Effects of dietary protein hydrolysates on growth performance, feed utilization, and intestinal function of juvenile mandarin fish (Siniperca chuatsi)

The substitution of live prey with formulated feed represents an inevitable trend in the green transformation of mandarin fish (Siniperca chuatsi) aquaculture. However, intestinal health issues induced by dietary transition have emerged as a critical bottleneck constraining its large-scale application. So, this study aimed to investigate the effects of dietary protein hydrolysates on the growth performance, feed utilization, and intestinal function of juvenile S. chuatsi . White fish meal, Brown fish meal, and blood meal were mixed at a fixed ratio and hydrolyzed using exogenous protease to obtain the composite protein hydrolysate. Based on this, five isonitrogenous and isolipidic diets were formulated by replacing composite protein with 0% (E0, control), 25% (E25), 50% (E50), 75% (E75), and 100% (E100) composite protein hydrolysate. Juvenile S. chuatsi (initial body weight: 33.72 ± 0.02 g) were reared for 8 weeks, with 30 fish per tank. Each treatment had three replicates, and fish were fed to apparent satiation twice daily. The results showed that final body weight, specific growth rate, protein efficiency, and protein deposition rate of fish in E25 and E50 groups did not differ significantly from those of the control group (P > 0.05). However, fish in the high protein hydrolysate groups (E75 and E100) exhibited significantly reduced condition factor and feed efficiency (P < 0.05). Body composition analysis revealed that the crude protein content in the liver was significantly higher in the E50 group compared to the control (P < 0.05), while the crude lipid content in the muscle was significantly increased in the E75 group (P < 0.05). In addition, moisture content in whole fish and viscera was significantly increased in the E100 group relative to the control (P < 0.05), whereas crude lipid content in the liver and viscera was significantly decreased (P < 0.05). Histological analysis of the midgut showed that villus height and muscle thickness reached their highest values in the E50 and E25 groups, respectively (P < 0.05). Gene expression analysis revealed that the mRNA levels of sodium-coupled neutral amino acid transporter 2 (snat2) and L-type amino acid transporter 1 (lat1) were significantly upregulated in the E25, E50, and E75 groups compared with the control (P < 0.05). The expression of peptide transporter 2 (pept2) was highest in the E75 group (P < 0.05). Meanwhile, the expression levels of tight junction proteins (claudin-4 and claudin-7) and mucin-2 were significantly upregulated in the E75 group compared with the control (P < 0.05), whereas these genes were significantly downregulated in the E100 group (P < 0.05). In conclusion, the appropriate inclusion level of dietary protein hydrolysates (50% replacement of mixed protein) can improve intestinal nutrient absorption and structural development, thereby enhancing the intestinal function of juvenile S. chuatsi without compromising growth performance. To provide theoretical basis and technical support for developing fish meal replacement technology and promoting the green and sustainable development of S. chuatsi culture.