Swimbladder inflation associated with body density change and larval survival in southern bluefin tuna Thunnus maccoyii

High mortality of southern bluefin tuna (SBT) Thunnus maccoyii larvae in captivity is a major problem hindering culture of this species. The relationships between body density of SBT larvae, swimbladder inflation and survival were investigated in this study. Swimbladder inflation and changes in volume have a direct effect on body density in larval SBT. Swimbladder inflation was first observed at 3 days post hatch, and larvae with successful swimbladder inflation were able to maintain their body density within a narrow range ({increment}ρ = 0.0006 g cm^-3). Although swimbladder volume increased with larval growth, it could not compensate for the increase in body density and did not prevent nocturnal sinking. The increase in body density was greater for larvae that did not inflate their swimbladder. Low percentages of swimbladder inflation (27.5 ± 3.5 %) coupled with negative body buoyancy of SBT might contribute to mortality as larvae sank in the dark phase and made contact with the tank bottom. Management strategies that maintain larvae within the water column and enhance swimbladder inflation are required to improve survival of SBT larvae reared in hatcheries.