Change in Number and Size of Eggs during Ovarian Maturation and Ovulation of the Amphidromous Type Ayu Plecoglossus altivelis in Chikugo River

In a previous paper, we described that oocytes developed asynchronously and only the first batch was ovulated. This paper presents the change in number and size of eggs during ovarian maturation and ovulation of the amphidromous type Ayu Plecoglossus altivelis captured in Chikugo River, Fukuoka Prefecture, Japan. On the basis of oogenetic stages of the most advanced group of oocytes in the ovary, the course of ovarian maturation was divided into six phases. GSI values corresponded to the maturity stages of this group. Number of eggs of the first batch decreased with the development of ovary. The number of ripe eggs laid in a spawning season corresponded to number of eggs of the first batch at phase III. Number of yolk-less eggs which are at the peri-nucleolus stage and less advanced stages decreased markedly with the development of ovary and increased again at the stage just after ovulation. It seemed that there was a multiplication period of oogonia before or after ovulation.