An experiment was performed to determine the effects of stress on non-specific defence mechanisms in rainbow trout fed diets containing different doses of glucan. Fish were fed with 0, 0.1, 0.5 and 1.0% glucan concentration in food. After 4 weeks of feeding, fish were stressed by 2 h transportation. The effect of stress was investigated by measuring changes in blood levels of cortisol, glucose, total protein and composition of their leukocyte population, as well as by changes in non-specific defence mechanisms of the fish. After 4 weeks feeding with glucan, elevated phagocytosis and oxidative radical production were observed in treated fish, but the levels did not correlate with the different doses of glucan. Stress induced by 2 h transportation caused high cortisol levels in plasma and hyperglycaemia in all groups, but the lowest level of glucose was measured in the group fed the low (0.1%) dose of glucan. Respiratory burst activity, phagocytosis, serum protein and lyzosyme levels were found to be significantly reduced by stress. The most dramatic reduction was observed in the control group, but the changes were not affected by glucan doses. One week post stress, hyperglycaemia was still observed in control and fish fed medium and high doses of glucan. Further reductions of total protein and intracellular oxidative radical production, were measured in all groups, but in fish fed with low dose of glucan the changes were less dramatic. The phagocytosis ratio increased in all groups, but did not attain the levels measured before stress in control and in the group fed the high concentration of glucan. A spontaneous infection with Flexibacter columnaris caused mortality in all groups except the group fed the low level of glucan. The results of the present study show that feeding of glucan in low doses several weeks before transportation can help to prevent negative effects of stress.
