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

Abstract

Otolith microstructure of larvae of southern bluefin tuna, Thunnus maccoyii (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 relative to body length, and there was a strong linear relationship between log otolith radius and standard length (R2 = 0.91). Growth trajectories of larvae were approximately linear, R2 ranging from 0.972 to 1.0, with a median value of 0.996. The distribution of individual growth rates was approximately normal, ranging from 0.20 to 0.47 mm d-1 and a mean of 0.32 mm d-1. Over the range examined, the growth of T. maccoyii larvae was relatively slow and linear, although the growth trajectories of oldest larvae exhibited a curvilinear 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.