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Grizzlies may be link between drops in cutthroat trout and elk calves

Grizzlies may be link between drops in cutthroat trout and elk calves

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Elk predation

A grizzly bear chases an elk calf in Yellowstone National Park near Mammoth in 2009. New research shows that prey-switching by grizzlies due to the loss of cutthroat trout is ‘capable of creating meaningful changes in the population dynamics of migratory elk’ in the greater Yellowstone area.

A drop in migratory elk calf numbers in and around Yellowstone National Park may be linked to a loss of cutthroat trout in Yellowstone Lake, according to research published Tuesday.

Grizzly bears may be the link.

As cutthroat trout numbers declined, some grizzly bears began switching from eating trout to preying more on elk calves, said researcher Arthur Middleton.

Middleton was the lead author on a paper called “Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone,” which was published in a science journal called Proceedings of the Royal Society B.

“We perhaps shouldn’t be so quick to departmentalize fisheries management and wildlife management because even though they seem like separate parts of an ecosystem they can turn out to be more closely linked that we thought,” Middleton said.

Drought still plays a role in declining elk calf production by reducing pregnancy rates, and increasing numbers of bears and wolves are also a factor. However, research suggests some bears are eating more elk calves while elk are still on their summer range near Yellowstone Lake, said Middleton, who worked with nearly a dozen other biologists on the paper.

Middleton used several research papers about grizzly bears and cutthroat trout as well as grizzly bears and elk calf predation to draw his conclusions. He worked on the paper while finishing his doctorate at the University of Wyoming and is now with the Yale School of Forestry and Environmental Studies.

Elk calf ratios in migratory herds around the park have decreased in varying degrees since the mid-1990s, and have been low since 2002, said Doug McWhirter, Cody district wildlife biologist for the Wyoming Game and Fish Department.

“This is one piece that could explain why the increase in elk predation is occurring,” McWhirter said. He was also a co-author on the paper and worked with Middleton on elk monitoring.

The increased predation has not decimated elk herds around Cody. All are still at or above population objectives. However, wildlife officials have limited hunting. Licenses dropped nearly 75 percent from 2002 to 2012 for elk in the Sunlight Basin and Crandall areas, he said.

The loss of cutthroat trout is not hurting grizzly bear populations, said Mark Bruscino, large-carnivore section supervisor for Game and Fish. Trends show increasing numbers and densities of bears in the greater Yellowstone ecosystem. Grizzly bears are opportunistic, which means if cutthroat trout aren’t around, they’ll eat something else.

In some cases, that other food may be elk calves, Middleton said.

Grizzly bears historically fed on some of the millions of cutthroat trout that moved up tributaries of Yellowstone Lake to spawn.

Problems started in the early 1990s when lake trout were discovered in Yellowstone Lake. An aggressive, nonnative fish, lake trout eat cutthroat trout but spawn deep in the lake bottom, which means they are rarely accessible to predators such as grizzly bears. Cutthroat trout numbers have declined up to 90 percent since the introduction of lake trout, Middleton said.

Yellowstone National Park officials have been fighting the lake trout invasion. They removed more than 300,000 lake trout in 2012, levels high enough to start suppressing the lake trout population, according to a release from Yellowstone National Park.

If cutthroat populations return close to their historic levels, it is possible some bears could begin eating trout again, Middleton said.

Reach Open Spaces reporter Christine Peterson at 307-746-3121 or Follow her on Twitter @PetersonOutside.



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