多摩川におけるモツゴPseudorasbora parvaの生殖年周期

抄録

Adult topmouth gudgeon Pseudorasbora parva (Cyprinid) were collected monthly for a year in the midstream of the Tama River, Tokyo. Gonadosomatic indices (GSI) and gonadal histology were used as criteria of gonadal maturation. During March and April male and female GSIs in-creased rapidly with the accumulation of yolk globules and active spermatogenesis, respectively. GSIs stayed high during April, May, and June, but decreased drastically in July showing the end of their spawning season. In the post spawning season (August) the ovaries and the testes were filled with yolkless oocytes and proliferating spermatogonia, respectively. During autumn and winter a gradual gonadal recrudescence which was represented by the accumulation of yolk vesicles or meiosis of spermatocytes occurred. These results indicate that the spawning season of P. parva extends from the end of March to the end of June in this sampling area. A possible environ-mental control on the annual reproductive cycles of this species was discussed.