Dietary olive leaf polyphenols enhance growth, immuno-hematological parameters, antioxidant capacity, immune and tight junction gene expression, and ammonia stress tolerance in Oncorhynchus mykiss

Over an eight-week feeding trial, this work evaluated the effects of olive leaf derived polyphenols (OLP) (Olea europaea L.) on growth performance, humoral and mucosal immune parameters, hematological and antioxidant status, the expression of immunity- and tight junction-related genes and resistance to ammonia stress in rainbow trout (Oncorhynchus mykiss). A total of 120 fish (average initial weight: 6.5 ± 0.1 g) were randomly assigned to four dietary treatments containing graded OLP levels of 0 (control), 0.5, 1, and 2 g kg⁻¹. OLP supplementation significantly improved growth parameters, including final weight, weight gain, specific growth rate, and feed conversion ratio (P < 0.05). Serum and mucosal immune factors (immunoglobulin and lysozyme) were significantly enhanced in fish fed 1 and 2 g kg⁻¹ OLP diets. Hematological indices increased at all OLP inclusion levels (P < 0.05). Moreover, elevated antioxidant enzyme activities were observed in fish fed OLP. Gene expression analysis revealed significant upregulation of immune-related genes (interleukin-1 beta and tumor necrosis factor alpha) and tight junction-associated genes (claudin-12, occludin, and zonula occludens) in fish fed 1 and 2 g kg⁻¹ OLP diets. Following ammonia exposure, OLP-fed groups demonstrated elevated hematological and antioxidant responses, and the 2 g kg⁻¹ OLP group achieved the highest post-stress survival rate. Overall, supplementation with 1–2 g kg⁻¹ OLP effectively enhanced growth, immunity, antioxidant capacity, and stress tolerance in rainbow trout, suggesting that OLP may serve as a promising natural dietary additive for improving fish health and performance.