To obtain basic knowledge on sinking death syndrome which has been considered to be one of the causes of early mortality during the seedling production of greater amberjack, ontogenetic changes of larval body density were examined. A rearing experiment was conducted to investigate the eŠects of aeration rate (0.1 2.0 L/min) on the vertical distribution, feeding, survival, growth and initial swim bladder in‰ation of larvae reared in 500 L tanks for 10 days post hatching. Greater amberjack larvae showed negative buoyancy. In tanks with aeration of 0.1 1.0 L/min, larvae were distributed in the upper layer in the daytime and on the tank bottom in the nighttime after 3 days post hatching. Excessive aeration (2.0 L/min) caused larvae to disperse in tanks. However, feeding amounts, and rates of survival, growth and initial swim bladder in‰ation of larvae were low in those tanks with rearing seawater of high turbulence. To prevent the larvae from sinking to the tank bottom, rearing techniques with less physical stress on larvae should be developed.

To understand the mechanism of the initial swim bladder in‰ation of greater amberjack Seriola dumerili larvae, we reared larvae from two days after hatching (DAH) in tanks in which the water surface was sealed with liquid para‹n. This treatment prevented the larvae from access to air through the water surface. Furthermore, we examined the ontogenetic development of larval swim bladders. Percentages of larvae with in‰ated swim bladders were very low in tanks with the sealed water surface compared with those in the tanks in which the oilˆlm was removed from the water surface. A swim bladder was formed as a protuberance from an upper portion of the intestine. A pneumatic duct which connected the swim bladder to the intestine was observed in larvae of 3 DAH. The pneumatic duct was obstructed and separated from the swim bladder in larvae of 5 DAH. Our experiments revealed that a mouth-opened larva of greater amberjack in‰ates its swim bladder by gaping air from the water surface and introducing it into the swim bladder through the pneumatic duct from 3 to 4 DAH.

Sinking death syndrome has been considered as one of the causes of early larval mortality during the seed production of seven-band grouper and kelp grouper. To elucidate the rearing period when larvae show strong minus buoyancy, ontogenetic change of body density was examined for larvae and juveniles of two species. Larvae tended to show nearly neutral buoyancy from hatching to growing to 3.5 3.9 mm in total length (TL) when the buds of the second dorsalˆn and pelvic-ˆn spines appeared. Then, body densities greatly increased with increasing the length ofˆn spines and reached a plateau at 1.060 1.070 g/cm 3 after the size of around 11 mm TL. Swim bladder in‰ation was not observed in the seven-band grouper; however, it was observed in the kelp grouper after 8.2 mm TL and the percentage of kelp grouper with in‰ated swim bladders largely increased after 18.6 mm TL.Body densities of kelp grouper with in‰ated swim bladders decreased to 1.024 1.035 g/cm 3 . It could be speculated thatˆn spines regulate the larval buoyancy by generating drag forces. It is therefore considered that larvae show strong minus buoyancy in rearing seawater at the initiation of great increase in body density whenˆn spines begin to elongate.

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