日本近海産小鰮鯨に就て

Abstract

1) The Lesser Rorqual, Balaenoptera acuto-rostrata Laeépède is commonly found in the adjacent waters of Japan and its distribution fairly coincides with those of fin whales; In summer it is caught at the southern seas of Kurile Islands (on both Okhotsk and Pacific sides), while in winter its whaling grounds are in Yellow Sea, Tusima Kaikyo and the adjacent waters of “Syanzen”, Korea. 2) The lesser rorquals get closest to the shore of all the whalebone whales and, as fin whales do, they also migrate far into Yellow Sea in the winter season where the salinity of the surface water is as low as 32.00‰ or even lower (euryhaline species). 3) The breeding appears to he in the south-western seas of Japan (Yellow Sea, Tusima Kaikyô and its adjacent seas) and judging from the data available on the foetuses measured at various shore-stations in the years 1932-1935, copulation may be presumed to take place chiefly in the season from December to March and the period of gestation covers about ten or eleven months. Newly born calf measures 2.4-2.7m. in body length. 4) The body length of the whales caught extends from 6.1m. to 9.1m. and the female 7.0m. long was found pregnant. The individuals captured in the northern regions are superior to those captured in the southern seas in body length, the fact was also observed in any other whalebone whales. Regarding the southwards migrations of the whales in the adjacent waters of Japan, the following two main courses may be presumed as in the fin wha'es; thus one herd descends Japan Sea, and the another one goes down Pacific coast of Honsyu, at winter advances.