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Warming autumn may delay Western Arctic Caribou Herd migration in Alaska

Yereth Rosen
Caribou from the Arctic Caribou Herd, the largest in North America, negotiate the Kobuk River during their fall migration to their winter grounds. In recent years, the herd has moved later and later in autumn. NPS

Temperature readings aren't the only things that have been out of whack in this unusually warm Alaska autumn. The annual southward migration of North America’s largest caribou herd has also been late and unusual, biologists say.

The Western Arctic Caribou Herd, with about 325,000 animals, apparently procrastinated en masse before starting the march from northernmost Alaska to its wintering grounds south of the Brook Range

After failing to show up for weeks at a key river crossing along the migration route, the caribou arrived almost at once near the end of September, said Jim Dau, a Kotzebue-based biologist with the Alaska Department of Fish and Game.

Dau has been collaring Western Arctic caribou for nearly three decades at the Onion Portage crossing site of the Kobuk River, a place used by migrating caribou for thousands of years. He and his colleagues used to travel to Onion Portage at the end of August. This year, the team went out on Sept. 26, a full month later. The biologists were slightly behind the leading edge of the migration, but they managed to catch and collar members of the herd as animals thundered across the river.

At that late date, Dau was worried about the river freezing up. Customary field trips for area high-school students -- who in past years have helped collar the animals -- were cancelled due to fears about dangers from river ice and conflicts with hunters.

No lake ice as temperatures soar into 40s

But soon after this year’s collaring operation ended, the ice was gone, and local temperatures soared into the 40s, he said. Lakes that used to be sufficiently frozen by October to support landing planes were still ice-free, Dau said.

“It turns out that this is the craziest fall I’ve ever seen. We got September weather into late October,” he said.

Though caribou are notoriously erratic in their migrations, Dau said a pattern has emerged since about 2000, with the animals showing up at the Kobuk River two to six weeks later than previous years. Statistics kept by the National Park Service show a similar pattern to what Dau saw on the ground, though officials caution that any conclusions are premature.

Records of three dozen collared caribou monitored by the Park Service show that the animals crossed the Noatak River far later this year than in any of the past three years, which is how long data has been collected, said Kyle Joly, a Fairbanks-based biologist with the National Park Service. “It’s pretty much a sure thing that it’s going to be the latest for the four years that we have GPS data,” he said.

Why are the caribou so late moving south? “I’d say that it’s pretty clear that it isn’t clear,” Joly said. The warm fall could have affected movement, or there might be some holdover effects from the cold spring, he said. “A lot of plants kind of got a late start,” meaning that the prime food availability in the northern part of the herd’s range might have been delayed, he said.

Warmer water, too

Joly and some colleagues are looking more closely at weather records, including data collected from sensors on caribou collars, to investigate possible links to migration patterns.

Dau, meanwhile, said he has noticed changes that go beyond caribou behavior.

The river water is warmer than in the past, he said. In his early days collaring caribou at Onion Portage -- a process honed to synchronized efficiency so that collars are fastened and blood samples drawn in just a few minutes per animal -- the workers grabbing the animals’ tails beneath the river’s surface would feel the shock of frigid water, he said.

“Back then, your hand would just ache from the cold,” he said.

No more. “We have people working on this project now, they have no idea. They’re not even wearing gloves,” he said.

Contact Yereth Rosen at yereth(at)alaskadispatch.com