ヤマメ卵巣卵数の周年変化

Abstract

The number of oocyte were determined in female masu salmon Onchorhynchus masou ishikawae collected monthly from January (13-month-old) to October (at the time of spawning). In the early period of the ovarian development (from January to April), about 3000 young oocytes in the primary growth phase could be counted in each individual. However, prior to the onset of rapid vitellogenesis, about 2/3 of the oocytes in the primary and secondary growth phase became atretic and disappeared between May and July (first reduction stage). During the period of rapid vitellogenesis, there was further reduction in the number of secondary oocytes due to atresia (second reduction stage). A relationship between body size and oocyte number was clearly demonstrated not only during the spawning season but also during the early reproductive cycle. This result indicates that fish showing fast growth during the juvenile stage may produce significantly more eggs at the onset of maturation (before 1 year age in this species). Decreased feeding rates during the first and second reduction stages resulted in lower number of oocytes in the fish. On the other hand, feeding until satiation enhanced body growth and slightly suppressed the incidence of atresia in the oocytes.