June 16, 2000

Grizzlies more dependent on cutthroat trout than believed, study shows

YELLOWSTONE NATIONAL PARK, Wyo. (AP) -- Yellowstone's cutthroat trout population may be a more important food source for grizzly bears than previously believed.

"We now have a clearer understanding of how much this grizzly bear population depends upon the trout as a food source," park Superintendent Michael Finley said Thursday. "Second, we have an additional reason to believe that protecting the trout population is an urgent priority."

Using recent advances in DNA technology, biologists determined that during a three-year study, a minimum of 84 different grizzlies visited streams around Yellowstone Lake during cutthroat trout spawning runs.

The new estimate is almost twice as high as previous estimates, which used bear track measurements.

Yellowstone's native cutthroat trout are threatened by the illegal introduction of lake trout, first discovered in 1994. Lake trout are efficient predators of cutthroats and without aggressive management could reduce the cutthroat population by as much as 90 percent, biologists say.

Lake trout spawn in deep water and would not serve as a replacement food for bears, biologists say.

"Dozens of species, from grizzly bears and eagles to small birds and mammals, will be affected if the cutthroat population declines and the annual spawning runs disappear," Finley said. "We don't yet know as much as we must to understand how a decline in the cutthroat trout population would affect the bear population, so this study is an important step in that understanding."

In 1997, with funding provided by Canon USA, the U.S. Geological Survey Biological Resources Division and the National Park Service, researchers arranged strands of barbed-wire near streams known to contain cutthroats.

Hair samples of bears snagged by the wires were analyzed, providing biologists with a unique DNA "fingerprint" of each bear and allowing them to arrive at a total number of individual bears.

Many black bear hair samples were also collected but have not yet been analyzed due to budget constraints.

Biologists estimate 15 percent to 20 percent of grizzly bears in the Greater Yellowstone Area eat cutthroat trout, one of the highest sources of digestible energy available to bears in the ecosystem.

Of the 124 Yellowstone Lake tributaries, at least 60 are known to contain cutthroat trout spawning runs. Researchers surveyed more than 45 percent of the streams known to be used by grizzlies.

Research suggests that lake trout are a probable cause for declines in spawning cutthroat trout in the West Thumb area of the Yellowstone Lake and biologists are concerned about potential for similar declines in the rest of the lake.

To suppress the lake trout population, the Park Service has been developing and refining a netting program and requiring anglers to keep any lake trout they catch.

The DNA fingerprinting technology was made possible due to an enzyme first discovered in one of Yellowstone's thermal pools in the 1960s.

"DNA fingerprinting has revolutionized the worlds of medicine, law enforcement, and many fields of research," Finley said. "It is both ironic and fortuitous that we can use this same technology here in Yellowstone, where it originated, to help us better manage and understand the park's wild resources."