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JUNEAU, Alaska — The size of a large caribou herd in Alaska’s Arctic region has dropped by more 50 per cent 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.

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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 caribou killed by hunters account for 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.