Effect of high dietary intakes of vitamin E and n‐3 HUFA on immune responses and resistance to Edwardsiella tarda challenge in Japanese flounder ( Paralichthys olivaceus , Temminck and Schlegel)

A feeding experiment was conducted to investigate the effects of high dietary intake of vitamin E (supplied as dl ‐α‐tocopheryl acetate) and n‐3 highly unsaturated fatty acid (n‐3 HUFA) on the non‐specific immune response and disease resistance in Japanese flounder Paralichthys olivaceus . Nine practical diets were formulated to contain one of three levels of vitamin E namely, 0, 80 or 200 mg kg −1 (the total α‐tocopherol contents in the diets were 21, 97 and 213 mg kg −1 based on analysis), and at each vitamin E level with one of three n‐3 HUFA levels i.e. 0.5%, 1.5% or 2.0%. Each diet was randomly assigned to triplicate groups of Japanese flounder (initial body weight: 40.5±1.0 g, mean±SD) in a re‐circulation rearing system. Fish were fed twice daily to apparent satiation at 07:00 and 18:00 hours for 12 weeks. During the experimental period, water temperature was maintained at 18±1°C, salinity 31–35 g L −1 , and pH 7.8–8.2. Dissolved oxygen was not less than 6 mg L −1 , and there were negligible levels of free ammonia and nitrite. The results showed that the increase in dietary n‐3 HUFA from 0.5% to 1.0% significantly decreased muscle α‐tocopherol contents in fish‐fed diets with 21 and 97 mg α‐tocopherol kg −1 diet ( P <0.05). In 1.0% HUFA groups, alternative complement pathway activity (ACH 50 ) of fish fed the diet containing the 213 mg α‐tocopherol kg −1 diet was significantly higher than noted for fish fed the diet containing 97 mg α‐tocopherol kg −1 diet ( P <0.05). Fish fed the diet with 213 mg α‐tocopherol kg −1 and 2.0% n‐3 HUFA had the highest lysozyme activity (131.7 U mL −1 ) among all the dietary treatments. Fish fed the diets containing 97 and 213 mg α‐tocopherol kg −1 with 1.0% n‐3 HUFA had significantly higher respiratory burst activity than those fed the diets containing 21 mg α‐tocopherol kg −1 with 0.5 and 1.0% n‐3 HUFA ( P <0.05). In the disease resistance experiment, high intake of dietary vitamin E with 213 mg α‐tocopherol kg −1 significantly decreased cumulative mortality and delayed the days to first mortality after a 7‐day Edwardsiella tarda challenge ( P <0.05). In addition, under the experimental conditions, dietary vitamin E and n‐3 HUFA had a synergistic effect on the non‐specific immune responses and disease resistance in Japanese flounder ( P <0.05).