FAIRBANKS - State wildlife biologists don't know why the populations of two of the state's largest caribou herds have jumped significantly in the past seven years while numbers in larger surrounding herds have declined, but nobody is complaining, especially the people who eat the caribou.
The Central Arctic Caribou Herd has more than doubled from 32,000 to 67,000 animals since 2002, becoming the state's third-largest herd. The Teshekpuk Caribou Herd has similarly increased from 45,000 to 64,000 animals during the same time and now ranks as the state's fourth-largest herd, based on photo censuses completed last summer.
At the same time, the Western Arctic and Porcupine herds, the state's largest two caribou herds that range on either side of the Teshekpuk and Central Arctic herds, appear to be decreasing, according to a press release issued by the Department of Fish and Game on Monday.
"It's very curious," ADF&G information officer Cathie Harms said.
But it's a good thing, Harms said. The two herds account for nearly one-quarter of Alaska's annual caribou harvest, which last year was about 20,000 animals.
"People that use those herds are pleased," Harms said.
The Teshekpuk herd, which roams the western North Slope, is the state's second most-hunted caribou herd behind the Western Arctic herd, the state's largest at 375,000 animals. The herd provides meat for subsistence hunters in several North Slope villages such as Barrow, Nuiqsut, Wainwright, Anaktuvuk Pass and Atqasuk. They kill about 4,000 Teshekpuk caribou a year, according to state harvest statistics.
The Central Arctic herd roams between the Brooks Range and Beaufort Sea and attracts resident and non-resident sport hunters - many of them bowhunters - who hunt off the Dalton Highway. Last year, about 1,500 hunters killed approximately 900 Central Arctic caribou, biologist Beth Lenart said.
As to why those two herds are faring better than other arctic herds is a mystery biologists are trying to solve, Harms said.
"We don't have an answer yet," she said.
Caribou herds are famous for their fluctuating populations, and the Central Arctic herd is a prime example. It numbered only about 5,000 animals when it was identified as a separate herd in the early 1970s. It increased to about 24,000 in the 1980s before starting another decline. It began increasing again in the 1990s and has been growing ever since, Lenart said.
Judging by the high pregnancy rates they had documented combined with high calf survival and a relatively low harvest rate, biologists figured the Central Arctic herd was growing but Lenart was surprised by how much.
"We expected them to increase but we didn't guess it would be that high," she said of the 13 percent annual growth rate.
Both herds will likely continue to grow "as long as the habitat is there," Harms said.
"Caribou herds tend to grow and decline," she noted. "Smaller herds tend to be governed by predation, and larger herds tend to be governed by habitat."
Food availability drives caribou populations in herds the size of the Teshekpuk and Central Arctic, Lenart said. Pregnancy rates go up when caribou are healthy, resulting in bigger calf crops.
"I think range use is what effects these big cycles," she said.
The Central Arctic herd has been expanding its winter range bit by bit in recent years and the animals appear to have plenty to eat, Lenart said.
"They're healthy, fat, good-conditioned caribou," she said. "They're doing really, really well. You look at them, and they look fat and healthy."
Based on radio-tracking data, biologists are confident the increases are not a result of caribou from neighboring herds joining the Teshekpuk and Central Arctic herds.
No radio-collared animals from neighboring herds were found within the Teshekpuk herd during the photocensus and only four collared caribou - two from the Teshekpuk herd and two from the Porcupine herd - were found in the Central Arctic herd.
"Even if each collared animal represented a thousand caribou, it wouldn't be enough to explain the increase in Central Arctic herd size," she said.
Biologists are confident their counts are accurate. In a photo census, biologists radio-track collared caribou to locate groups of animals, which are then photographed by a large-format camera in the belly of another plane. The photos are examined under magnifying glasses and individual caribou are counted. Ideally, biologist do a photo census every two or three years, Lenart said. However, conditions haven't allowed a count of the Teshekpuk or Central Arctic herds since 2002.