Lipid metabolism, growth performance, antioxidant ability and intestinal morphology of rainbow trout (Oncorhynchus mykiss) under cage culture with flowing water were affected by dietary lipid levels

The present study was conducted to evaluate the effects of dietary lipid levels on lipid metabolism, growth performance, antioxidant ability and intestinal morphology of rainbow trout (Oncorhynchus mykiss) under cage culture with flowing water. O. mykiss (average initial body weight 17.64 ± 0.05 g) were fed with six diets (L1, L2, L3, L4, L5 and L6) containing six levels of lipid (10.03 %, 12.97 %, 17.22 %, 20.16 %, 23.19 %, 26.06 %) for 8 weeks. Result showed that weight gain (WG), specific growth rate (SGR) and feed intake (FI) of fish were increased with increasing of dietary lipid levels (P < 0.05). Fish fed D6 diet had higher viscerosomatic index (VSI) and mesenteric fat index (MFI) compared with fish fed other diets. The moisture and crude protein contents in whole body and muscle decreased with increasing of dietary lipid levels, but crude lipid content showed the opposite trend. The hepatic superoxide dismutase (SOD) activity was increased with increasing of dietary lipid levels (P < 0.05), while no significant differences were found in hepatic malondialdehyde (MDA) content (P> 0.05). Correspondingly, the mRNA relative expression of SOD in liver was increased with increasing of dietary lipid levels (P < 0.05). Fish fed the diet (L6) containing 26.06 % lipid level showed the optimal intestinal morphology not only in villus amount and height but also in muscular layer thickness. The lipid metabolism related genes expression such as fatty acid synthetase (FAS), lipoprotein lipase (LPL), were significantly affected by dietary lipid levels (P < 0.05). The mRNA relative expression of FAS was increased from L1 to L4 and then decreased with the increasing of dietary lipid levels. The mRNA expression of LPL in fish fed with L5 and L6 diets were significantly higher than other treatments. These results indicated that the suitable lipid requirement of rainbow trout under cage culture with flowing water was not less than 26.06 %.