This study aimed to evaluate the effects of dietary probiotic supplementation of Lactobacillus helveticus on growth, digestive enzymes, and hematological, biochemical, immune, and antioxidant parameters, as well as intestinal microbiota of rainbow trout (Oncorhynchus mykiss). Fish (35.46±0.9 g) were fed with different levels of dietary L. helveticus: control, 1 × 106, 1 × 107, 1 × 108, and 1 × 109 at high stocking density (80 kg m−3) for 60 days. Results indicated that growth performance significantly improved in probiotic supplemented fish (P<0.05). Digestive enzyme parameters revealed that supplementation could significantly increase amylase, protease, and lipase (P<0.05). The treated groups showed significant improvements in serum immune parameters including lysozyme (LYZ), alternative complement (ACH50), respiratory burst activity (RBA), and myeloperoxidase (MPO) (P<0.05). Total protein (TP), albumin (ALB), and globulin (GLO) increased in fish fed experimental diets (P<0.05). Lactate dehydrogenase (LDH) activity was significantly lower in fish fed dietary additives (P<0.05) while white blood cells (WBC), lymphocytes, neutrophils, hematocrit (Hct), red blood cells (RBC) were significantly enhanced (P<0.05). Fish fed with supplemented diets showed significantly enhanced antioxidant status, catalase (CAT) and superoxide dismutase (SOD). Malondialdehyde (MDA) content was significantly lower in fish fed dietary additives (P<0.05). Lactic acid bacteria (LAB) in the treatment groups were significantly increased (P<0.05). In conclusion, dietary supplementation of L. helveticus reduced detrimental effects of high stocking density on growth performance and immune response. It appears that L. helveticus can be recommended as a beneficial probiotic feed additive for rainbow trout.
