Effects of dietary size-fractionated fish hydrolysates on growth, activities of digestive enzymes and aminotransferases and expression of some protein metabolism related genes in large yellow croaker (Larimichthys crocea) larvae

• PUFH replacing 40% fish meal seems optimal for large yellow croaker larvae.
• PUFH replacing 40% fish meal can enhance the amino acids metabolism.
• PUFH replacing 40% fish meal can enhance the digestion and absorption of protein.

The present study was conducted to investigate the effects of size-fractionated fish hydrolysates on growth, activities of digestive enzymes and aminotransferases and expression of some protein metabolism related genes in large yellow croaker (Larimichthys crocea) larvae. Fish (initial body weight 3.15 ± 0.15 mg) were fed for 30 days with four diets: the control diet was produced using fish meal (FM) as the main protein source and other three diets were formulated with permeate after ultra-filtration of fish hydrolysate (PUFH), retentate after ultra-filtration of fish hydrolysate (RUFH) or non-ultrafiltered fish hydrolysate (NUFH) replacing approximately 40% FM. The results showed that specific growth rate and survival rate were significantly lower in fish fed diets with NUFH and RUFH than that of fish fed the control diet (P < 0.05) and there were no significant differences between PUFH-40 group and the control group (P > 0.05). Specific activities of digestive enzymes and the ratio “pancreatic enzyme in intestinal segment/pancreatic enzyme in pancreatic segment” were not significantly different between fish fed diets with PUFH and fish meal and they were both higher than that of fish fed diets with NUFH and RUFH (P < 0.05). Specific activities of both alanine and aspartate aminotransferases were significantly higher in fish fed the diet with PUFH than that of fish fed other diets (P < 0.05). Transcription of peptide transporter 1 (PepT1), cholecystokinin (CCK) and trypsin was significantly down-regulated in fish fed diets with NUFH and RUFH (P < 0.05). No significant differences were observed in the target of rapamycin (TOR) gene expression of fish among dietary treatments (P > 0.05). These results suggest that when replacing 40% FM, PUFH seems optimal for large yellow croaker larvae compared to NUFH and RUFH and size-fractionated fish hydrolysates could significantly affect digestion and absorption of protein in large yellow croaker larvae by altering the mRNA expression levels of CCK, PepT1 and trypsin.