Seasonal changes in plasma levels of steroid hormones in an asynchronous fish the gudgeon Gobio gobio L. (Teleostei, Cyprinidae).

Annual changes in plasma of estradiol-17 beta, testosterone, and 17,20 beta-dihydroxy-4-pregnen-3-one concentrations were measured, by radioimmunoassay, in female gudgeon Gobio gobio a fish which has an asynchronous-type ovary containing oocytes at various stages of development and spawns several times during the reproductive period. The gonadosomatic index and the relation between stages of maturity and steroid concentrations were also followed during the reproductive cycle. Plasma levels of estradiol-17 beta, testosterone, and 17,20 beta-dihydroxy-4-pregnen-3-one were low from October to April and increased rapidly in May to reach 0.61 +/- 0.31; 2.3 +/- 0.42; and 3.17 +/- 0.68 ng/ml, respectively. Elevated levels were maintained during spawning when vitellogenic oocytes are present alongside oocytes in final maturation. Histological analysis of the ovary indicated that an important number of spawnings has occurred since the proportion of oocytes in final maturation stage was very low (less than 1%). Fish in the regressive phase also presented high steroid levels. The vitellogenic oocytes in preovulatory atresia and the postovulatory follicles may be responsible for these events.