The specific gravities of the ayu Plecoglossus altivelis larvae were measured in a series of saline solutions. The volume of swim bladder and wet and dry fish weight were also measured.
After decresasing from 1.038 to 1.032 through the prelarval period, the specific gravity of postlarvae with uninflated swim bladder increased to 1.050. The larvae began to inflate their swim bladder while 15 to 20mm TL during the nighttime and 21 to 35mm TL during the daytime. With inflation of the swim bladder, the specific gravity decreased remarkably and its diel change became to be marked, by day-increase and night-decrease. Buoyancy regulation was effected by the change of the swim bladder volume, which was induced by a photocycle, viz. light-deflation and dark-infla-tion. The gas in the swim bladder originated from the air swallowed at the water surface.
These results suggest that ayu larvae display diel vertical migration, viz. dusk-upward and dawn-downward, similar to several physostomous species such as anchovy and herring.
