Abstract
Pterodroma nigripennis and P. longirostris were observed in the Northwest Pacific during several cruises on R. V. Hakuho Maru from May to November between 1980 and 1984 (the north of 7°N, the west of 180°E). It can be deduced that P. nigripennis and P. longirostris fly regularly every year in the Northwest Pacific, and during these observations there were no species of smaller petrels to migrate in these areas except the above species including P. hypoleuca. With regard to the season of migration of P. nigripennis to the Northwest Pacific, they migrate to these areas in June-July, and in August-September there are great numbers in the waters southesast of Honsyu, then they return to the breeding islands after November. The favoured water temperature of P. nigripennis is in the range 25°-28°C, so it is surmised that they do not move in the waters off Sanriku to the north of 38°N. P. longirostris migrate to the waters far east of Honsyu in May-June and concentrically distribute in the waters off Sanriku between late July and September, in this season they were feeding mainly at the waters in front of Kuroshio Current, and then they beign to return to their breeding islands. The three smaller petrels occurred in areas where surface water temperatures ranged from 16°C to 31°C in June-September, P. nigripennis favour in areas with the highest water temperature in these species and P. hypoleuca next, but when these species distribute in the Northwest Pacific, they devide a feeding area for keeping away from competition. In the Northwest Pacific P. hypoleuca and P. nigripennis formed mixed groups with sea birds which the breeding islands or distribution areas are same with them, on the other hand P. longirostris formed large flocks with the storm petrels in having same feeding areas.
