about a year of migrations all around the islands of Japan, then gathers to spawn at
sites in the East China Sea, near Kyushu and in the northern Sea of Japan. The
nearshore zone off Hokkaido is the site of a fishery based on attracting T. pacificus to
lights hung from boats over the water. Dahl’s porpoise, Phocoenoides dalli, is
endemic to the region, living mostly well offshore, and both killer whales (Orca) and
several smaller porpoise live around the perimeter, as do Steller sea lions (Eumetopias
jubatus), harbor seals (Phoca phoca), and sea otter (Enhydra lutris). Humpback
whales (Megaptera novaeangliae) that overwinter in subtropical waters, many of
them around the Hawaiian Islands, migrate north in spring, then swing east or west on
reaching subarctic latitudes, feeding through summer months in Alaskan and Russian
fjords. Gray whales (Eschrichtius robustus) migrate along the coast from Mexican
wintering areas, basically bypassing the subarctic to feed in the northern Bering and
Chukchi Seas. Thus, the region has a rich nektonic fauna, richer than listed here, and
many of these larger animals have whole populations of experts devoted almost
exclusively to studying them.
Subarctic Atlantic
(^) The contrast of the subarctic Pacific with the North Atlantic north of the Gulf Stream
and south of the polar front has four main parts. In the North Atlantic, (i) winter
mixing is deep, to ∼200–400 m (Fig. 11.6), and phytoplankton stocks are reduced
then to very low levels (<0.2 μg Chl liter−1). (ii) Spring blooms are typical, reaching
2–4 mg Chl m−3 from New England to Norway. Large diatoms dominate Atlantic
blooms, whereas diatoms are only occasionally a major fraction of phytoplankton
biomass in the oceanic Pacific and always small (<7 μm) species. (iii) Proximity to
continents apparently provides enough iron to fuel nearly complete exhaustion of
major nutrients from surface layers, particularly nitrate. It is important, also, that the
concentration of nutrients at depth is about half that in the Pacific, so depletion is
more readily reached before trace metal limitation is severe. (iv) The large grazing
copepods (the dominant mesozooplankton) are from the genus Calanus, species that
ascend to the surface and generate egg biomass from active feeding, unlike the
subarctic Pacific Neocalanus species.
Fig. 11.6 Profiles comparing summer (August) and winter (January) temperature (T)
and salinity (S) profiles from the subarctic Atlantic. The summer water column is
strongly stratified. The winter profile shows deep mixing (to 360 m). These are Argo
float data from the World Ocean Database maintained at the US National Ocean Data
Center (NODC).