An 84-day feeding trial was carried out onfingerling (4.44 ± 0.06 g) rainbow trout (Oncorhynchusmykiss, Walbaum, 1792) to evaluate the effect of dietary supplementation with a commercial prebiotic GroBiotic ®-A (G-A) on the growth, feed efficiency, haematology and immunlogical parameters. Ttreatments containing various inclusions of G-A (0 %, 0.5 %, 1.0 %, 1.5 %, 2.0 %, 2.5 %, and 3.0 %) were added to a commercial fish dietand were fed twice daily at 2-6 % of body weight. The highest weight

more »

... (WG), specific growth rate (SGR) and average daily gain (ADG) were obtained in fish fed the diet containing 2.5 % G-A followed by 3.0 % inclusion (P<0.05). The highest feed efficiency (FE), protein efficiency ratio (PER) and net protein utilization (NPU) were also recorded in the 2.5 % G-A inclusion (P<0.05). Survival was significantly higher (P<0.05) in fish fed with 2 % and 2.5 % G-Asupplement (P<0.05). Although higher Hb, haematocrit, RBC, WBC, MCH, MCHC, MCV, lymphocytes, and neutrophils were observed at all G-A supplemented diets, the differences among themwerenot significant (P>0.05). On the other hand,significantly higher differencee (P<0.05) in lysozyme and immunoglobulin (IgM) concentrations wereobserved in 2.5% G-A inclusion. The results of this study indicated that 2.5 % G-A inclusionhad a better performance on growth and haematoimmunological parameters in rainbow trout fingerling.