We examined the effect of feeding ration and refeeding time on plasma biochemical components and hepatic insulin‐like growth factor gene (IGF) mRNA expression to evaluate nutritional indicators in juvenile Yellowtail Seriola quinqueradiata (also known as Buri). Fish were fed a diet for 10 d with the following feeding rations: 80, 40, 20, and 10% satiation. Satiated feeding was 4.65% of body weight. At day 10, the plasma concentrations of albumin, calcium, inorganic phosphate, and total protein, activities of alkaline phosphatase and amylase, and hepatic IGF1 mRNA and IGF2 mRNA expression were significantly correlated with feeding ration. In the analysis of refeeding time, fish were refed after a 24‐h fasting period and plasma biochemical components and hepatic IGFs mRNA were analyzed at 0, 3, 6, and 9 h after refeeding. Plasma concentrations of total bilirubin, inorganic phosphate, creatinine, glucose, and potassium, and activities of amylase were significantly correlated with refeeding time. The correlation between refeeding time and gene expression was significant for IGF2 mRNA (r = 0.70, P < 0.05) but not for IGF1 mRNA (r = 0.67, P = 0.07). Comparing the results of the two experiments, the plasma concentrations of total protein, albumin, and calcium reflected only the feeding ration. The plasma concentration of inorganic phosphate, activities of alkaline phosphatase and amylase, and hepatic IGF1 mRNA and IGF2 mRNA reflected both the feeding ration and refeeding time. These observations suggest that these components reflected the nutritional status of Yellowtail, but sampling time should be carefully examined because of the fluctuating feature of these components.

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We investigated the effect of dietary supplementation with a branched-chain amino acid (BCAA) mixture (valine, leucine and isoleucine) on juvenile yellowtail (Seriola quinqueradiata) growth and hepatic insulin-like growth factor (IGF) gene expression. Total dietary BCAA content was 8.1, 8.5, 9.0 and 9.8 g kg À1 for the control, 0.4%, 1.0% and 2.0% BCAA experimental groups, respectively. Body weight was measured on day 77, after which fish were fasted for 3 days and refed; the livers were then removed 3 or 9 h later for analysis of hepatic gene expression. Body weight significantly increased in the BCAA-supplemented groups. Hepatic expression of IGF1 and IGF2 significantly increased in the 2% BCAA group at 3 and 9 h after refeeding. In the 1% BCAA group, hepatic IGF1 expression tended to be higher at 9 h than in the control group after refeeding. Also, hepatic IGF2 significantly increased at only 9 h after refeeding in the 1% BCAA group. In conclusion, dietary supplementation with crystalline BCAAs increased growth and hepatic expression of IGF1 and IGF2. These results suggest that dietary crystalline BCAA supplementation would be a valuable addition to yellowtail aquaculture practices.

We examined the effect of dietary supplementation of leucine on body weight and hepatic insulin‐like growth factor (IGF) and IGF‐binding protein (IGFBP) gene expression in juvenile Japanese Amberjack Seriola quinqueradiata. Crystalline leucine was dissolved in deionized water and added to the basal diet. Deionized water only was added to the basal diet as a control. The dietary concentration of free leucine was 0.03, 0.85, and 2.43 g/100 g diet for the control, 1% leucine‐supplemented, and 3% leucine‐supplemented diets, respectively. A growth trial was conducted for 77 d, after which the body weight was measured and the fish were then fasted for 3 d and refed. At 3, 9, and 24 h after refeeding, the liver and muscle were dissected for gene expression analysis. The mean body weight significantly increased in the 1% leucine‐supplemented group during the growth trial. The expression of hepatic IGBFP‐1 significantly increased at 3 and 9 h after refeeding in the leucine‐supplemented groups. The expression of hepatic IGF1 significantly decreased at 9 h and that of IGF2 significantly decreased at 9 and 24 h after refeeding in the 3% leucine‐supplemented group. The expression of hepatic IGFBP‐2, IGFBP‐3, and IGFBP‐5 were similar among the groups and were only affected by feeding time. In conclusion, we demonstrated for the first time that leucine supplementation increases body weight and influences the hepatic growth factor system in Japanese Amberjack.

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