Spawning period of ocean sunfish Mola mola in waters of the eastern Kanto region, Japan

The spawning period of ocean sunfish Mola mola was estimated using changes in gonad index (GI) and histological observations of gonads. GI was calculated for 183 ocean sunfish, captured from 1981 to 2006 in the waters off eastern Japan's Kanto region. GI maximum value occurred in August for both sexes of wild specimens. Gonads of 9 female specimens (total length 185-272 cm) and 4 male specimens (168-190 cm) were examined histologically. Oocytes in the ovaries revealed four distinct maturational stages. Based on the seasonal changes of GI and the gonad maturational phases, the spawning period for ocean sunfish is estimated to occur from August to October. The discovery of asynchronous oocyte development in the ovaries also suggests that ocean sunfish are multiple spawners.