Historical review of exploitation of streaked shearwaters on Oshima Ohshima Island by seaweed gatherers

Abstract

Oshima Ohshima Island, located at 41°30′N, 139°22′E in the Sea of Japan, is the northernmost breeding place of streaked shearwaters (Calonectris leucomelas) in Japan. At present, the breeding population size is estimated to be as small as only a few hundreds. To clarify past human exploitation of the shearwater on this island, information obtained from 25 elderly fishermen who once engaged in seaweed gathering was examined. Between 1921 and 1967 on Oshima Ohshima Island, a total of about 300, 000-410, 000 streaked shearwaters were killed for food by fishermen who were gathering seaweed "wakame" (Undaria pinnatifida). Although the heavy annual exploitation of streaked shearwaters on this island stopped in 1967, no definite signs of recovery of the breeding shearwater population have been observed since. This is probably due to occasional poaching, breeding disturbance by feral rabbits (Oryctolagus cuniculus), which compete for burrows and prevent the development of bushes and forests, and predation by Norway rats (Rattus norvegicus). Of these, the feral rabbit seems to have had the strongest effect in preventing the recovery of favourable woody nesting sites for the streaked shearwaters. To assist in the recovery of the breeding streaked shearwater population on Oshima Ohshima Island, removal of feral rabbits and Norway rats from the nesting sites of shearwaters is thus recommended.