Investigation of the Oral Effects of Alcoholic Extract of Wild Yarrow ( <i>Achillea wilhelmsii</i> ) on Growth Performance, Immune, and Biochemical Serum Responses in Rainbow Trout ( <i>Oncorhynchus mykiss</i> )

<i>Achillea wilhelmsii</i> (AW), a plant rich in flavonoids, including lutein, apigenin, rutin, and phenolic compounds with antioxidant, antibacterial, and antifungal properties, is used in traditional medicine. In this study, the impact of AW extract on the growth, immune response, and biochemical indices of rainbow trout (<i>Oncorhynchus mykiss</i>) was investigated. Over 8 weeks, fish were fed diets supplemented with varying concentrations of AW extract (0%, 0.5%, 1%, or 2%). No significant differences were observed in growth performance, glucose levels, or key enzymes such as lactic acid dehydrogenase, alanine transaminase, or aspartate aminotransferase (AST) between the AW-supplemented groups and the control group. However, fish that received AW supplementation showed significantly higher levels of total serum protein, lysozyme activity, superoxide dismutase (SOD) activity, and immunoglobulin M (IgM). Moreover, the AW-fed groups exhibited lower mortality after exposure to <i>Yersinia ruckeri</i>. In conclusion, AW supplementation could enhance immune function in rainbow trout and decrease mortality after exposure to <i>Y. ruckeri</i>. Therefore, using this plant (1% and 2%) in aquaculture could be justified as a means to increase resistance to pathogens and improve the immune system performance of fish.