Annual Reproductive Cycle of a Bitterling, Rhodeus ocellatus ocellatus

Abstract

In an attempt to study the long-term reproductive rhythm on a bitterling, Rhodeus ocellatus ocellatus, adult fish were sampled using bottle traps about once a month at the Kayanuma Pond for one year and at the Shintone Canal for three consecutive years.
From monthly changes in the GSI and gonadal histology, it was ascertained that their spawning season extended from the end of March to the middle of September. The GSI, for both female and male, showed a rapid rise, a peak in May and/or a plateau, and then a sudden decline.
The female ovipositor grew to the anal fin height in March, along with yolk accumulation in the oocytes, and made cyclic changes in length during the spawning season. Toward the end of the season, the ovipositor dwindled, concurrent with ovarian regression. The testicular lobules contained cysts filled with spermatogonia and primary spermatocytes just before the spawning season, while the testis underwent shrinkage and the spermatogenesis stopped abruptly at the season's end.
Such sudden changes in the gonadal histology suggest possible environmental control of the spawning season at its termination as well as intiation. At Kayanuma, the gonadal maturation of this species was delayed probably due to unfavorable nutrition indicated by low condition factor persisting through winter months.