Fewer cutthroat trout in Yellowstone Lake could be part of the reason that elk herds migrating out of the park are declining.
Two recently published studies conducted in Yellowstone National Park point to the connectivity between the decline of spawning cutthroat trout from Yellowstone Lake and a resulting shift in grizzly bear diets to elk calves.
“We were surprised to see this connection,” said Arthur Middleton, lead author of a study published in the Proceedings of the Royal Society B, in a telephone interview. The study was released Tuesday. Middleton wrote the article based on his doctoral dissertation at the University of Wyoming. He is now on a two-year postdoctoral fellowship at Yale University.
The shift in diet has reduced elk calf recruitment by 4 to 16 percent, Middleton postulated, and has hindered the population growth of migratory elk herds that use the Yellowstone Lake area by 2 to 11 percent.
“It might be worth adding that we don’t think this is the answer to where the elk calves went, but it’s something we should think about with changes in bear diets and the change in elk calf numbers,” he said.
The studies' conclusions are no surprise to David Mattson, a U.S. Geological Survey wildlife biologist who spent 14 years studying grizzly bear foraging and diets in the Greater Yellowstone Ecosystem.
"I think it highlights what should be the obvious for any observer of Yellowstone, that it's a complex system," he said. "And grizzly bears are kind of the consummate connector of all of the species in that system."
Mattson said during his years studying bears in the mid-1980s and mid-1990s, it was a time of an enriched ecosystem with large herds of elk, bison and plentiful cutthroat trout.
"All of that's reversed," he said. "We're watching probably the first state of impoverishment in the ecosystem as far as grizzly bears are concerned."
Middleton was helped in linking the aquatic and terrestrial food webs by a study authored by Jennifer Fortin of Washington State University that detailed the diets of 27 GPS-collared bears in Yellowstone. Fortin’s study was published in February in the Journal of Wildlife Management. She worked with fellow WSU student Charles Robbins tracking the bears and analyzing their scat.
Her study showed a 70 to 95 percent decline in trout consumed by grizzly and black bears between 1997 and 2009. The study also found that grizzly bears killed an elk calf every two to four days in June while black bears killed an elk calf every four to eight days.
“They’re the ones who did the really detailed studies following grizzly bears around and studying what they ate to drill into bear diets,” Middleton said. “Without their work we wouldn’t have arrived where we did.”
By the numbers
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Possibly only in Yellowstone could such a connection be established by data. That’s because there’s a rich source of current and past studies on everything from the Yellowstone cutthroat trout’s precipitous decline by 90 percent since predacious lake trout were illegally introduced, to cow elk pregnancy rates and cow-to-calf ratios for migratory elk herds in the park.
“I was really synthesizing these studies,” Middleton said. “An important part of what I did with the co-author (Thomas Morrison) was to pull together studies on bear diets and elk populations and as we did we saw evidence for links.”
Looking at the numbers gives credence to what seems only logical: once cutthroat trout numbers declined, bears that were eating those fish had to supplement their diet with some other food source. Elk calves just happened to be available.
To understand the effect of the trout decline on fish-eating bears, consider this: Clear Creek, one of 124 tributaries to Yellowstone Lake used for spawning, saw a migration of more than 54,000 cutthroat trout in 1988. By 2007, that number had plummeted to just over 500 fish. Although nonnative lake trout have been blamed for most of the decline, cutthroat also suffered through drought, which dewatered some of the spawning streams, and infection by whirling disease.
Like cutthroat trout, Yellowstone’s northern range elk herd has also seen a significant population decline. In 1988, the elk population hit 19,000. This winter the herd was estimated at just over 3,900 animals. The elk have also suffered from drought, increasing infection from the disease brucellosis, hunting pressure, the reintroduction of wolves that mainly eat elk and an increase in the number of other predators, including bears and cougars. Although 80 percent of the northern range’s cow elk are getting pregnant, according to aerial surveys conducted in July and August near Yellowstone Lake, only 10 percent of the cows still had calves.
One study cited by Middleton estimated that 68 grizzly bears – roughly 14 to 21 percent of the Greater Yellowstone Ecosystem’s population – relied on cutthroat trout as part of their spring diet. The fish spawn between mid-May and early August.
It was estimated that grizzly bears that dined on cutthroat trout ate more than 7,800 fish during the spawning season – more than 12,400 pounds. With the fish’s decline, that amount dropped to about 300 fish a year, or about 690 pounds. With elk calves weighing an estimated 39 pounds, grizzly bears would have to eat almost 300 elk calves to supplement the loss of those 7,500 cutthroat trout, Middleton estimated.
Another study estimated the elk calf predation by bears rose from 12 percent in the late 1980s to 41 percent by the mid-2000s.
Middleton’s paper notes that because grizzly bears are only one of 28 species that are believed to have depended on spawning cutthroat trout, “the broader ecological consequences of lake trout invasion are potentially tremendous.”
“It’s worthwhile to emphasize the uncertainty of what the size of the effect might be,” Middleton said.
The paper argues that the findings are relevant to wolf management plans for the states that surround Yellowstone National Park – Montana, Wyoming and Idaho. The migratory nature of elk from Yellowstone into the surrounding states may indicate that “elk calf recruitment may not be as sensitive to wolf removal on some outlying winter ranges as to the number of grizzly bears and the availability of alternative grizzly bear foods on elk summer ranges in and around YNP.”
Middleton said his study is a “surprising connection and one that goes back to some fisherman’s decision that it was important” to illegally introduce lake trout into Yellowstone Lake. If lake trout can be suppressed through the park’s efforts to net adults and kill the eggs they have laid – an effort that began in 1998 -- perhaps the cutthroat trout can rebound and once again benefit the other animals of the ecosystem, including elk, Middleton said.
The other lesson, he noted, is that “perhaps we shouldn’t be so quick to compartmentalize fish and wildlife management. They may seem separate, but they are more closely linked than we thought. Yellowstone National Park has long known that, but it’s been slow to enter the popular realm.”