en Biology & physiology Biology & physiology پژوهشي Orginal research papers This study aimed to evaluate the effect of&nbsp;<strong>Myrtle, </strong><em>Myrtus communis,</em><strong> extract on hematological, immuno-physiological, antioxidant responses, bactericidal activity, and tissue histomorphology of gill and liver in juvenile Siberian sturgeon, </strong><em>Acipenser baerii</em><strong>.</strong> <strong>Siberian sturgeon were exposed to 4 doses of myrtle extract including 25% (67.4 mg/L; M₂₅), 50% (134.9 mg/L; M₅₀), 75% (202.0 mg/L; M₇₅), and 100% (269.8 mg/L; M₁₀₀) of the maximum allowable concentration and a control treatment (without exposure myrtle extract).</strong> Hemoglobin and red blood cell&nbsp; values were significantly increased in fish exposed to the myrtle extracts (<em>p</em><0.05). The white blood cell was lower in M₂₅ and M₇₅, while the highest value was found in M₁₀₀treatment (<em>p</em><0.05). Myrtle extract did not affect the lymphocyte value in the course of exposure (<em>p</em>>0.05). The highest albumin and total protein levels were observed in M₂₅ and M₅₀ groups. The highest values of lysozyme and total immunoglobulin (Ig) activities were observed in M₅₀, M₇₅ and M₂₅, M₅₀, respectively (<em>p</em><0.05). Superoxide dismutase and catalase activities of those fish exposed to M₅₀ and M₇₅ were significantly higher than the control and M₁₀₀ groups (<em>p</em><0.05). The lowest glutathione peroxidase value was observed in the control group compared to the others (<em>p</em><0.05). <strong>The severe changes such as adhesion and curling of gill lamella discern were observed in fish exposed to different levels of myrtle extracts.</strong> <em>Moreover, in the control group, severe hepatocyte destruction was accompanied by nucleus pyknosis, but the severity of atrophy was observed in M₇₅ and M₁₀₀treatments.</em> <strong>Overall, the<em> r</em></strong><em>esults suggested that </em>myrtle <em>in the </em>range of 67.4-202 mg/L <em>could be applied as a stimulant agent to Siberian sturgeon aquaculture.</em> Exposure, Myrtle, Immuno-physiological, Siberian sturgeon 1409 1425 http://jifro.ir/browse.php?a_code=A-10-3956-1&slc_lang=en&sid=1 R. Barzegar barzegar.17655@gmail.com 100319475328460041337 100319475328460041337 No -Department of Fisheries, Lahijan Branch, Islamic Azad University, Lahijan, Iran M. Farokhrouz dr.farokhrouz@gmail.com 100319475328460041338 100319475328460041338 Yes Department of Fisheries, Lahijan Branch, Islamic Azad University, Lahijan, Iran H. Khara h.khara1974@yahoo.com 100319475328460041339 100319475328460041339 No Department of Fisheries, Lahijan Branch, Islamic Azad University, Lahijan, Iran A. Shenavar Masouleh shenavar1969@gmail.com 100319475328460041340 100319475328460041340 No International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran M. Ahmadnezhad m_ahmadnezhad@yahoo.com 100319475328460041341 100319475328460041341 No - Inland Waters Aquaculture Research Center. Iranian Fisheries Sciences research, Agricultural Research, Education and Extension Organization (AREEO), Bandar Anzali, Iran