The current research evaluated the effects of dietary hesperidin (HSP) on growth parameters, digestive enzyme activities, innate immune markers, and antioxidant responses in rainbow trout, Oncorhynchus mykiss after the 30- and 60-day feeding trial. Then, specimens were subjected to Yersinia ruckeri infection for 14 days. For this, six hundred rainbow trout juveniles (initial weight; 25.49±0.40 g) were fed with different levels of HSP including 0 (HSP0; control), 50 (HSP50), 100 (HSP100), 150 (HSP150), and 200 (HSP200) mg/kg feed. After 60 days, dietary HSP100 supplementation significantly improved growth and feed efficiency indices. The optimal dose of HSP based on the regression test for WG and FCR detected 122 and 131mg/kg, respectively. At the end of the 30th day, dietary HSP150 supplementation markedly boosted serum lysozyme (LYZ), myeloperoxidase (MPO), complement component C3, and immunoglobulin (Ig) levels, as decreased malondialdehyde (MDA) content. In the same period, dietary administration of HSP at the different concentrations markedly increased complement component C4 (HSP150 and HSP200), superoxide dismutase (SOD), skin mucus alkaline phosphatase (ALP) (HSP100- HSP200), skin mucus LYZ and catalase (CAT) (HSP100 and HSP 150), and skin mucus Ig level (HSP50- HSP200). After 60 days, all dietary HSP supplementation significantly improved lipase, serum C3, glutathione peroxidase (GPX), and skin mucus ALP, LYZ, and Ig levels, but decreased MDA, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) values. After 60 days, dietary administration of HSP150 induced a significant discrepancy in the activity of serum LYZ, MPO, respiratory burst (RB), and lactate dehydrogenase (LDH) compared to the control group. In the same period, dietary HSP supplementation at different levels induced a remarkable increase in digestive protease activity (HSP100), serum ACH50 and CAT activities (HSP100 and HSP150), skin mucus peroxidase and serum SOD levels (HSP100-HSP200), and serum C4 value (HSP50, HSP150, and HSP200), but markedly decreased ALP activity (HSP150 and HSP200). In addition, dietary HSP supplementation, especially HSP150, markedly boosted disease resistance against Y. ruckeri infection. The current data demonstrated that the oral administration of 100 - 150 mg/kg HSP has a high potential to promote growth performance, disease resistance, and faster induction of immune and antioxidant responses.
