Scientific Reports of the Whales Research Institute (1978)


The geographical variation of the colour types of the species indicates three local stocks in the northwestern North Pacific and western Bering Sea (WNP/BS), off the Pacific coast of Japan (PJ), and in the Sea of Japan and Okhotsk Sea (SJ/OS).
The catches by the salmon gillnet fishery in WNP/BS and the present and past harpoon fishing in PJ and in SJ/OS, and the age structure based on the cemental growth layers give the following estimation of the life history parameters. The calves are born in August to September at the length of 100 cm after the gestation of 11.4 months, and nursed for about 2 years. Sexual maturity is attained in males at the age of 7.9 years and the length of 195.7 cm, and in females at 6.8 years and 186.5 cm. The mean calving interval is about 3 years, and the annual natural mortality rate of the adult female will be below 0.1. Data are insufficient to indicate the degree of between stocks difference of the parameters.
The weaned immature individuals are distributed in the south in the winter (PJ, WNP/BS, and SJ/OS) and possibly in the summer too (WNP/BS). The segregation of pregnant females by the size of the fetus is expected (WNP/BS). The harpoon fishing mainly exploits the weaned immature and newly matured individuals (PJ and SJ/OS), however the salmon gillnet fishery kills the calves of suckling age and newly matured individuals (WNP/BS). The lactating females are underrepresented in the both fisheries.