ONTOGENIC CHANGES IN SPECIFIC-GRAVITY DUR ING EARLY DEVELOPMENT OF BLACK-SEA BREAM

The specific gravity of black sea bream Acanthopagrus schlegeli larvae and juveniles was measured in a series of saline solutions. The volum e of the swim bladder and wet and dry fish weights were also measured. During the egg and prelarval periods, they have a slightly positive o r neutral buoyancy in sea water of ca. 1.024 specific gravity. During the postlarval period, specific gravity increased to a level ca. 1.028 and began to change diurnally. After inflation of the swim bladder, v oluminal changes in it caused the postlarvae to be negatively buoyant during the day and neutlally buoyant at night. These changes could be induced by manipulation of the photocycle. No diel change in specific gravity could be detected in black sea bream of 20 mm. These results i ndicate that levels in the specific gravity of black sea bream in the specific gravity of black sea bream during the larval and juvenile per iods are much lower than those reported for the red sea bream during t his same period. It has been suggested that this difference influences the differences in behavior of these two sparids which results in dif ferences in the vertical distribution from the pelagic to benthic life .