A 63-day experiment was carried out under controlled conditions to compare the effects of fructooligosaccharide (FOS) at four levels (5, 10, 20 and 30 g kg-1) on the growth performance, nutritional efficiency indices, haemato-immunological parameters, stress resistance, digestive enzymes and cultivable autochthonous intestinal microbiota of juvenile (68.52±1.52 g) Khaju fish Schizothorax zarudnyi. Fish fed the diet containing 20 g FOS kg-1 had the significantly (p<0.05) highest final weight. Dietary FOS supplementation (5-20 g kg-1) showed significant effects on SGR compared with control treatment. Hb, Haematocrit, MCV, MCH and lymphocytes in fish fed the diets containing 20-30 g FOS kg-1 increased significantly than those of fed control treatment. After 63-day feeding trial and also, 5-min air exposure challenge test, the activities of IG, LYZ and ACP in serum fish fed the diets containing 10-30 g FOS kg-1 showed a significantly increasing trend than other treatments. The ratio of lactobacillus count to total autochthonous intestinal microbiota in fish fed 10-30 g FOS kg-1 was significantly higher than that of other treatments. Furthermore, dietary FOS supplementation significantly increased survival rate of juvenile Khaju fish. Polynomial regression of SGR, FCR, PPV and PER suggested that the optimum dietary FOS level could be higher than 18.2 and <23.8 g kg-1 in fish reared in culture conditions. These results boost conducting further research on the use of FOS and other prebiotics in Khaju fish studies.

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