Scientific Reports of the Whales Research Institute (1988)


Results of nine whale sighting cruises in the Japanese coastal waters in 1983 through 1986 and some additional cruises provided following conclusion. Dall’s porpoises inhabit waters below 24°C, but the upper temperature limit is lower in winter. Dalli-type individuals occur through out the range of the species, but the majority of them winter in the Sea of Japan, and summer in the Okhotsk Sea and Pacific coast of Japan. Truei-type individuals inhabit only waters off the Pacific coast of northern Japan and southern Kuril Islands. They winter off the Pacific coast of Japan, and summer mainly in the waters between 40° and 45°N and west of 155°E. A minimum estimate of the number of dalli-type that winter in the eastern Sea of Japan is about 46,000 individuals, of these about 15,000 migrate through the Tsugaru Strait to the Pacific coast of Japan with the major portion of the stock summering in the Okhotsk Sea and northern Sea of Japan. The number of truei-types in the nearshore surveyed area is estimated at about 26,000, and the extraporation of the density to the offshore range of the type provides an estimation of total truei-type population of about 58,000 individuals. The continuation of current Japanese harpoon fishery will further deplete the stock(s) of Dall’s porpoises.