JUNEAU, Alaska — The size of a large caribou herd in Alaska’s Arctic region has dropped by more than 50 percent over the last three years, and researchers who have tentatively ruled out hunting and predation as significant factors for the decline are trying to determine why.

The state’s Central Arctic herd, which roams an area of north-central Alaska about the size of Ohio, hit a peak of about 70,000 caribou in 2010.

It fell to 50,000 in 2013. That year, spring arrived late, meaning caribou had to trudge through snow later than usual at a time when their bodies are already stressed and not getting the grasses they need for nutrition.

Surveys by the Alaska Department of Fish and Game suggest the herd dwindled to about 22,000 caribou this year. There has been a higher than normal rate of death among adult female caribou tracked with radio collars but the reason for that is unclear, said state wildlife biologist Beth Lenart.

The department does not believe hunting is a factor in the decline, saying that caribou killed by hunters account for only a small portion of overall deaths. The department doesn’t think predation by wolves and bears plays a large role in regulating Arctic caribou herds, and biologists so far have not detected diseases affecting the herd, Lenart said.

Pregnancy rates for female caribou are down slightly since 2013, but not alarmingly so, she said. Fewer adult males are tracked by tracking collars, making it more difficult to spot trends in their numbers. Researchers conclude that caribou have likely died when the sensors on their collars indicate the animals have not moved for at least 12 hours.

Associated Press