The Mode of Ovarian Maturation and Spawning of the Landlocked Dwarf Ayu Plecoglossus altivelis in Lake Biwa

The changes in number and size of eggs during ovarian maturation and ovulation of the dwarf Ayu in Lake Biwa, so-called “Koayu”, taken in 1980 were examined. Differences of egg diameter between the Koayu and the amphidromous type Ayu became remarkable in the eggs developed over the tertiary yolk globule stage. There were no significant differences in the numbers of mature eggs and the GSI values between the ovaries at the first maturation and those at the second maturation, but those at the third maturation had a smaller egg number and GSI value than the previous two. The number of yolk-less eggs increased after the first ovulation, but this phenomenon was not recognized in the ovaries after the second and the third ovulations. It seemed that there was no possibility of the fourth maturation in the fish because no oocytes were found in the ovaries after the third ovulation. These facts suggest that the multiplication of yolk-less eggs after the first ovulation makes the third ovulation possible and the fish uses up all oocytes in a spawning season. The mode of ovarian maturation and spawning of the fish corresponds to the annual life of the fish.