Fingerlings of the rainbow trout, Oncorhynchus mykiss (n=300, 10.63 Æ 0.6 g), were fed tarragon (Artemisia dracunculus) essential
oil (TGO) for 2 months to examine its effects on growth properties, immunity, and resistance to Yersinia ruckeri infection. The
treatments were control or TG1, TG2 (fed 0.5% TGO), TG3 (1% TGO), and TG4 (2% TGO). According to the results, an
improvement was observed in growth parameters in all TGO-treated groups compared to the control (P<0:05). The digestive
enzyme activities (protease and lipase) were significantly elevated in response to dietary TGO (P<0:05). The immune system of the
fish was enhanced by TGO, as it stimulated the immune parameters in serum (lysozyme, myeloperoxidase (MPO), alternative
complement (ACH50), Ig) and mucus (lysozyme, protease, ACH50, Ig) (P<0:05). The treatments, TG3 and TG4, showed more
immune performance in response to TGO (P<0:05). The fish in TG2 treatment had a higher levels of serum total protein than
other groups (P<0:05). The concentration of triglycerides (TRIG) and cholesterol (CHOL) in serum significantly decreased
(P<0:05) in response to TGO, as the lowest levels were observed in the treatment, TG3. The antioxidant enzymes (superoxide
dismutase (SOD) and catalase (CAT)) of serum elevated in TGO-treated fish, with the maximum values for the TG4 group
(P<0:05). TGO reduced (P<0:05) alanine aminotransferase (ALT) and alkaline phosphatase (ALP) levels in serum. After bacterial
challenge, the TGO-treated fish showed lower mortality compared to the control, where the lowest mortality was observed in TG4
(P<0:05). In conclusion, TGO improved growth, immunity, and survival after bacterial challenge in the rainbow trout, with more
performance in fish fed 1%–2% TGO