An overview of the Oyashio ecosystem

Abstract

The Oyashio shelf region and the seasonally ice-covered areas north of Hokkaido are highly productive, supporting a wide range of species including marine mammals, seabirds and commercially important species in the western subarctic Pacific. The fishes include gadids, such as walleye pollock and Pacific cod, and subarctic migratory pelagic fishes such as chum salmon and pink salmon. It is also an important summer feeding ground for subtropical migrants such as the Japanese sardine, Japanese anchovy, Pacific saury, mackerels, Japanese common squid, whales and seabirds. In recent decades, some components of the Oyashio ecosystem (i.e., phytoplankton, mesozooplankton, gadid fish, and subtropical migrants) have shown changes in species abundance or distribution that are correlated with environmental changes such as the 1976/77 and 1988/89 regime shifts. The First Oyashio Intrusion moved northward from the mid 1960s until the late 1970s when it moved southward until the 1980s, after which it returned to the north again after the mid 1990s. The sea surface temperature in spring decreased after the late 1970s, increased after the late 1980s, and remained high during the 1990s. The extent of ice cover in the Sea of Okhostk also decreased during the latest warming in the 1980-90s but has increased again since the late 1990s. These and other kinds of variability are described in this overview of the status of the Oyashio ecosystem and the surrounding region.

Foraging areas of streaked shearwaters in relation to seasonal changes in the marine environment of the Northwestern Pacific: inter-colony and sex-related differences

“…Zooplankton biomass is generally low in the warm Kuroshio water, except along the frontal or coastal regions, while primary productivity and zooplankton biomass are generally high in the cold Oyashio water (Odate 1994, Kasai et al 2001, Sakurai 2007. For this reason, the Oyashio is the main feeding grounds for many pelagic fish species (Odate 1994, Sakurai 2007. However, the distribution of these pelagic fishes is directly affected by water temperature (Oozeki et al 2007, Takasuka et al 2008b.…”

“…The northward seasonal movements appeared to match the seasonal development of oceanographic processes in the Kuroshio-Oyashio region. Zooplankton biomass is generally low in the warm Kuroshio water, except along the frontal or coastal regions, while primary productivity and zooplankton biomass are generally high in the cold Oyashio water (Odate 1994, Kasai et al 2001, Sakurai 2007. For this reason, the Oyashio is the main feeding grounds for many pelagic fish species (Odate 1994, Sakurai 2007.…”

“…The area between the Kuroshio and Oyashio is known as the Kuroshio-Oyashio transition area, where the warm Kuroshio water and cold Oyashio water converge, generating a variety of complex hydrographic features (Kawai 1972, Olson 2001, Yasuda 2003. As a consequence of this highly dynamic ocean environment, the Kuroshio-Oyashio transition area represents one of the most productive zones in terms of carbon in the World Ocean (Kimura et al 1997, Qiu 2001, Yasuda 2003, Sakurai 2007. Streaked shearwaters from both Sangan (SA) and Mikura (MK) Islands -located ca.…”

“…The Kuroshio is the western boundary current of the North Pacific subtropical gyre that is characterized by warm and nutrient-poor surface waters, while the Oyashio is the western boundary current of the North Pacific subArctic gyre that is characterized by cold and nutrientrich surface waters (Kawai 1972, Qiu 2001, Sakurai 2007Fig. 1).…”

Synchronous marine pelagic regime shifts in the Northern Hemisphere

“…Regime shifts have been reported for the North Pacific (1976/1977 and 1989) [6,[26][27][28][29], specifically the northeast Pacific (1976/1977) [30,31], (1998/1999) [32,33] and the northwest Pacific (1976/1977, 1988/1989 and 1998) [34][35][36]; the Humboldt Current in the southern Pacific (1968/1970 and 1984/1986) [37,38]; and the northwest Atlantic (1989Atlantic ( /1990 [39][40][41][42][43]. In addition, regime shifts were observed in the late 1980s around all European seas [8]: the North Sea [44,45], the Baltic Sea [9,46], the northwest European shelf seas (1987/1990) [47,48], the western [49] and eastern (Adriatic) Mediterranean Sea [8,50,51] and the Black Sea [52,53].…”