More grizzlies making meals of cutthroat trout

More grizzly bears in Yellowstone National Park are feasting on cutthroat trout, posing a challenge to wildlife managers

June 20, 2000
Web posted at: 11:55 a.m. EDT (1555 GMT)

By Environmental News Network staff

Wildlife managers have another reason to protect Yellowstone National Park's dwindling cutthroat trout population: More endangered grizzly bears are feasting on Yellowstone trout than previously believed, according to a recent study.

Using DNA technology, biologists from the National Park Service and the Interagency Grizzly Bear Study Team determined that at least 84 grizzly bears visited streams in Yellowstone during spawning runs of cutthroat trout.

Previous estimates put the dinner count for cutthroat at 35 to 48 grizzlies.

"This is an important finding for several reasons," said Yellowstone superintendent Michael Finley. "First, we now have a clearer understanding of how much this grizzly bear population depends upon the trout as a food source. Second, we have an additional reason to believe that protecting the trout population is an urgent priority. And third, we have a terrific example of how one Yellowstone research project can benefit others."

To determine how many grizzly bears are feeding on trout, the biologists prepared barbed wire corrals to snag a small amount of the bear's hair. DNA analysis of the hair samples provided the biologists with a fingerprint of each grizzly that visited more than 45 percent of the known streams used by the bears.

Grizzly bears depend on cutthroat trout as a major food source

Introduced by anglers to Yellowstone in 1994, lake trout pose the greatest threat to cutthroat trout. Lake trout are efficient predators of cutthroats and, in absence of aggressive management, can reduce native cutthroat populations by as much as 90 percent, according to the National Park Service. Whirling disease, recently discovered in Yellowstone Lake, has also contributed to the loss of cutthroat trout.

Because lake trout spawn in deep waters, they are less accessible to bears than cutthroats and do not serve as a replacement food.

"Dozens of species, from grizzly bears to eagles, will be effected if the cutthroat trout population declines and these annual spawning runs disappear," said Finley.

Cutthroat trout are one of the highest sources of energy available to bears in the Greater Yellowstone Ecosystem. Bears must eat enough food each fall to take them through hibernation, and if females don't receive enough fat in their caloric race against winter, their fertility diminishes.

Cutthroat trout are one of four essential food sources for grizzlies.

Other major sources of nutrition, including the whitebark pine seed, army cutworm moths and bison, are also facing dramatic declines, said Louisa Willcox, the project coordinator for the Sierra Club's Grizzly Bear Ecosystems Project.

When food shortages occur, bears do not usually starve. In search of alternative foods, they comb new territory, Willcox said. This behavior usually brings the bears in closer proximity to people, resulting in increased conflicts.

Increased interaction with humans translates to increased mortality for grizzlies. In Yellowstone, more human-caused grizzly bear mortalities occur in years of food shortage than in years when bear foods are abundant.

With fewer than 400 grizzlies remaining in Yellowstone, according to estimates, a modest increase in deaths could have a serious impact on the bear population.

The current Fish and Wildlife Service Grizzly Bear Recovery Plan does not account for the loss of cutthroat trout or any other key food sources for grizzlies.

"This study underscores the importance of giving greater consideration to cutthroat trout in the recovery plan," said Willcox. "All of this argues for a bigger recovery area which provides greater flexibility for the bears."

Copyright 2000, Environmental News Network, All Rights Reserved

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National Park Service
Interagency Grizzly Bear Study Team
Grizzly Bear Ecosystems Project
U.S. Fish and Wildlife Service
   •Endangered Species Act
   •grizzly bear recovery
Grizzly Bear Page

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