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 (ACH₅₀) 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).