Vitamin E acts as a strong inhibitor of lipid peroxidation with RRR-α-tocopherol being the most effective form in animals. For this reason, vitamin E requirements show strong interactions not only to dietary levels of polyunsaturated fatty acids, but also with that of natural or synthetic dietary antioxidants. Considering major changes in raw ingredients for aquafeeds , this study aimed to re- evaluate the α-tocopherol (aTOH) requirement of Atlantic salmon ( Salmo salar) during smoltification when fed diets following a modern feed formulation. In addition, the fish were exposed to a n environmental challenge as dietary antioxidant requirements, including vitamin E, might be higher under stressful environmental conditions as they can occur during seawater transfer.
The experimental design followed a regression model. Seven diets were formulated on a basal feed mix (α-tocopherol: 42 mg/kg, VE42) by supplementation of graded levels of synthetic dl-α-tocophenyl acetat e to receive diets of 100 mg/kg, VE100; 200 mg/kg, VE200; 300 mg/kg, VE300; 400 mg/kg, VE400; 600 mg/kg, VE600 and 800 mg/kg, VE800. Sixteen tanks in a flow- through system were stocked with 110 Atlantic salmon (initial weight: 81 ± 2 g), respectively, corresponding to duplicate tanks per treatment, except for VE42 and VE800 that were run in triplicate. Twelve fish per tank were sampled a fter 12 weeks of rearing in freshwater at standard conditions (12 ˚C, >75 % DO) . The remaining fish were subsequently exposed, for an additional 6 weeks, to an environmental seawater challenge under combined hypoxia and temperature stress (14 ˚ C, 60 % DO, salinity: 22 ppt) prior to sampling.
Dietary aTOH level had no significant effect on final body weight in freshwater or seawater challenged fish. Liver aTOH levels increased with dietary aTOH levels following a 2nd order polynomial. Fish fed VE800 showed significantly higher liver aTOH levels at freshwater stage compared to seawater challenged fish (Figure 1). In addition, in stressed fish, l iver malondialdehyde (MDA) levels were significantly higher in VE42 compared to VE200 , VE400, VE600 and VE800.
MDA levels indicate that a dietary aTOH level above 100 mg/kg is required to prevent the accumulation of lipid peroxidation products in the liver of stressed fish.