Dietary l-ascorbic acid affects growth, nonspecific immune responses and disease resistance in juvenile grouper, Epinephelus malabaricus

An 8-week feeding trial was conducted to estimate the adequate dietary l-ascorbic acid (AA) requirements of juvenile grouper and its effect on nonspecific immune responses and disease resistance when challenged with Vibrio carchariae. AA supplemented at 0, 20, 50, 80, 150, 250, 400 and 800 mg/kg diet in the basal diet providing of 0, 3, 14, 27, 46, 76, 135 and 288 AA mg/kg diet, respectively. Each diet was fed to triplicate groups of grouper with initial body weight of 7.58±0.05 g. Fish fed diets supplemented with ≥46 mg AA/kg had significantly (P<0.05) greater weight gain (WG) and feed efficiency (FE) than fish fed diets with ≦14 mg AA/kg. After the final weighing, 10 fish were randomly taken from each aquarium and injected with V. carchariae. Mortality was recorded daily for 7 days and nonspecific immune parameters were monitored in the remaining fish. Higher cumulative mortality was in fish fed the unsupplemented diet or the 3-mg AA/kg diet than other dietary groups. Fish fed the 288-mg AA/kg diet had higher respiratory burst activity (superoxide O2−) of head kidney leukocytes and alternative complement activity than fish fed the unsupplemented diet and diets with ≦76 mg AA /kg, respectively. Lysozyme activity was higher in fish fed diets with ≥76 mg AA than fish fed diets with ≦27 mg AA/kg. These data indicated that the adequate dietary AA concentration for optimum growth of juvenile grouper is 45.3 mg AA/kg diet, and six times the optimum dietary AA level was needed to enhance the nonspecific immune responses and maintain survival of the fish infected with bacteria.