Effects of dietary rapeseed meal on growth performance, digestion and protein metabolism in relation to gene expression of juvenile cobia (Rachycentron canadum)

Abstract

A 60-day feeding trial in seawater floating cages (1.5×1.5×2.5m) was conducted to investigate the effects of dietary rapeseed meal (RM) levels on feed intake, growth, survival, digestion and protein metabolism in relation to gene expression of juvenile cobia (initial body weight 94.6g). Five isonitrogenous (crude protein 450gkg−1 of dry matter) and isoenergetic (20kJg−1) practical diets were formulated by replacing 0 (the control), 125, 250, 375 and 500gkg−1 fish meal protein with RM protein. Each diet was randomly fed to triplicate groups of fish, and each cage was stocked with 20 fish. Fish were fed twice daily (06:00 and 18:00) to apparent satiation. The survival ranged from 96.7 to 98.3%, and no significant difference was observed among dietary treatments (P>0.05). With increasing dietary RM levels, feed intake (FI), specific growth rate (SGR) and feed efficiency (FE) decreased. Fish fed the diet with 250gkg−1 or more protein from RM had significantly lower SGR and FE than the control group (P<0.05), but there was no significant difference in FI at this level compared with the control group (P>0.05). Apparent digestibility coefficients (ADCs) of dry matter (DM), crude protein and energy significantly decreased with increasing dietary RM levels (P<0.05). Fish fed the diet with 250gkg−1 or more protein from RM had significantly lower ADC values of crude protein and energy compared with the control group (P<0.05). Whole-body crude protein and crude lipid decreased with increasing dietary RM levels. Fish fed the diet with 500gkg−1 protein from RM had significantly lower whole-body crude protein and crude lipid compared with the control group (P<0.05). However, whole-body moisture and ash showed opposite trends with crude protein and crude lipid. Moisture, crude protein and crude lipid contents in cobia muscle showed similar trends with those in whole body. There were no significant differences in plasma ammonia, urea, cholesterol and amino acids among fish fed the experimental diets (P>0.05). Fish fed the diet with 500gkg−1 protein from RM had significantly lower aspartate aminotransferase (AST) activity in liver than the control group (P<0.05). Hepatic insulin-like growth factor I (IGF-I) gene expression level was significantly decreased in fish fed the diet with 500gkg−1 protein from RM compared with the control group (P<0.05). However, IGF-I gene expression level in dorsal muscle was significantly increased in fish fed this diet compared with the control group (P<0.05). No significant differences were observed in target of rapamycin (TOR) expression levels in cobia liver and dorsal muscle at different RM levels (P>0.05). Results of the present study indicated that protein from RM could substitute 125gkg−1 fish meal protein without influencing the growth, feed utilization and protein metabolism in cobia. The higher substitution levels of RM induced negative influences on feed intake, growth and hepatic IGF-I expression level.