Aspects of the reproductive biology of the landlocked large type ayu Plecoglossus altivelis in Lake Biwa, “ooayu”, were investigated from August to October in 1980. In more than half the number of ovaries after the first spawning, degeneration of developing eggs or collapse of the whole ovary was observed. Hence, after the first spawning, a small number of fish seemed to attain the second maturation. All the fish which attained the second maturation were skinny and half of them had lower gonadosomatic indexes and smaller number of mature eggs compared to those in the first maturation. No significant differences of egg diameter for various maturation stages of egg were indicated between the dwarf type ayu, “koayu”, and the ooayu. The pattern of oocyte development of ooayu showed the “trimodal twice spawning type”. Resting ocoytes in the peri-nucleolus stage which are usually present all the year round in the general serial spawners, were not observed in the ovary of ooayu after the second spawning. This fact suggests that ooayu have an inherent regulating mechanism of oocyte development adapted to the annual life of the fish, i.e. they discharge all oocytes in a single spawning season in spite of their serial spawning habit.
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