Present status and future guideline for conservation of the streaked shearwater breeding population on Oshima Ohshima Island, Hokkaido [Japan]

Oshima Ohshima Island is the largest uninhabited volcanic island in Japan and is located in the northern Sea of Japan near the southwest part of Hokkaido, Japan. The latest eruption on the island during 1741 - 1759 made a new central cone named Kanpo Mountain. Due to erupted volcanic rocks and ashes, the present flora of the island consists mostly of herbaceous plants, with a few woody plants. Oceanographically, the island is located in the flow of the Tsugaru Warm Current, causing spring to arrive earlier on the island than in the southwest part of Hokkaido. The breeding population of streaked shearwaters (Calonectris leucomelas) on Oshima Ohshima Island had long been harvested during two main periods: first during 1905 - 1920 by commercial hunters who exported the feathers to foreign countries, and second during 1921 - 1967 by seasonal seaweed gatherers who used the bird meat for food while living on the island. Seaweed gathering, which was performed annually from late March to late April, was conducted by fishermen in Era village. The target seaweed was wakame (Undaria pinnatifida), which had a high commercial value before 1960. However, after 1961, the price of wakame decreased gradually due to increase of wakame culture in coastal sea areas of Japan, and finally wakame gathering at Oshima Ohshima Island ceased to be unprofitable in 1967. As for the streaked shearwater, a total of about 300,000 - 400,000 individuals were killed in the above period by fishermen. However, since 1967, a breeding population size of the streaked shearwater on the island has not shown any sign of recovery. At present, the breeding population size is consisting of only about 80 - 85 pairs at Yamasedomari on the island. The possible causes of keeping lower population level of the nesting birds for over 30 years contribute to the presence of brown rats (Rattus norvegicus) as a predator for eggs, chicks and adults of the streaked shearwater and feral rabbits (Orycolagus cunicuhis) as a destroyer for vegetation at the breeding colony. Terrestrial plant community on the island has been always confined to the early stage of succession by feral rabbit's ring barking for woody plants and selective eating for herbaceous plants. As a result, nesting sites of the streaked shearwater seem to be restricted to unaccessible cliff sides and steep rocky slopes not extending to flat and gentle slope areas. Therefore, in order to conserve the breeding population of streaked shearwater on the Oshima Ohshima Island in the future, brown rats and feral rabbits on the island must be culled from the island.