Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
To clarify the maturation process of female fat greenling Hexagrammos otakii, histological observations were carried out on the ovaries of samples collected in Kikonai Bay, southern Hokkaido, from June 1999 to May 2000. The maturation process within the size range examined (170-460mm in standard length) was divided into 6 phases: immature (recovery), oil droplet, yolk formation, maturation, spawning, and spent. Based on changes of the development stage and diameter of oocytes, the development form of fat greenling was thought to belong to the “group-synchronous oocyte development” type. The spawning season in this region was assumed to occur from November to January, with a peak occurring between mid-November and late December.