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The effect of docosahexaenoic acid (DHA) on the growth performance, survival and swim bladder inflation of larval Seriola dumerili during the rotifer feeding period was investigated in two feeding experiments. Amberjack larvae at 3 day post hatching were fed rotifers enriched with (1) freshwater Chlorella (Chlo), (2) a mixture (2:1, v/v) of Chlo and DHA-enriched Chlorella (DHA-Chlo), (3) DHA-Chlo and (4) DHA-Chlo and commercial DHA emulsion, in triplicate for 7 days. The average DHA contents of the rotifers were 0.0, 0.4, 1.0 and 1.9 mg g À1 DM respectively. The survival rate was improved by the enrichment of rotifers with DHA-Chlo alone, and DHA-Chlo and emulsion. Growth and swim bladder inflation of fish fed rotifers enriched with DHA-Chlo were significantly (P < 0.05) improved, however, with increased levels of DHA further improvement was not found. DHA content in the larval whole body proportionally increased with the DHA level in the rotifers. These results suggest that DHA enrichment of rotifers is effective to improve the growth, survival rate and swim bladder inflation of amberjack larvae. The DHA requirement of amberjack larvae is estimated to be 1.5 mg g À1 on a dry matter basis of rotifers.

Yellowfin tuna Thunnus albacares larvae were reared from hatching beyond metamorphosis in May and June 1996. The larval size was 2.65mm SL at just after hatching and 27.68mm SL on Day 37. Transformation to juveniles occurred around 30 days after hatching at about 13mm SL. The larvae ini tially fed on rotifers since Day 4, on fish larvae and Artemia nauplii since Day 16, and then on frozen fish and minced fish meat. The primitive digestive system differentiated on Day 4. The gastric gland and pyloric caeca first appeared on Day 14 and 16, respectively. The pharyngeal and jaw teeth became fully functional synchronized with gastric gland differentiation. The number of gastric glands and pyloric caeca and volume of the gastric blind sac increased markedly toward the juvenile stage. Although exter nal morphological development of yellowfin tuna resembled to that of other marine fishes hatched from pelagic eggs, the digestive system developed precociously. The rapid development of the digestive sys tem allows the early appearance of piscivory which can support the high growth potential as shown in other scombrid fishes.

Aquaculture is currently a major source of fish and has the potential to become a major source of protein in the future. These demands require efficient aquaculture. The intestinal microbiota plays an integral role that benefits the host, providing nutrition and modulating the immune system. Although our understanding of microbiota in fish gut has increased, comprehensive studies examining fish microbiota and host metabolism remain limited. Here, we investigated the microbiota and host metabolism in the coral leopard grouper, which is traded in Asian markets as a superior fish and has begun to be produced via aquaculture. We initially examined the structural changes of the gut microbiota using next-generation sequencing and found that the composition of microbiota changed between fasting and feeding conditions. The dominant phyla were Proteobacteria in fasting and Firmicutes in feeding; interchanging the dominant bacteria required 12 hours. Moreover, microbiota diversity was higher under feeding conditions than under fasting conditions. Multivariate analysis revealed that Proteobacteria are the key bacteria in fasting and Firmicutes and Fusobacteria are the key bacteria in feeding. Subsequently, we estimated microbiota functional capacity. Microbiota functional structure was relatively stable throughout the experiment; however, individual function activity changed according to feeding conditions. Taken together, these findings indicate that the gut microbiota could be a key factor to understanding fish feeding conditions and play a role in interactions with host metabolism. In addition, the composition of microbiota in ambient seawater directly affects the fish; therefore, it is important to monitor the microbiota in rearing tanks and seawater circulating systems.

To improve the nutritional quality of live foods and dry feeds ordinarily used for the seed production of amberjack Seriola dumerili, the nutrient contents of rotifers, Artemia nauplii and commercial feeds used in two larval production stations were evaluated. For comparison of the nutrient contents, artificially produced larvae, wild-caught juveniles and wild zooplankton samples were also analyzed. The proportions of 22:6n-3 in the polar lipid of the cultured larvae increased by feeding the dry feeds. The taurine contents of the cultured larvae reflected the contents of their foods (rotifers < dry feed < Artemia nauplii). The taurine content and the proportion of 22:6n-3 in Acartia spp. were higher than in foods fed to the larvae. These parameters in the wild juveniles were higher than the cultured ones. The A/E ratios [(each essential amino acid/total essential amino acids) ¥ 1000] of the total amino acids of the live foods and dry feeds were similar to those of the cultured larvae, except for the lower ratios of histidine, arginine, threonine and lysine in the live foods. The mucosal folds of the intestine of the cultured larvae did not show typical signs of dietary phospholipid deficiency. These results suggest that requirements of nutrients such as 22:6n-3 and taurine should be determined for mass production of amberjack seeds.

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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.