Ovarian Maturation and Ovulation of the Amphidromous Type Ayu Plecoglossus altivelis in Chikugo River Based on Histological Observations

The process of oocyte maturation and the seasonal changes of ovarian development were observed with the amphidromous type Ayu captured at Chikugo River, Fukuoka Prefecture, Japan. In the oocytes of the fish, three kinds of the vitelline elements, i.e. yolk vesicles, yolk globules and oil droplets appeared in this order. In the mature eggs, yolk globules never formed a massed yolk in spite of increasing in size. Accumulation of yolk rapidly proceeded and was completed in about three weeks. The oocytes showed the development of asynchronous type and the mature ovaries contained groups of oocytes in various maturation stages at the same time. Therefore the ovarian maturity of the fish is adequately represented by the histological maturation stage of the oocytes that have the modal value of diameter in the first batch (the most advanced group of oocytes). After the first batch was ovulated, the second batch developed up to secondary yolk globule stage at most. But the fish seemed to die before long without ovulation of the second batch.