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Otolith microstructure of larvae of southern bluefin tuna, Thunnus rnaccoyii (Castelnau, 1872), collected from the east Indian Ocean in January/February 1987 was examined using Light and scanning electron microscopy. Daily formation was verified by examining the growth of the marginal increment on otoliths of larvae collected on 6 successive days. Most of the larvae sampled came from a single cohort which spawned over 2 d. The daily progression of increment number for this cohort was further evidence that increments were formed daily. Larvae ranged from ca 7 to 18-d-old; the larval stage appeared to last ca 20 d. Growth curves could not be fitted to plots of size-at-age due to a large variance in size-at-age and violation of assumptions for parametric tests. Back-calculation of individual growth trajectories was possible because growth of sagittae was exponential relatlve to body length, and there was a strong linear relationship between log otolith radius and standard length ( R ~ = 0 91) Growth trajectories of larvae were approximately linear, R' ranging from 0.972 to 1.0, with a med~an value of 0.996. The lstribution of lndiv~dual growth rates was approximately normal, ranging from 0.20 to 0.47 mm d-' and a mean of 0 32 mm d-l. Over the range examined, the growth of T. maccoyii larvae was relatively slow and linear, although the growth trajectories of oldest larvae exhibited a c u ~ l i n e a r Increase in the days immediately preceding collection. The fast growth often attributed to young stages of tuna must occur in the late larval/early juvenile stage rather than the early larval stage of this species.

Age, growth rate, and growth trajectory determined from otolith microstructure of southern bluefin tuna Thunnus maccoyii larvae