Kutum (Rutilus frisii kutum; Kamensky, 1901) is an economically important Cyprinid species endemic to the Caspian Sea. In this study, the plasma levels of estradiol-17β (E2), 17α-hydroxyprogesterone (17-OHP), testosterone (T), and progesterone (P4) and oocyte developmental stages were studied during the reproductive cycle of Rutilus frisii kutum. These hormones were assayed by radioimmunoassay (RIA), and histological features of developmental stages of oocytes were described in detail using light microscopy. The results showed that plasma levels of E2 and T began to increase during the cortical alveolus stage and this trend continued during the vitellogenesis process. The highest plasma levels of E2 and T were measured at the end of the vitellogenic stage (133.4 ± 19.7 and 7.0 ± 1.4 ng/ml respectively) in March. Once oocytes entered the maturing phase in April, E2 and T levels both declined sharply. By contrast, plasma 17-OHP levels started to increase in early April and reached their maximum value in the final maturation stage in mid April (4.0 ± 2.2 ng/ml). Plasma levels of P4 were very low before initiation of the maturational stage but increased notably during maturation by early April (2.6 ± 0.4 ng/ml) and declined again later. These results indicate that in Kutum, the two hormones E2 and T were functionally important during the vitellogenic phase while progestogens were probably associated with the maturational phase of ovarian growth.
