In July, 2002, grizzly bears on the Firth River in Ivvavik National Park received a major boost to their summer diets. More than 20 caribou died while swimming across the river in high water, and an almost an equal number of bears quickly moved in to feed on the carcasses.

This natural event was notable for several reasons. While it is not unusual for bears to tear into the carcasses of unlucky caribou that have died while crossing the river, the usual victims are calves. In this case, 19 of the 22 drowned animals were bull caribou.

Also, during the caribou migration grizzlies are often seen along the banks of the Firth, looking for their next meal, but normally there are not 19 of them there at the same time. The bear concentration was high enough that for six days the park service did not allow any river traffic on the 45-kilometre stretch of river in which the bears were feeding.

Grizzly bears are known for their huge home ranges, which can be more than 60 square kilometers in this part of the North. So what sort of grizzly grapevine was at work here, getting out the news to so many bears that dinner was served on the Firth?

John Hechtel, an Alaskan bear biologist who worked for the Yukon government for several years, thinks they were probably just following their noses.

"I'm sure they just keyed into the smell," he says.

Hechtel has heard many anecdotal stories about polar bears and grizzlies following scent trails for as much as 25 to 30 kilometres, and he witnessed one incident which convinced him that bears have this ability.

On the North Slope, he watched a grizzly sow with two cubs suddenly leave the area where they has been feeding and start traveling in a straight line, continuing on for several miles. As Hechtel followed them, he could not figure out what they were up to until the bears finally arrived at the carcass of a dead cow caribou, whose live calf was still in the area.

"If I had not seen it, I would have assumed that the bear killed the cow caribou," he says, particularly since the caribou had died recently and the carcass was not particularly smelly.

Hechtel says there's another reason why so many bears would key in on an unexpected food source so quickly. "Bears are just such supreme opportunists; their whole strategy is to take advantage of anything like that."

"They travel around a lot and they are spread out, but their territories overlap so it is not that unusual to get denser concentrations. Socially it is not a problem because they will not exclude each other from an area the way that wolves do."

Not all of the bears were adults; in one spot the wardens spotted a sow with three cubs feeding about 40 metres away from another full-grown bear.

"But there were five carcasses close by, so the bears were probably more tolerant of each other," says Ron Larsen, the chief park warden for Ivvavik National Park.

Grant MacHutchon, a B.C. bear biologist, saw similar scenes along the Firth when he studied grizzlies there several years ago. "We saw places where there were a few caribou carcasses in the same area, and there would be four or five bears on them."

MacHutchon says that most of the females he saw had three cubs per litter, and he credits the caribou for the healthy state of the grizzly bear population in the park. "The caribou are a very important part of their yearly food budget," he says.

The Firth is well known as a river where water can rise rapidly because permafrost prevents rain and snowmelt from easily absorbing into the ground. But while high water is suspected to have caused the caribou to drown, the Ivvavik wardens are still speculating on why so many bull caribou died in the river.

Earlier in the migration, several wardens had watched a group of bull caribou make a particularly difficult crossing, and that episode raised the theory that the bulls' huge antlers might have caused their downfall.

"They saw the caribou crossing above the Sheep Slot rapid, and many of them ended up swimming through the rapid. When they were in the rapid, occasionally their antlers would get caught in the current, and then they would seem to lose all control," says Larsen.

Larsen does not think that all 22 caribou died while crossing at the same time as the carcasses were spread out over different sections of the river.

In any event, bad luck for the caribou spelled good luck for the bears, particularly since a bull caribou typically weighs well over 100 kilograms. Larsen says the bears seem to have taken full advantage of the sudden windfall.

When wardens patrolled the closed section of river shortly before reopening it, there were no bears to be seen along the banks of the Firth, but all of the caribou carcasses they saw were totally picked clean.