Effects of Dietary Oregano ( <i>Origanum vulgare</i> ) Essential Oil Supplementation in Hybrid Red Tilapia ( <i>Oreochromis</i> spp.) Diet on Growth Performance, Survival, Intestinal Health, Reproductive and Serum Biochemical Indices

Feed additives are fundamental components of aquaculture, modulating growth performance, nutrient utilisation and physiological responses in cultured fish. Phytogenic compounds, particularly plant‐derived essential oils (EOs), are increasingly explored for their functional benefits in fish. Although commonly used oils such as sunflower, cottonseed and corn oil have been widely studied, oregano EO (OEO) remains less investigated, despite evidence of its positive effects in zebrafish, Nile tilapia and rainbow trout. Its efficacy in hybrid red tilapia ( Oreochromis sp.) has not been documented. This study evaluated dietary OEO supplementation at 0, 0.4, 0.8 and 1.2 g/kg feed on growth, survival, fish health and gut morphology in hybrid red tilapia fingerlings with a mean weight of 9.5 g. A completely randomised research design with three replicates of 15 fish per treatment was employed, and fish were fed to satiety twice daily for 13 weeks. The results indicate that fish‐fed 1.2 g/kg OEO demonstrated significantly higher survival, weight gain (WG) and feed efficiency indices (feed conversion ratio [FCR] and protein efficiency ratio [PER]) compared to controls ( p &lt; 0.05). Haematological analysis revealed elevated haemoglobin (Hb), haematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular Hb (MCH) and MCH concentration (MCHC), alongside reduced aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, indicating improved metabolic and liver function. Serum biochemical parameters, including alkaline phosphatase (ALP), albumin (ALB), globulin (GLO), total protein and cholesterol, were markedly enhanced at 1.2 g/kg feed OEO inclusion. Organ indices (hepatosomatic index [HSI], viscerosomatic index [VSI], stomach index [SI] and gonadosomatic index [GSI]) were significantly elevated, while gut histomorphology showed longer, wider and more branched villi with increased goblet cell counts and intervilli spacing in the highest OEO treatment group (1.2 g/kg feed). The study concludes that dietary OEO at 1.2 g/kg significantly improved growth performance, feed utilisation, blood health and gut morphology in hybrid red tilapia. Its gonad‐stimulating properties further highlight its potential application in broodstock diets to support sustainable fingerling production.