L-hydroxyproline (N99% pure) was obtained from Hengyuan Biotech (Shanghai, China) Co., LTD. Fishmeal, soybean meal, corn gluten meal,wheat gluten meal, peanut meal, and beer yeast were used as the primary protein sources. Fish oil and soybean lecithin were used as the lipid sources. Wheat flour was used as the carbohydrate sources. Seven isonitrogenous and isoenergetic diets were formulated: a reference diet (FM) containing 60% fish meal and three other diets (40I, 50I and 60I) in which 40%, 50% and 60% fish meal was substituted. Based on the supplementation levels used in reports (Asknes et al., 2008; Zhanget al., 2013), experimental diets with plant protein substitutions were either supplemented with 0.6% hyp or not. All ingredients were grounded into fine powder through 178 μm mesh. Ingredients were blended with oil and water then added. Pellets (3 mm × 4 mm) were made automatically by pellet-making machine and dried for 12 h in a ventilated oven at 40 °C. The hyp concentrations after diet preparation were quantitated as 0.23% (FM), 0.16%(40I), 0.58%(40I HYP), 0.18% (50I), 0.62% (50I HYP), 0.14% (60I) and 0.57% (60I HYP). DL-Methionine, L-Threonine,L-Histidine, and Lys-H2SO4 (Crystalline amino acids) were supplemented to meet the essential amino acid (EAA) requirements of juvenile turbot based on the amino composition of FM diet. All the compositions of experiment diets were shown in Table 1.
