Maturation and Reproductive Cycle of Female Pacific Cod in Waters Adjacent to the Southern Coast of Hokkaido, Japan

Abstract

The maturation process and reproductive cycle of female Pacific cod Gadus macrocephalus were examined in the waters adjacent to the southern and southeastern coasts of Hokkaido, Japan, by collecting fish between April 1989 and September 1990.
Histological examination was made of the ovaries. During the course of ovarian maturation, a portion of the oocytes became isolated from immature oocytes at the yolk vesicle stage (less than 0.3mm in diameter) and gradually developed into a group of yolky oocytes. When these oocytes reached the migratory nucleus stage (0.5-0.7mm in diameter), they began to change into transparent mature eggs (0.8-0.9mm in diameter) accompanied by hydration and yolk fusion. Following this, all of the mature eggs were simultaneously ovulated into the ovarian cavity. The maturity of female Pacific cod was histologically divided into nine grades from yolkless phase (I) to spent phase (IX). Ovaries gradually developed to the yolk vesicle phase from spring to summer. The onset of yolk formation and the most active yolk formation occurred from August through November. Females with ovaries at the migratory nucleus phase appeared during December and January. From the changes in maturity states and the gonadsomatic index (GSI values), the peak of spwaning in this region was assumed to occur during the period of late December through January. Also, the age of first maturation of female cod was estimated to be four years old.