春産卵魚アカヒレタビラの生殖年周期〔魚類の生殖リズムに関する研究-2-〕

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Adult bitterling, Acheilognathus tabira, were sampled at intervals ranging from one to three months with a set net in Lake Kasumigaura for two years. Maturity of fish was estimated from gonadosomatic index (GSI) and gonadal histology. In females ovipositor length and egg diameter were also examined. Gonads stayed regressed during high summer months, when most oocytes were at perinucleolus stage, and all of male germ cells were spermatogonia, From this state, maturation progressed in two phases. In the first phase that began in September, accummulation of yolk vesicles and formation of spermatocytes occurred accompanying slow increase of GSI and egg diameter. In the second phase that began in March or early April, accummulation of yolk globules and active spermatogenesis occurred accompanyig rapid increase of GSI and egg diameter. Judging from elongation of the ovipositor and appearance of the ovulated eggs, the spawning period of this species extends from late April to late June, though it may somewhat fluctuate by years. After the period gonads regressed rapidly. Different environmental factors may effect the initiation of each phase of maturation and the termination of the spawning season. Activity of gonadotrophs corresponded to each maturation phase, which suggests that both phases are controlled by gonadotropin.
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