A paper published Tuesday says invasive cutthroat-eating lake trout in Yellowstone Lake have caused grizzly bears to shift their diet, resulting in a decline in Yellowstone National Park’s elk calves.

Cutthroat trout that spawned during spring in shallow tributaries of the lake used to make up a substantial portion of grizzlies’ diets, researchers said. Sixty-eight grizzly bears gobbled up 12,469 pounds of spawning trout every year when cutthroat were abundant, researchers estimated.

But with the proliferation of lake trout, believed to be illegally stocked in Yellowstone Lake in the late 1980s, cutthroat spawning declined drastically, the authors said. By 2009 grizzlies were eating as little as 692 pounds of cutthroat annually, the paper said.

Grizzlies couldn’t turn to lake trout as an alternative because those fish live only in deep water. Instead, “the resulting loss of trout biomass would be replaced with approximately 297 elk calves,” principal author Arthur D. Middleton and co-authors estimated.

The implications are significant for the control of lake trout in Yellowstone Lake, where park contractors netted and killed more than 300,000 of the fish last fall, Middleton and co-authors said. Yellowstone has budgeted $2.3 million in 2013 for cutthroat rehabilitation and killing lake trout, also known as mackinaw, in the 139-square-mile lake.

The paper discounts the popular notion that wolves are largely responsible for declines in calves among Yellowstone’s migrating elk herds. Researchers also disputed proclamations that the restoration of predators “heralds a return to a historical condition of the [Greater Yellowstone Ecosystem,] providing evidence of conservation successes.”

“[O]ur work suggests that important effects of human disturbance and grizzly bear predation on migratory elk are being overlooked,” the paper said.

The study looks at four elk populations that roam the country occupied by grizzlies that once fed on cutthroat. Researchers put the Northern Yellowstone, Clarks Fork, Cody and Jackson herds in that grizzly range.

Grizzlies seeking new food

“This synthesis suggests that even in a core wilderness area like Yellowstone, a fisherman’s blunder in the aquatic system many years ago can have far-reaching effects by forcing an omnivorous predator to seek new foods in the terrestrial landscape,” Middleton said in a statement. “These surprisingly broad ecological consequences underscore the importance of identifying new methods to suppress lake trout and the value of preventing such invasions elsewhere.”

His paper, “Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone,” was published in Proceedings of The Royal Society B. A recent University of Wyoming graduate, Middleton now is a post-doctoral fellow at the Yale School of Forestry and Environmental Studies.

Middleton and his fellow authors considered the effects of drought and whirling disease on cutthroat trout, as well as mackinaw predation. The disease- and beetle-caused decline of another grizzly food, whitebark pine nuts, also is part of the equation, as is the scavenging of winter-killed ungulates by wolves, the authors wrote.

Researchers also considered the growing number of bears in the ecosystem. That growth has occurred outside the Yellowstone Lake area of interest, they said.

Elk calf mortality up

Fringe bears also had an effect on elk calf declines, authors agreed. Consequently, calf-eating grizzlies around Yellowstone Lake and beyond act together to put a pinch on the calves of migrating Yellowstone elk herds.

“Although we cannot rule out effects of these latter changes, we expect that their consequences have not been as dramatic as the loss of a diet item (i.e., cutthroat trout) that coincides both spatially and seasonally with the calving of many migratory elk,” the paper said.

Researchers say cutthroat spawning in shallow Yellowstone Lake tributaries declined by 90 percent as a result of invasive lake trout. Cutthroat used to make up more than three times the “dietary proportion” of grizzly food compared to ungulates, including elk, the study said.

After the cutthroat decline, trout virtually disappeared from the mix, according to the paper. In their place, grizzlies turned to elk calves from herds that summer in Yellowstone.

Elk calf mortality increased after the crash of cutthroat trout, researchers said.

“[G]rizzly bears far outpace wolves and other predators as a cause of summer elk calf mortality, and reductions in [elk] pregnancy do not appear large enough to explain the decreases in summer calf-cow ratios that have recently been observed,” the paper said.

Meanwhile, the number of elk calves consumed by grizzlies in the Yellowstone Lake spawning area increased from a median of slightly more than three a year per bear before cutthroat decline to about seven today, the paper said.

Using another method of estimating the changes, including historic studies and radio telemetry, authors reckoned “68 individuals in the Yellowstone Lake watershed would have killed 245 calves annually” before cutthroat declined.

“In the past decade in the Yellowstone Lake watershed, the same number of grizzly bears are estimated to kill 476 calves annually for an estimated increase of 231 calves,” researchers said.

The second method of estimating calf losses results in a conclusion that “broadly agrees” with calculations researchers made based on how many elk calves grizzlies would have to eat to replace lost cutthroat, the paper said.

Middleton was a student of Matt Kaufman, head of the Wyoming Cooperative Fish and Wildlife Research Unit and assistant professor in the Department of Zoology and Physiology at University of Wyoming.

Kaufman co-authored the report along with a team of researchers from Washington State University, Iowa State University, Yellowstone National Park, the Wyoming Game and Fish Department, and Montana Fish, Wildlife and Parks.