ANCHORAGE - Alaska's largest caribou herd fell by 20 percent between 2003 and 2007, according the latest count by the state Department of Fish and Game.
The Western Arctic Caribou Herd declined by 113,000 animals after years of steady growth.
The reasons are not clear, said Jim Dau, the lead state biologist on the herd since 1988, but warm spells in the middle of recent winters may have played a role.
The herd ranges from the North Slope to Eastern Norton Sound and from the Chukchi Sea to the Koyukuk River. It remains twice the size of any other caribou herd in Alaska.
The herd is important for subsistence hunters in dozens of villages. It's also a major moneymaker for businesses that cater to sport hunters and a key link in the area's food chain, Dau said.
The population of the herd fell 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.
The decline could be a natural occurrence due to disease, predators and a shortage of food, Dau said.
"I'm not absolutely sure that that's what happened here," he said.
The herd suffered at least one tough winter since the previous census.
In 2005, just before Christmas, temperatures grew unseasonably warm for four days and it rained for two of those days, Dau said.
After the rain came freezing temperatures, which covered the ground in a hard crust of snow, and caribou struggled to find food, he said.
In 2007, there was another warm winter spell but extended warm temperatures and high wind swept away the snow and the herd thrived.
Climatologists predict midwinter thaws will become more common, Dau said.