Studies on reproductive rhythms of fishes. XVII Annual reproductive cycle in an autumn-spawning bitterling Acheilognathus rhombea.

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Abstract
An autumn-spawning bitterling, Acheilognathus rhombea was sampled at 1-2 month intervals from an outdoor pond kept under natural conditions, and their gonadal maturity and plasma steroid levels were examined. In underyearling fish, apparent increases of GSI, accumulation of yolk globules, and active spermatogenesis occurred during September. Decreases of GSI due to degeneration of yolk globule stage oocytes and termination of spermatogenesis occurred in late November. Judging from these results and changes in reproductive activities in aquarium-reared fish, spawning period of underyearling fish extends from early (males) or middle (females) September to late November. In yearling fish, spawning period was initiated 2 months earlier than underyearling fish, whereas it ended in the same month as underyearling fish. There may be sharp difference in initiating factor(s) of the spawning period between underyearling and yearling fish. Considerable plasma levels of estradiol-17β in females and testosterone and 11-ketotestosterone in males were detected during gonadal recrudescence and spawning, but these steroids were un-detectable in late November suggesting that vitellogenesis and spermatogenesis terminate im-mediately at the end of spawning period.

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