Abstract: The present study aimed to investigate the effect of Foeniculum vulgare essential oil on the health of the common carp (Cyprinus carpio). A total of 120 healthy fish were provided with feed containing 200, 400, and 600 mg/kg of F. vulgare oil for 60 days. Findings revealed that the oral administration of 200 mg/kg of F. vulgare oil significantly increased final weight, weight gain, and specific growth rate compared to the control group (p < 0.05). Plasma biochemical enzyme levels remained unchanged in 200 and 400 mg/kg treatments, whereas they were found to be significantly increased in treatments with 600 mg/kg. Although no significant alterations in glucose, triglyceride, and cholesterol levels were observed, the treated groups exhibited significant increases in total protein, albumin, globulin, and creatinine levels. Results also indicated significantly decreasing levels of glutathione peroxidase, whereas superoxide dismutase activity increased. The gonadosomatic index showed notable improvement in the 200 and 400 mg/kg groups. Furthermore, plasma concentrations of estradiol and testosterone were significantly affected by doses of 400 and 200 mg/kg. Findings suggest that, following the administration of F. vulgare extract, the reproductive and general health of the fish appears to be improved. Nevertheless, it is recommended to supplement fish diets with up to 200 mg/kg of F. vulgare extract to improve their reproductive and general health. Concentrations above this limit can potentially cause harm.

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