Effects of fishmeal replacement with soy protein hydrolysates on growth performance, blood biochemistry, gastrointestinal digestion and muscle composition of juvenile starry flounder (Platichthys stellatus)

The nutritional value of soy protein can be improved by enzymatic modification into hydrolysates (SPH), which provide an alternative ingredient for replacing fishmeal protein (FMP) in feed. This study assessed the feasibility of replacing FMP with SPH in diet for juvenile starry flounder (Platichthys stellatus Pallas). The changes in growth performance, blood biochemical indices, gastrointestinal enzyme activities, and muscle composition of fish were examined after 9 weeks of feeding with iso-energetic and iso-proteic diets, in which 0 (control), 15, 30, 50, 70, 85, and 100% of FMP were replaced by SPH (SPH0-100). Results showed that diets with low to moderate SPH inclusion levels (SPH15-50) significantly reduced feed conversion ratio and increased average body weight gain, weight gain ratio, specific growth ratio, daily feed intake, and protein efficiency ratio (P < 0.05), whereas complete FMP replacement (SPH100) caused poor growth of fish compared to the control diet. Condition factor was significantly enhanced in fish fed the diets SPH30-85 (P < 0.05), while hepatosomatic, viscersomatic, intestine-somatic indices were significantly reduced in fish fed the diets containing SPH (SPH15-100, P < 0.05). Dietary inclusion of SPH (SPH15-100) significantly elevated serum superoxide dismutase activity (P < 0.05), but only diets SPH15-70 significantly elevated total antioxidant capacity (P < 0.05), and diets SPH15-85 significantly enhanced serum lysozyme activity (P < 0.05). Diets with low SPH inclusion levels (SPH15-30) significantly reduced serum aspartate aminotransferase and alanine transaminase activities (P < 0.05), whereas those with moderate to high SPH inclusion levels (SPH50-100) significantly decreased serum cholesterol, low-density lipoprotein cholesterol, and triglyceride (P < 0.05). Dietary inclusion of SPH (SPH15-100) had no strong effects on pepsin, amylase, lipase, or alkaline phosphatase activity (P > 0.05), but diets SPH15-70 significantly elevated trypsin activity (P < 0.05) and diets SPH50-100 significantly elevated ATPase activity in fish (P < 0.05). Proximate and indispensable amino acid compositions of muscle had no significant changes with dietary FMP replacement by SPH (P > 0.05), whereas the contents of aspartic acid, glutamic acid, serine, glycine, and total indispensable amino acids in muscle significantly declined in fish fed the diets SPH85-100 (P < 0.05). Replacing up to 70% of dietary FMP with SPH did not hamper growth or reduce feed efficiency of juvenile starry flounder, optimal replacement level restricted to 38.32%.