1Department of Marine Biosciences, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, 2Shibushi Station, National Center for Stock Enhancement, Fisheries Research Agency, Shibushi, Kagoshima 899-7101, 3Stock Enhancement Technology Development Center, National Research Institute of Aquaculture, Fisheries Research Agency, Saiki, Oita 879-2602, Japan
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 dorsal fin and pelvic-fin spines appeared. Then, body densities greatly increased with increasing the length of fin spines and reached a plateau at 1.060-1.070 g/cm3 after the size of around 11 mm TL. Swim bladder inflation 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 inflated swim bladders largely increased after 18.6 mm TL. Body densities of kelp grouper with inflated swim bladders decreased to 1.024-1.035 g/cm3. It could be speculated that fin 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 when fin spines begin to elongate.