Whales take Northwest Passage as Arctic sea-ice melts
Bowhead whales, the giants of the Arctic, are using the Northwest Passage to move across the top of the Americas.
Skeletons, DNA samples and harpoon heads have all suggested that bowhead populations living on each side of the continent did meet and mingle.
Now, research published in the journal Biology Letters has used satellite tags to provide confirmation.
The work may provide insights into the development of Arctic cultures in which bowhead hunting plays a central role.
Mads Peter Heide-Jorgensen from the Greenland Institute of Natural Resources led a team that has satellite-tagged more than 100 bowheads over the last decade - much of the work funded by oil and gas companies interested in the Arctic's new mineral wealth.
Last August, as the Arctic sea ice neared its annual minimum, the satellite data told them that one whale from the Greenland side and one from the Alaskan side had arrived in pretty much the same area, north of the Canadian mainland.
They spent about 10 days circling in the same patch of water before heading back to their respective home ranges.
This is the first time such mingling has been seen directly, although there is plenty of circumstantial evidence that it has happened sporadically in the past.
Bowhead skeletons are found on elevated beaches through the Canadian archipelago. In 19th Century Alaska, whales were caught containing tips of harpoons that had been thrown by whalers in the Atlantic.
Passage may have been easier in the last four summers than in preceding decades, as the Arctic ice has shrunk to a smaller size than at any time in the satellite record.
"I'm pretty sure that the low sea ice in the summer has triggered this migration through this area," Dr Heide-Jorgensen told BBC News.
"I'm pretty sure that when it occurred in the past, when we got all these skulls on the beach, that was during a warm period.
"During any climatic period, there could have been years with less ice in the Northwest Passage; and I'm pretty sure the bowheads can find cracks [in the ice] that are too small to show up on satellite images."
The other fascinating line of evidence for bowhead migration along northern Canada lies in the human societies that depend on the whales, where communities are structured around the process of whaling.
The best known example today are the Inupiat of Alaska.
They are descended from the Thule culture, which appears to have emerged in Alaska about 1,000 years ago.
Thule people spread across Canada to Greenland in the three centuries following.
They ate meat and blubber from the bowhead, built houses using their bones and burned their oil.
Given their dependence on the single species, could they have made the migration without access to the whales?
Anne Jensen, an archaeologist who has lived in the northern Alaskan town of Barrow for many years and is a leading authority on the Thule people, said the routes taken by the two whales last year passed close by sites that date the Thule expansion.
"This supports the idea that the expansion was connected to whaling," she told BBC News.
"People were moving - presumably after having done some small-group exploring that is archaeologically invisible - to places where they knew whales went, perhaps in a period when the ice cover was reduced for some reason.
"Certainly, they were not settling in places where whales weren't available at that point.
"Since the Thule seem to have been able to hunt whales from the ice at leads, it doesn't mean that there was an ice-free Northwest Passage - just one that bowheads could get through, with enough leads for hunters to see and catch the whales."
Of all the great whales, the bowheads are the best adapted to life in the harsh Arctic environment, with a domed bony head that can force a way through relatively thin sheets of ice, and a thick layer of blubber.
"They have their lunch package on them - they can move for months without eating because they have blubber almost half a metre thick, so so they can move through the Northwest Passage without eating," said Dr Heide-Jorgensen.
Genetic evidence also suggests that in periods past, bowheads also moved along the northern coast of Siberia.
They do not have a set migration pattern, unlike some other whale species - which raises questions about how they may adapt in future, if the projected declines in summer Arctic sea ice materialise.
"I'm sure they will be going all over the area when there's open water," said Dr Heide-Jorgensen.
"They're capable of moving relatively long distances in a short period of time, and they can venture into areas where we haven't seen them for a very long time - so I'm sure they will use the open water if the ice disappears."
The one question mark would be over their food supply, with major changes in Arctic sea ice likely to disrupt food webs that include the swarms of copepods (tiny crustaceans) that are the key ingredient of their diet.
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