Distribution of Pelagic Elasmobranchs Caught by Salmon Research Gillnets in the North Pacific

Abstract

The distribution of pelagic elasmobranchs was analyzed using bycatch data from the high seas salmon surveys by research gillnets from 1981 to 1991 in the North Pacific Ocean and the Bering Sea. Five species of sharks and one stingray were reported. Blue and salmon sharks and spiny dogfish were dominant and constituted 98% of the total shark catch. It is considered that the research area includes the southern part of the distribution for spiny dogfish and salmon sharks and the northern part of that for blue, shortfin mako, cookiecutter sharks and pelagic stingrays. These elasmobranchs showed spa tial segregation by species in distribution according to oceanographic conditions. Intraspecific sexual and growth dependent segregations were also confirmed. Length frequency distributions of blue and salmon sharks suggest that their nursery grounds exist around the transitional domain of the subarctic boundary. The Japanese high-seas salmon fishery ceased in 1992 af ter some 40 years of industrial fishing from 1952 because of international concern about the impact of this fishery upon the oceanic ecosystem. The Fisheries Agency of Japan has conducted salmon research for the rational use of salmon stocks for a long time. It also started bycatch research in the 1970 s using several research vessels. Bycatch research data collected by the research vessels of Hokkaido University and chartered vessels for scientific research from 1981 to 1991 were used for the present analy sis. The research area is widely distributed from 35?N 63°N from the northern coast of Japan to the Gulf of Alaska and the Bering Sea. The spatial distribution and migration of pelagic elas mobranchs are still poorly known, although several sur veys were conducted in the North Pacific Ocean.1-6) Many of those studies partly described their distributions and migrations due to the limitation of research area, season and gear selectivity. Therefore, more detailed biological and ecological information is necessary for the conserva tion and stock management of pelagic elasmobranchs. This paper describes the distribution of pelagic sharks and rays in the North Pacific, north of the subarctic boundary.