Comparative study on the utilization of protein hydrolysates of fish and Rana catesbeiana by turbot (Scophthalmus maximus)

Cod and bullfrog scraps were selected as protein sources to prepare fish protein hydrolysate and bullfrog protein hydrolysate, and four isonitrogenous and isolipidic diets were formulated. Scophthalmus maximus with an initial body weight of (8.00±0.01) g was fed for a period of 56 d in order to explore the utilization efficiency of protein hydrolysates from different sources by S. maximus. The experimental designed two control groups with no protein hydrolysate, the positive control group contained 35% fish meal and the negative control group including 26.5% fish meal. Two experimental groups had different sources of protein hydrolysates, the FPH group contained 26.5% fish meal and 8.0% fish protein hydrolysate, and the BPH group contained 26.5% fish meal and 9.5% bullfrog protein hydrolysate. The results showed as follows: the final body weight, weight gain rate and special growth rate of the FPH group were significantly higher than those of the BPH and NC groups (P<0.05), but there was no significant difference between the FPH and PC groups (P>0.05). The concentrations of essential amino acids in chyme including lysine, arginine, threonine and valine in the BPH group were significantly higher than those in the PC group (P<0.05). There was no significant difference in the contents of other non-essential amino acids except for taurine and tyrosine, and the concentrations of most amino acids were the highest in the BPH group. The mRNA levels of proton-coupled amino acid transporter PAT1 and small peptide transporter PepT1 in the FPH and BPH groups were significantly higher than those in the PC and NC groups (P<0.05). There was no significant difference in the mRNA levels between cationic amino acid transporter CAT1 and Y⁺L-type amino acid transporter Y⁺LAT2 among all treatment groups (P>0.05). In conclusion, the addition of protein hydrolysates in the feed could promote growth performance of S. maximus, and fish protein hydrolysate showed more beneficial effects on the growth than that of bullfrog protein hydrolysate. The responses of amino acid concentrations in chyme and the expressions of amino acid and small peptide transporters also indicated that the feed utilization efficiency of S. maximus supplemented with fish protein hydrolysate was higher than that with bullfrog protein hydrolysate.