A COMPARISON OF THE EFFECT OF SILICONE COATED ASCORBIC-ACID AND ASCORBYL PHOSPHATE ON THE COURSE OF ICHTHYOPHTHIRIOSIS IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM)

The aim of the present study was to compare the effect of high and low doses of different vitamin C formulations on the course of ichthyopht hiriosis (Ichthyophthirius multifiliis infection) in rainbow trout. Af ter a depletion period, trout fingerlings were fed diets deficient in vitamin C(AA 0), or containing different levels of silicone coated asc orbic acid (AA; 50/200 mg kg feed(-1)) or ascorbyl phosphate (AP; 50/2 000 mg AA equivalent kg feed(-1)). Fish infected with I. multifiliis d emonstrated a significant reduction in mortality when fed high levels of AA or AP.A correlation between the parasite burden and the vitamin C content in the diet was not found. Specific immunity-measured as imm obilization titre of sera-was not influenced by the level or the formu lation of vitamin C in the diet. The most probable mechanism of high l evel doses of vitamin C is an increase of the general health status ex pressed as an improved nonspecific resistance and ability to cope with different stressors. In rainbow trout infected with I. multifiliis, t he beneficial effect of ascorbyl phosphate is equal to that of silicon e coated ascorbic acid, and therefore, the more stable AP may be a val uable alternative for use in fish diets.