other Biology & physiology Biology & physiology پژوهشي Orginal research papers The objective of this study was to compare the biological parameters such as length, weight, age, caviar production indices, caviar/weight, condition factor and sex ratio of Persian sturgeon (<em>Acipenser persicus</em> Borodin, 1897) and Russian sturgeon (<em>A. gueldenstaedtii </em>Brandt & Ratzeburg, 1833) in the Caspian Sea during 1990-2014. All these characteristics (except condition factor) of Persian sturgeon were greater than the Russian sturgeon. In the long period, annually average (&plusmn;SD) fork length ranged from 129.3 (&plusmn;11.87) to 140.9 (&plusmn;15.99) cm for Russian sturgeon and 145.7 (&plusmn;17.61) to 157.4 (&plusmn;19.31) cm for the Persian sturgeon. The annually caviar harvest for the Russian sturgeon ranged from 3.86 (&plusmn;1.05) to 5.02 (&plusmn;1.82) kg with a caviar/weight ratio were ranged from 16.9 (&plusmn;4.12) to 20.1% (&plusmn;5.12), while comparatively for the Persian sturgeon was greater and ranged from 4.84 (&plusmn;1.62) and 7.04 (&plusmn;2.58) kg with a caviar/weight ratio of 16.2 (&plusmn;3.99) to 21.1% (&plusmn;5.20). The age of both species ranged between 6 to 40 years but the modal age of Russian sturgeon was less than Persian sturgeon. The male:female sex ratio of Russian and Persian sturgeons were 0.23:1 and 0.57:1, respectively. The sex ratio of Russian sturgeon was significantly different from sex ratio of Persian sturgeon (<em>p</em><0.001). The condition factor of Russian and Persian sturgeons were 0.92 (&plusmn;0.16) and 0.74 (&plusmn;0.13), respectively. Moreover, results of the MANOVA revealed significant differences in population structure of two species (<em>p</em><0.001). Therefore, different conservation and management activities need for these two valuable sturgeons in the Caspian Sea. Acipenser persicus, Acipenser gueldenstaedtii, Age, Length, Weight, Sex, Caspian Sea 552 563 http://jifro.ir/browse.php?a_code=A-10-2161-2&slc_lang=other&sid=1 M. Tavakoli m_tavakoli_e@yahoo.com 100319475328460029562 100319475328460029562 Yes International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran H. Fazli 100319475328460029563 100319475328460029563 No Iranian Fisheries Science Research Institute, Caspian Sea Ecology Research Center, Agricultural Research, Education and Extension Organization, Sari, Iran Iranian Fisheries Science Research Institute, Caspian Sea Ecology Research Center, Agricultural Research, Education and Extension Organization, Sari, Iran M. Moghim 100319475328460029564 100319475328460029564 No Iranian Fisheries Science Research Institute, Caspian Sea Ecology Research Center, Agricultural Research, Education and Extension Organization, Sari, Iran Iranian Fisheries Science Research Institute, Caspian Sea Ecology Research Center, Agricultural Research, Education and Extension Organization, Sari, Iran M.R. Behrooz Khoshghalb 100319475328460029565 100319475328460029565 No International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran