Replacing fish oil with plant or animal lipid sources in fish feed has become a necessity as the global production of fish oil is encountered with constraints. Therefore, the aim of present study was to investigate the effects of plant oils such as canola oil and linseed oil (Flaxseed) replacement with fish oil in diet, on hematological and blood biochemical parameters, serum lysozyme activity, and resistance to stress factors of fingerling rainbow trout Oncorhynchus mykiss . For this purpose, in a complete randomized design, with seven treatments and three replicates, two hundred and ten fingerling trout (mean weight 16.49 ± 0.46 g) were used in a eight weeks feeding trial. Treatment were as follows: treatment 1, 100% fish oil; treatment 2, 100% canola oil; treatment 3, 100% linseed oil; treatment 4, 1:1 combination of fish oil and canola oil; treatment 5, 1:1 combination of fish oil and linseed oil; treatment 6, 1:1 composition of canola oil and linseed oil; treatment 7, 1:1:1 composition of fish oil, canola oil and linseed oil) were prepared. All diets had similar protein (30.66 ± 0.47 percent) and fat (16.09 ± 0.29 percent) contents. Experiment was carried out in a semi-closed system with a 10 percent replacement of water per day and fish were fed at fixed rate of 3% live weight /day during experimental period. Results showed that dietary source of fat in diets had no significant effect on hematocrit, hemoglobin, and total count of white blood cells ( P 0.05), but the number of lymphocytes and neutrophils were significantly affected by fat sources ( P 0.05). The highest number of neutrophils and the lowest number of lymphocytes obtained from treatments two and three, respectively. AST enzyme levels among different treatments showed no significant difference, but ALT level in fish receiving treatment four showed significant difference from other treatments ( P 0.05). Serum lipoproteins including HDL and LDL levels were not affected significantly by different oil sources. Similarly, differences between cholesterol, triglyceride, glucose, and cortisol levels in serum of fish receiving different treatments were not significant. Total protein, albumin and globulin level in serum of fish receiving Key words: Rainbow trout, Oncorhynchus mykiss , Fish oil, Canola oil, Flaxseed oil, Hematology, Blood biochemical parameters, Serum lysozyme activity and Stress
