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Are Yellowstone Grizzlies Effected by Cutthroat Trout Decline?

Posted by struscott | January 22, 2008

Ever wonder what grizzly bears in Yellowstone National Park really eat? If you ask Jennifer Fortin, a zoology Ph.D. student at Washington State University, she can tell you.

Fortin is conducting a three-year dietary analysis in the park focusing on the impact of the population decline of cutthroat trout on the grizzly diet. The study is currently in its first year.

Historically, cutthroat trout have been an important food resource for grizzly bears; however, with the illegal introduction of lake trout in the late 1980’s the cutthroat trout population has been decimated, according to Fortin.

“There is one stream that feeds Yellowstone Lake and in the past it had over 70,000 spawning cutthroat trout – now there are only 500,” said Fortin.

Twenty-two species feed on cutthroat trout, including bears for which the fish used to be a major food resource. While cutthroat trout spawn in streams and are available for bears to eat, lake trout spawn in the lake and are not available for bears and other species to eat. The result is a serious impact on the species that depend on fish as a food resource, said Fortin.

Fortin uses naturally occurring mercury in the cutthroat trout to monitor the bears’ consumption of the fish. Mercury is present because of natural thermal features in Yellowstone Lake, said Fortin.

Because bears excrete mercury efficiently through their hair, Fortin sets hair snares around the stream and lake from May to mid-August. This allows for a population estimate, in addition to determining cutthroat trout consumption without having to physically watch the bears eat the fish, said Fortin.

The knowledge that bears excrete mercury through their hair was established in a previous study done at the WSU Bear Research Center.

“They were fed fish from Yellowstone Lake, which were provided by Yellowstone National Park, to monitor mercury levels in their hair and correlate that with the bears’ cutthroat trout intake,” said Fortin.

Fortin also studies the whole diet of grizzlies and black bears by using stable isotopes and hair analysis to determine the different types of plants and animals that were consumed in the bears’ diet, in addition to randomly selecting a 24-hour set of locations provided by a grizzly wearing a global positioning satellite collar.

“We downloaded the bear’s locations and then visited the sites to see what the bear was eating in that area,” Fortin said.

Fortin is also studying another important at-risk food resource. The Whitebark pine tree is being affected by blister rust and pine beetle. By visiting the sites where bears feed, Fortin gains a better understanding of the effect that diseased Whitebark pines are having on the bears’ diet. Another aspect of the study is a complete habitat and dietary comparison between grizzly bears and black bears.

“We won’t know what the impact of the food resource decline is until we’ve completed all three seasons,” said Fortin. “So I’ve got two more summers to work in Yellowstone.”

Fortin’s supervisor Charlie Robbins, a professor and head of the WSU Bear Center, appreciates her ability to organize a complex project and oversee a large field crew while keeping her supervisors well informed.

All of the information gathered through studies at the WSU Bear Center and Fortin’s field study in Yellowstone National Park is aimed at helping with management decisions for the grizzly bear population in Yellowstone.

Fortin has also done grizzly bear research in Alaska. While in Alaska, Fortin walked streams filled with spawning salmon, counting as she went, even though she knew there were both black bears and grizzly bears in the immediate area, said Robbins.

“She’s fearless,” Robbins said. “I’ve seen males twice her size hesitate to do the things she does.”

Yellowstone National Park is one of only two current ecosystems in the lower 48 states with a healthy population of grizzly bears.

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