Caribou cross a river in this undated handout photo. Six of the eight western Arctic caribou herds appear to be in serious, long-term decline. (Canadian Press)

A new study of one of Canada's largest caribou herds seems to confirm fears that, like other such herds in the western Arctic, its population is undergoing a steep and mysterious decline.

The massive Beverly herd, which roams the tundra from northern Manitoba and Saskatchewan and well into the Northwest Territories and Nunavut, once numbered about 276,000. But a just-released survey suggests the number of caribou cows on the calving grounds of the massive Beverly herd have fallen by 98 per cent over the past 14 years.

Aerial spotting teams found only 93 cows as they flew over the calving grounds on the tundra west of Baker Lake, Nunavut, this summer.

In 1994, crews counted 5,737 cows in the same region. The numbers have been declining ever since: 2,639 in 2002 and 189 in 2007.

The herd's birth rate is now less than one-fifth its traditional levels. Spotters counted only 15 calves for every 100 cows instead of the usual 80.

"It's a very, very sharp downward trend," said Ross Thompson of the Beverly and Qamanirjuaq Caribou Management Board, which oversees the herd. "It is a concern that the herd has shown that few numbers."

The survey wasn't intended to provide a population count, but does provide an index of herd health. The Beverly herd now appears to have almost certainly joined five of the eight main western Arctic eight herds that appear to be in serious, long-term decline.

The Bathurst herd, 472,000 strong in 1986, has lost about three-quarters of its strength since then. Since 1989, the 178,000-animal strong Porcupine herd has fallen off by 30 to 40 per cent.

The smaller Cape Bathurst, Bluenose East and Bluenose West herds have all fallen by at least a third. Studies are now being completed on the Qamanirjuaq and Ahiak herds.

Nobody has a good explanation, said Thompson.

"There isn't any one factor that anybody can identify," he said. "Something has happened over the years."

Some suggest climate change is playing a role, altering the delicate timing of spring thaw and calving. Others point to increasing industrial activity on the tundra, noting the Beverly and Qamanirjuaq's calving grounds are home to nearly 1,000 mineral leases. And some say modern hunting methods are crimping the ability of the herds to recover.

The N.W.T. conducted surveys on herds adjacent to the Beverly herd and found no evidence that large numbers of animals were simply shifting from one herd to the next, said Thompson.

Monte Hummel of the World Wildlife Fund, who has been closely involved with caribou research, agrees that a variety of factors are likely at play. But he says the timing of the declines is suggestive.

"It's a strange co-incidence to me that all this industrial activity happened at the same time as this caribou decline." Although about half of the Beverly's calving grounds are located inside the Thelon Wildlife Sanctuary, the half most heavily used falls outside the preserve.

Hummel says governments should adopt a "precautionary principle" and not add any new stressors to the herds in the way of new development until the declines are better understood.

Nunavut environment minister Dan Shewchuk, a former member of the Beverly caribou management board, was not available for comment Sunday.

Caribou herds are vital to both the diet and culture of thousands of Inuit, Dene and Métis across the North. Estimates put the value of the Beverly herd alone at $20 million a year to the people who depend on it.

The apparent collapse of the western caribou herds led to a "caribou summit" in Inuvik in January 2007. One of the main recommendations was the protection of calving grounds.

No new calving grounds have been set aside from industrial development since then.