Alaska's largest caribou herd shrank by more than 20 percent between 2003 and 2007, according to a new count from the state Department of Fish and Game.
The Western Arctic Caribou Herd -- which ranges from the North Slope to Eastern Norton Sound and from the Chukchi Sea to the Koyukuk River -- declined by 113,000 animals after years of steady growth.
The herd is still twice the size of any other caribou herd in Alaska. It's a crucial source of subsistence hunting for dozens of villages, a major moneymaker for businesses that cater to sport hunters and a key link in the area's food chain, said Jim Dau, the state's lead biologist on the herd since 1988.
So why did it shrink?
That's not quite clear, but warm spells in the middle of recent winters may have played a role, Dau said.
The herd dwindled to as few as 75,000 animals in the mid-1970s, but had recovered to an estimated 490,000 in 2003. New estimates based on aerial surveys in July 2007 put the number at 377,000.
It could be that the recent decline is a natural occurrence as disease, predators and a shortage of food combined to thin the high numbers.
But, Dau said, "I'm not absolutely sure that that's what happened here."
That's because the herd suffered at least one particularly tough winter since the last census.
In 2005, just before Christmas, temperatures grew unseasonably warm for four days -- including two days of rain, Dau said.
After the rain came freezing temperatures, which covered the ground in an iron-hard crust of snow, he said. The caribou struggled to find food and died in droves.
In 2007 came another warm spell during the winter. This time, however, instead of another large die-off, the herd thrived as extended warm temperatures and high winds swept away the snow.
Climatologists predict these mid-winter thaws -- which present a fine line between good times and disaster for caribou -- will only become more common, Dau said.
When the Western Arctic herd reached nearly half a million caribou a few years ago, everyone held their breath wondering how long the herd could continue to grow, said Sue Steinacher, a wildlife education information specialist for the state.
So while this recent decline didn't come as a surprise, Steinacher said it's the first big dip in 20 years.
"It's enough to get everybody's attention."
Caribou counting is a painstaking process, and the state only does it every few years. The next census is planned for 2010.
To tally the Western Arctic herd, the state flew a deHavilland Beaver with an old U.S. Geological Survey mapping camera in its belly over the caribou in July, taking hundreds of photos.
The state estimates it caught 99 percent of the herd on camera.
Last fall, a veterinarian tested tissue and organ samples from the herd, giving the caribou high marks for overall health, according to Fish and Game.
The herd's death rate was low and calf survival rate was high in 2007, Dau said, but the Western Arctic Caribou Herd isn't the only herd in decline.
Others in Alaska and Western Canada are thinning as well.
"Taken collectively, these declines may merely be coincidence," Dau said in a written statement. "Alternatively, we may be entering a phase when conditions throughout North America are less favorable for caribou than during the past 30 years."
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