Distribution and migration of pink salmon juveniles in the coastal waters of eastern

Pink salmon is considered to be specialized species of Pacific salmon, because they have a rigid two-year anadromous life cycle, a short freshwater residence, and two genetically isolated broodlines in North America and Asia. The geographical distribution of adult pink salmon migrating upstream in Japan includes part of the Pacific coast of northern Honshu and the Sea of Japan coast of Hokkaido; however, the main spawning grounds are located in rivers of the Okhotsk Sea coast of eastern Hokkaido. In Japan, pink salmon fry are produced by stock enhancement programs. Although the number of fry released in Hokkaido has been similar over the past 15 years, i.e., approximately 140 million fish, the commercial harvest of pink salmon has a unique biennial abundance cycle. Unfortunately, the exact cause of this fluctuation is unknown, because of the lack of information about Hokkaido pink salmon biology in relation to ocean conditions. Moreover, to elucidate the ecology of pink salmon in terms of growth and abundance, information about their distribution and offshore migration during early sea life is important, because high mortality is often suggested to occur soon after juveniles enter the ocean (Manzer and Shepard 1962; Parker 1968). In 2002, we began to investigate the distribution and migration patterns of juvenile pink salmon in the coastal waters of Abashiri on the Okhotsk Sea coast of eastern Hokkaido. Twelve study sites were established, including four at 1 km fro