ANCHORAGE, Alaska — As scientists observed climate warming in the Bering Sea, they suspected valuable commercial fish species such as Pacific cod and walleye pollock would move north toward the Bering Strait and into the Arctic Ocean.
But that’s likely decades off, according to one surprising result from a study of the sea north of the Aleutian Islands.
Scientists say a pool of cold water in the northern Bering Sea has been a locked door to the northward migration of pollock and cod, the fish harvested for America’s fish sticks and fast food sandwiches.
“Our original hypothesis was wrong, and we think they won’t have habitat to occupy northward in the northern Bering Sea,” said Mike Sigler of Juneau, a marine biologist with the Alaska Fisheries Science Center, part of the National Oceanic and Atmospheric Administration.
Water along the ocean floor where pollock live has been kept cold by the layer of sea ice that forms every winter on the surface of the northern Bering Sea. That ice is expected to persist even with climate warming. Cold water sets up below the ice layer and remains cold throughout the summer.
“What it looks like at the moment is that the northern Bering Sea — and north to us is north of St. Matthew Island — looks like it is going to stay cold,” said physical oceanographer Phyllis Stabeno of NOAA’s Pacific Marine Environmental Laboratory in Seattle.
Sigler and Stabeno are two of more than 100 principal investigators taking part in a $52 million study of the eastern Bering Sea ecosystem. Supported by the National Science Foundation and the North Pacific Research Board, scientists are focusing on creatures from plankton to walrus.
“We’re in the analysis and synthesis stage to complete the project,” Sigler said, of the study that began in 2007..
Commercial fishermen in the Bering Sea and Aleutian Islands this year were allowed to catch 2.8 billion pounds of pollock, and nearly 510 million pounds of Pacific cod.
Researchers had documented sub-arctic species including cod and pollock moving north within the Bering and assumed that would continue, until the discovery of the cold pool.
“The hypothesis that we had worked with for years was that as warming occurs, the Bering Sea would kind of warm uniformly,” Stabeno said, meaning that fish species would move north. And that would have complicated matters for the commercial fishing fleet, increasing calls for a Coast Guard base, a deep water port and other infrastructure in northwest Alaska, which has remained remote because it’s covered by ice so much of the year.
Temperature readings collected for more than a decade from fixed buoys and from cruises show an environment that remains inhospitable to pollock, cod and arrowtooth flounder, which do not like temperatures below 2 degrees Celsius (35.6 degrees Fahrenheit).
The key is ice cover, which forms in the Bering Sea in December or January.
“In a cold year, like 2010, it will cover basically the whole eastern shelf,” Stabeno said. “That’s that broad area that’s less than 200 meters (656 feet) deep. It stays until March, April. In the north, it stays until even into June sometimes.
Water below the ice chills to minus 1.73 degrees Celsius (28.9 degrees Fahrenheit), Stabeno said, the freezing point of seawater. In the north, after ice retreats in spring, a warm layer forms at the top that isolates cold water on the bottom.
The northern Bering Sea is hemmed in by Russia on the west and Alaska on the east, Stabeno said, likely contributing to colder water.
“Land cools faster than water does,” Stabeno said. “You tend to get colder temperatures.”
The Bering Sea south of St. Matthew Island, a speck of land 500 miles north of Dutch Harbor in the Aleutians, is more variable. It starts out warmer. Some years, it gets far less ice cover and water along the bottom never gets as cold, she said.
The absence of abundant cod and pollock in the north was confirmed by shipboard surveys.
“They used acoustics and didn’t find much pollock,” Sigler said.
The scientists detected the cold pool throughout five warm years in the early 2000s that were followed by five cold years. They project it’s likely to last for 30 years, perhaps through 2050, Stabeno said
“What happens after that, none of us really know,” she said. “But the models look really good up to then.”
As for warm-blooded species moving north, Sigler said, scientists need more information.
“We’re just kind of in the speculation stage that if currents drive their food north, then the whales, for example, will follow them,” Sigler said.
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