Growth performance and tissue fatty acid change in rainbow trout (Oncorhynchus mykiss) feed with different energy levels

A feeding trial was conducted in rainbow trout juveniles to assess the growth performance and feed utilization after fed the two diets with different energy levels (medium energy-ME, (18.4 kJ/kg) and low energy-LE (17.7 kJ/kg). Twenty fish (10 females and 10 males) males and females are distinguished through the head and anal fin, were randomly distributed into each cylindrical tank (300-L rearing volume) with four replicates. The fish were fed twice daily with each experimental diet until apparent satiation for 60 days. At the end of the feeding trial, the mean final weight in the LE group ranged from 385.7±21.4 to 406.0±23.6 g and the mean final weight in the ME group ranged between 424.1±22.0 g and 434.2±37.3 g. Overall, the fish fed with LE achieved the highest percentage of body weight gain of 290.34±1.99% among both treatments, with a statistically significant difference. Irrespective of gender, there was no significant difference in feed intake and FCR. No significant difference was also found in VSI which ranged from 3.83 to 4.08%. However, female fish fed with LE diet had the highest VFI than male fish fed with ME counterparts. In terms of specific fatty acids such as EPA and DHA, no significant differences were detected between experimental diets and there were no significant interactions between two factors (energy levels and gender) in EPA and DHA profile of the fillets of the experimental fish. A significant difference between genders was found in triglyceride, total protein, ALT and AST whereas the experimental diets had a significant impact on cholesterol levels in both genders. Liver histology scores in fish fed ME and LE showed no specific pathologies, normal liver structure, and no fibrous tissue increase. In the study, the expression of IL-1β and TNF-α was reduced in the liver of fish fed on ME diet, while the highest expression was found in fish fed on LE diet. Also, the expression of glucokinase increased in the liver of fish fed the LE diet. In contrast, the highest expression of IL-18 was found in the liver of fish fed the ME diet. While the lowest expression of IL-18 was in the liver of fish fed the LE diet. Key Words: Dietary energy, Growth performance, Feed intake, Liver histology, Gene expression, Immunology