The Arctic Ocean is so cold that only a handful of fish and marine mammals can survive there. Subsurface temperatures range from 37.4 degrees Fahrenheit on a warm summer day to 28.76 degrees, the freezing point of seawater.
In those extreme conditions, one fish species in the center of the Arctic food web is uniquely equipped to thrive: the Arctic cod.
A slender and smaller cousin of the Pacific and Atlantic cod, the Arctic cod is often seen near the underside of the ice, feeding on pteropods, copepods, krill, worms, and small fish. It uses cracks and seams in the ice much as tropical fish use a coral reef: as a refuge from predators.
Its survival in these heat-sapping waters depends on two things: blood and fat.
Arctic cod blood is a biological marvel. The fish survives thanks to a special protein that acts as an antifreeze, preventing the blood from crystallizing at temperatures below freezing. As for the fat, it is hard to overstate its importance to the health of the entire Arctic food web. Pound for pound, Arctic cod contain nearly twice the energy of groundfish like pollock, which thrive in the subarctic region of the North Pacific. For animals in the Arctic, where every calorie is dearly earned and spent, that's a massive bang for the buck.
The Bering Strait acts as the border between the Pacific and the Arctic Ocean. But man-made distinctions mean little in the biological world. What really separates Arctic from subarctic species is the Bering Sea cold pool, a tongue of near-freezing seawater that constantly expands and recedes south from the strait. During warmer summer months, some subarctic species like pollock and flounder can move into Arctic waters, but the winter cold pool eventually drives them back south.
Since the early 1980s, though, the cold pool has been in retreat. Rising air temperatures and the shrinking ice pack have pushed warmer waters more than 140 miles north of the cold pool’s mid-century baseline. At least 23 species in the Bering Sea have marched north, following the warming water. Pollock and arrowtooth flounder migrated 30 miles north. Arctic cod retreated, unable to compete with larger subarctic species.
Not all the news is bad, however. Arctic cod represent a most critical nexus point in the Arctic food web, and their place in the ecosystem has so far been well protected by both human and natural systems. In the past four years, the cold pool has regained some of its lost ground (possibly because of a shift in the Pacific Decadal Oscillation, an El Niño–like pattern of climate variability). But over the long term, fisheries scientists expect the cold pool to continue its northward retreat.
There is a limit, though. "Above a certain latitude it still gets dark and cold enough in winter for seasonal ice to form, and that creates the cold pool," says Franz Mueter, a fisheries biologist who studies the state's Arctic and subarctic marine systems. "It's going to be a long time before you see the full year-round expansion of Bering Sea groundfish into the Arctic."
Arctic Cod Fishing Ban Could Forestall Trouble
Also in the Arctic cod's favor: a fishing ban. To prevent a free-for-all in newly ice-free waters, the North Pacific Fishery Management Council in 2009 banned all large-scale commercial fishing in American territorial waters above the Bering Strait.
Perhaps most important, the ban covers not just fish but "all other forms of marine animals and plant life." That may forestall the kind of trouble now brewing in the waters around Antarctica, where a fast-growing industrial krill fishery (it's sold as food for farmed salmon and pressed into oil for omega-3 supplements) threatens the base of the food chain in the southern ocean. Krill are less abundant in the Arctic, but the growing demand for the crustaceans could lead krill processors to turn their eyes to northern waters.
"Fishermen are frontiersmen," said Caleb Pungowiyi, former president of the , who was among those who fought for the ban. "They want to expand their territory. Before you allow any industrialized fisheries in the Arctic, you need to know the science on the stocks, how they can be sustainably fished."