PREVENTION OF STRESS IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) FED DIETSCONTAINING DIFFERENT DOSES OF GLUCAN

An experiment was performed to determine the effects of stress on non- specific defence mechanisms in rainbow trout fed diets containing diff erent 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 measurin g changes in blood levels of cortisol, glucose, total protein and comp osition of their leukocyte population, as well as by changes in non-sp ecific defence mechanisms of the fish. After 4 weeks feeding with gluc an, elevated phagocytosis and oxidative radical production were observ ed in treated fish, but the levels did not correlate with the differen t doses of glucan. Stress induced by 2 h transportation caused high co rtisol levels in plasma and hyperglycaemia in all groups, but the lowe st 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 ch anges were not affected by glucan doses. One week post stress, hypergl ycaemia was still observed in control and fish fed medium and high dos es of glucan. Further reductions of total protein and intracellular ox idative radical production, were measured in all groups, but in fish f ed with low dose of glucan the changes were less dramatic. The phagocy tosis ratio increased in all groups, but did not attain the levels mea sured before stress in control and in the group fed the high concentra tion of glucan. A spontaneous infection with Flexibacter columnaris ca used mortality in all groups except the group fed the low level of glu can. The results of the present study show that feeding of glucan in l ow doses several weeks before transportation can help to prevent negat ive effects of stress. (C) 1997 Elsevier Science B.V.