Ontogenetic Change in Buoyancy in the Early Stage of Red Sea Bream.

The densities of red sea bream Pagrus major larvae and juveniles were measured in a series of saline solutions of different densities. The volume of swim bladder and body weight on a wet and dry basis were also measured.After the embryonic and prelarval period with slightly positive or virtually neutral bouyancy, the density of postlarvae increased to a level of ca. 1.028g/cm3 and began to change diurnally. After the inffation of the swim bladder, a diel change of buoyancy became marked, viz. day-negative and night-neutral. Buoyancy regulation was effected by a voluminal change in the swim bladder, which was induced by a photocycle, viz. light-deflation and dark-inflation. No such diel change of density could be detected in the transition from larva to juvenile of 12 to 13mm TL, and the density increased toward ca. 1.035g/cm3 at 25mm TL.From these results, it is suggested that the diel and ontogenetic change in buoyancy in larval and juvenile red sea bream controls their vertical distribution and habitat transition.