Recent years have pushed the extent of Arctic sea ice back to record lows, and so chased thousands of massive walruses, which often rest on the ice, to the coast, where they congregate, waiting for the ice to creep back south. The result can be lethal stampedes of the behemoths as they haul out onto land.
In the long-term, it's not yet clear how these unprecedented migrations will affect the massive animals, but scientists hope a satellite tracking program will help them better understand what's happening.
This year brought a record, or near record, low for sea ice, and an equally impressive migration of walruses from the Chukchi Sea to one spot on the northwest coast of Alaska. [Video: Walrus Haulout]
The last bits of sea ice disappeared around the beginning of August, and the walruses began to move in different directions looking for a place to haul out, either on sea ice or land. They finally hauled out on the beach, near Point Lay later in the month, according to Chad Jay, the program lead for walrus research at the United States Geological Survey (USGS).
One aerial survey estimated upward of 20,000 of the animals had congregated there, he said.
Walruses normally come to shore; however, these mass migrations first appeared in 2007, a year of unprecedented ice melt.
Retreating ice
Although sea ice has always retreated north in the summer, the edge has usually remained very near or over the continental shelf. The walruses can feed in this shallower water, diving down to retrieve clams, snails and worms from the sediment at the bottom of the seafloor. Females and calves in particular rest on the ice during the summer and look for food beneath it, he said.
"What we are seeing now and for the last five years, we have seen the edge of the sea ice recede far north of the continental shelf and over the deep waters of the Arctic Basin, and if walruses were to stay with that sea ice they would find themselves over the deep water and unable to forage effectively," Jay said.
The past five years have been the five lowest years for sea ice on record since 1979, according to Walt Meier, a research scientist at the National Snow and Ice Data Center (NSIDC). This year brought the sea ice to its second-lowest point in the same period, according to the NSIDC. A group of German researchers, however, reported 2011 as the record low.
Scientists blame a combination of natural fluctuations and climate change caused by human emission of greenhouse gases, like carbon dioxide, for the recent record lows in sea ice.
An unnatural migration
So, with ice in the sea receding, walruses head to the shores of Alaska and Russia — where they haul out in even greater numbers. In 2007 and 2009, the walruses spread themselves out in smaller groups along the Alaskan coast, but in 2010 and this year they massed together. (There was no massive haul out in 2008.) This year as with last, all the animals settled in one giant group near Point Lay. But this summer, they arrived a couple weeks earlier.
"I think they're still establishing some sort of pattern in how to respond to the loss of sea ice," Jay said. "There is plenty of space, they are funny that way. They are very gregarious and like to haul out near one another."
But the cramped quarters can lead to problems: If startled, the walruses can stampede, killing calves. As a result, coastal communities and officials are attempting to reduce disturbances with measures such as a flight restriction to keep planes from getting too close, he said.
The melting sea ice raises a host of questions about the walrus, according to Jay. Will it affect the calves' survival? Are they spending more energy to swim further offshore to feed? Is the availability of their prey changing?
How to track a walrus
Scientists at the USGS and their Russian counterparts are attempting to get a handle on the walrus activities by tagging them with satellite transmitters. It seem a bit odd that the USGS is studying these animals, but the Pacific walrus is one of four marine mammal species managed by the U.S. Department of the Interior, and the USGS is the research arm of the DOI, according the USGS website.
In July, the scientists, including Jay, tagged 40 walruses from a boat using a crossbow with a harpoon head that attaches tag to the walrus by embedding into the animals' blubber. Then, in late August, they tagged 34 more. Russian researchers are tagging an additional 25. [Giants on Ice: Gallery of Walruses]
Prior to picking up the crossbow a few years ago, the scientists had strapped the tags on the walruses' tusks, an effort that required them to trap and sedate the enormous animals — the females, on which they are focusing, can weigh more than 1 ton (0.9 metric tons). A male can weigh as much as 2 tons (1.8 metric tons).
Although the crossbow-applied tags seem to fall off within about two months, this method has allowed scientists to tag more animals and generate more tracking data.
Since the transmitters are heavy and aerodynamic, they must be shot from within 10 yards (9.14 meters) of the target animal. On the water, this means approaching in a boat, and on land this means crawling on your belly to the edge of the herd downwind of the walrus, Jay said.
"I don't know if I would say it is dangerous, you've got to kind of watch what you are doing," Jay said. "Usually their response is to move away."
While the researchers were out on the water in July, two walruses — most likely young males — surface and jabbed at the inflatable pontoons of the research boat, he said.
You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.
