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This study was conducted to investigate the effect of fish protein hydrolysate on growth performance, insulin-like growth factor I (IGF-I) levels and the expression levels of liver IGF-I mRNA in juvenile Japanese flounder (Paralichthys olivaceus). Fish hydrolysate was produced by enzymatic (alcalase and flavourzyme) treatment and size-fractionated by ultrafiltration. The permeate after ultrafiltration (UF) and the non-ultrafiltered fish hydrolysate were tested as feed ingredients using high plant protein diets. Fish meal was used in the control diet (FM). The feeding trial lasted for 60 days, and fish fed with 37 g kg )1 UF showed the best growth, feed efficiency, digestibility and protein utilization. Plasma IGF-I level was examined with radioimmunoassay, and the expression levels of liver IGF-I mRNA were evaluated using real-time PCR normalized against the 18S rRNA gene. Plasma IGF-I levels were significantly increased by inclusion of fish protein hydrolysate. Liver IGF-I mRNA expression was significantly higher in fish fed with 37 g kg )1 UF diet than fish fed with control diet. The results indicated that small molecular weight compounds from fish protein hydrolysate showed a positive effect on growth and feed utilization in juvenile Japanese flounder. Dietary fish protein hydrolysate could improve plasma IGF-I levels and liver IGF-I mRNA expression in Japanese flounder.

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Four experimental diets were fed to turbot to examine the effect of fish hydrolysate and ultra-filtered fish hydrolysate on growth performance, feed utilization and non-specific immune response. Fish hydrolysate was produced by enzymatic treatment and size fractionated using ultra-filtration (UF). The permeate (molecular weight <1000 Da) after UF and the non-ultra-filtered fish hydrolysate (NUF) were tested as feed ingredients. Diets UF1, UF2 contained 3.7%, 1.2% ultra-filtered fish hydrolysate to replace fish meal protein respectively. The diets UF1, NUF were identical in composition except that the molecular weight of fish hydrolysate in the diet. Fish meal was used in the control diet. All diets were made equal in protein, lipid and energy. Each experimental diet was fed to juvenile turbot (27.87 ± 0.04 g) in triplicate for 8 weeks. Results of this study indicate that the best overall growth and feed utilization of turbot juveniles were obtained with a diet containing higher dose of the small molecular weight compounds in fish hydrolysate. Acid phosphatase, alkaline phosphatase, lysozyme and superoxide dismutase activity in serum were not affected by diet. Total antioxidant capacity was improved with increasing level of low molecule weight fish hydrolysate (UF1).

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