–early summer, around early September for L. erythropterus and L. johnii, late September for L. malabaricus and late October for L. sebae. Results from the model were only preliminary for the small samples of L. argentimaculatus, L. bohar, L. carponotatus, L. monostigma, L. rivulatus, and L. vitta, although the common validation approach indicated annual periodicity of opaque zone completion for these species.