Requirement of access to an air–water interface for initial swim bladder inflation and ontogenetic and day–night change of body-specific gravity in Japanese amberjack, Seriola quinqueradiata, larvae

Swim bladder dysplasia is closely related with larval mass mortality due to sinking syndrome and juvenile vertebral malformations in many cultured fishes. This study revealed the appropriate environmental condition for initial swim bladder inflation (ISI) of transiently physostomous Seriola quinqueradiata and the ontogenetic and day–night change of larval swim bladder volume and body-specific gravity (BSG) which elucidates a part of sinking syndrome mechanism. 77.7% of larvae reared with a clearer water surface had inflated swim bladders, while those reared with oil sealed had a very low percentage lower than 6.7%. This indicates their requirement of a clearer air–water interface to initially inflate swim bladders by gulping air and the necessity of treatments to remove the surface film of rearing water. Larval BSG was estimated from their sinking speed through the bilayer sea water column which consisted of 1/2 and 1/1 sea water in the upper and in the lower layer respectively. Larval BSG increased from 1 to 4 days post hatch (dph), thereafter decreased until 9 dph and increased a little again from 10 dph, and averaged larval BSG was higher than that of sea water. This suggests the critical period of larval sinking on 3, 4 and 10 dph.