Soy peptide inclusion levels influence the growth performance, proteolytic enzyme activities, blood biochemical parameters and body composition of Japanese flounder, Paralichthys olivaceus

Processing alternative protein sources such as soybean into hydrolysates through enzymatic and chemical modification can produce bioactive compounds that could be utilized as an ingredient in fish feed. The objective of this study was to investigate the efficacy of soy peptide hydrolysate (SPH) at different inclusion levels in fish diets to clarify the utilization of peptide by fish species. A 56-day feeding trial was conducted on Japanese flounder, Paralichthys olivaceus with an initial mean body weight of 1.01 ± 0.02 g. Five treatment diets were formulated to contain 0 (SP0, control), 10 (SP10), 20 (SP20), 30 (SP30) and 40% (SP40) SPH. All test diets were formulated to be isonitrogenous (55% crude protein) and isolipidic (16% crude lipid). The diets were assigned to 15 tanks (15 fish tank− 1) with each treatment having three replicates. Results of the feeding trial indicated that growth rates, feed intake, and feed efficiency significantly (P < 0.05) improved when 20% of SPH was supplemented in the diets. However, there was a significant (P < 0.05) reduction of growth, feed intake and feed efficiency when fish fed the diets with high inclusion levels of SPH (SP30 and SP40). There were no significant differences (P > 0.05) in the proximate composition (crude protein, crude lipid, ash and moisture), amino acid of the fish body, and blood biochemical parameters (blood urea nitrogen, glutamyl oxaloacetic transaminase, glutamic pyruvate transaminase, total cholesterol and total bilirubin) in fish fed all the test diets. However, blood total cholesterol value in fish fed moderate level (SP10 and SP20) of SPH had a lower numerical value compared to the rest of the treatments. Protease activity in liver and digestive tract of fish in all dietary treatments did not differ significantly (P > 0.05). These results indicate that SPH can be efficiently utilized by flounders and can promote growth performance and feed efficiency at a moderate inclusion level of 20%.

► Growth parameters improved when 20% of soy peptide hydrolysate (SPH) was supplemented in the diets.
► Further increase of SPH was not effective.
► SPH can be efficiently utilized by flounders.