From osprey and river otters to maybe even elk, the long-running lake trout invasion of Yellowstone Lake is having far-reaching and surprising impacts on the ecosystem below and above the lake's surface.

While a 40-pound lake trout can offer a big, juicy meal, they typically swim too deeply in the 136-square-mile lake for bears and other predators to reach. That leaves them free to feast on the much smaller Yellowstone cutthroat trout, the prized species native to the lake.

Yellowstone National Park biologists say an adult lake trout "can eat up to 41 cutthroat trout per year and can consume cutthroat trout up to 55 percent their own size." At the same time, a 12-pound female lake trout can produce roughly 9,000 eggs per year, they add.

Exactly how lake trout found their way to Yellowstone Lake isn't known, though it's been suspected that an angler hoping to boost the lake's catch dropped some in back in the 1980s and 1990s, with the first confirmed catch of one in 1994.

Since then the impact to the lake's cutthroat trout has been significant, as bears that used to pull spawning cutthroats from the lake's tributaries have had to look elsewhere, as have osprey, which can't normally dive deep enough to snare a lake trout. The lake waters have become more clear because the downfall in cutthroat trout numbers led to a boost in zooplankton. The fallout of that, according to researchers, has been a slight increase in water temperature.

"I’d say of all these cutthroat trout consumers, the osprey are definitely the ones that took the hardest hit in the Yellowstone Lake area," Todd Koel, a fisheries biologist at Yellowstone, said during a conversation earlier this week. "Once the cutthroat declined in the 2000s, the osprey declined in the lake area dramatically with them. So before the cutthroat decline, before the lake trout invasion, we would have an average of 38 osprey nests every year, from 1987-1991. Now, the current condition is we have about three. So greater than a tenfold loss of osprey nesting.

"And the nesting success crashed, as well. So not only did the (sheer) numbers, but then their ability to produce young greatly declined as well. They’re not fledging chicks."

The spiraling impacts of lake trout were the subject of a paper that appeared this week in Science Advances. It noted that organisms affected by the big fish range from the microscopic level in the lake to large animals in the region.

How can a fish impact a terrestrial animal, or a bird that flies overhead? The decimation of the cutthroat fishery that the lake's osprey population long relied on is one example. How grizzlies reacted to the drop in cutthroats also stands out.

"There’s strong evidence that suggests that the prey switching that bears exhibited in the absence of cutthroat, they’re going to eat something, so they switch," explained Koel, one of the paper's authors. A graduate student at the University of Wyoming back in 2013 suggested in a paper that the prey grizzly bears switched to following the great decline of cutthroat spawning was elk calves, the biologist said.

According to Koel and his fellow authors, "By 2007-2009, grizzly bears had shifted to alternative prey, and the proportion of cutthroat trout in their diet had declined to zero. Elk then accounted for 84 percent of all ungulates consumed by bears in the Yellowstone Lake area, suggesting lake trout had some level of indirect, negative impact on migratory elk using this area when spawning cutthroat trout were rare."

Lake trout spawning in Shoshone Lake, Yellowstone National Park/NPS, Jay Fleming

During our conversation, Koel pointed out that, "Everything’s tied together. But then, to what extent does that affect those populations on a bigger scale? That’s not known. So (the hypothesis that) lake trout come in and indirectly affect something like elk on the landscape, that's pretty interesting.”

Another interesting, though disconcerting observation, is that bald eagles, as with the grizzlies, have looked elsewhere for their meals.

"So in the absence of cutthroat trout, what’s been observed is that they’re feeding more upon winter kill and carcasses left by wolves and other animals, bears, in the Yellowstone Lake area," the biologist said. "Unfortunately, we’ve also seen bald eagles more often preying upon a few other animals that are also relatively rare in the park, like trumpeter swan cygnets, and common loons, as well. Which is a result of the cutthroat decline.

"So you get these shifts that impact other species, which are also important in the park.”

There are two concerning examples that reflect how great the lake trout invasion has been the past quarter-century:

* Clear Creek, a tributary on the eastern side of the lake, saw upwards of 70,000 cutthroat trout heading into the stream to spawn in 1980, pointed out Koel. The number has dropped to hundreds, maybe. In their paper, the researchers noted that the "estimated number of spawning cutthroats consumed annually by grizzlies declined from 20,910 in the late 1980s to 2,266 in the late 1990s to only 302 in the late 2000s."

* In 2017 a lake trout netting operation, which began in 1994, snagged roughly 400,000 of the fish in 2017, the biologist said. On average, from 2012 to 2017, the netting operations captured nearly 309,000 pounds of lake trout. Once removed from the nets, the fish have their air bladders popped so they'll sink deep into the lake.

But there is a glimmer or two of good news. In 2018, the netting operation captured 297,000 lake trout, a reduction of roughly 25 percent from the year before, he said.

"The statistical modeling that we’ve been doing has been saying the lake trout population’s in decline, but on the lake we’re seeing all these fish every year," Koel said. "This last year was the first time all the crews were saying they’re hard to find. And this is despite even greater effort in netting last year than the year before. So it’s not a consequence of less effort or crews that weren’t experienced."

Additionally, more cutthroats have been seen spawning in tributaries on the west side of Yellowstone Lake. Bears also are returning to the streams for meals, he said, and juvenile cutthroats are becoming more and more visible in the lake.

"We’re seeing more small cutthroat trout in the sampling of the lake that we’re doing and in the catches by anglers. We’re seeing small cutthroat trout coming back into the population, where for quite a while they were almost totally absent," Koel said. "We had all these big cutthroat swimming around out there, but no small ones to replace them. Now we’re seeing that again. So with the lake trout predation suppressed, reduced, that allows that rebound of the cutthroat population to come back.”

With continued success in lake trout netting, it's hoped the cutthroat population will rebound significantly, something that could not only bring osprey back to the lake's shores for nesting and roosting, but also benefit river otters that have suffered from diminished cutthroat meals.

Surveys show about one river otter for every eight miles of the lake's 110 miles of shoreline. “That’s among the lowest (population density) reported in the literature for river otters anywhere," Koel pointed out, though he noted that there was no baseline from before the lake trout invasion to measure against.

What must be kept in mind is that the loss of ospreys from Yellowstone Lake, and the move from cutthroat to trout for grizzly meals, doesn't reflect a decline in those two species, said the biologist. Just a shift in their patterns.

"There’s uncertainty about what’s going to be the end result of all this," Koel said just the same. "We are seeing responses in bears to the spawning cutthroat trout. That’s probably, ecologically, the one indicator that we’re seeing, along with the cutthroat recovery.

"But within the lake itself, the trophic cascade that occurred within the lake when the cuttnroat declined, then the plankton community shifted as a result of that, and we have yet to see a shift back in the plankton community related to a cutthroat increase," he added. "We have yet to see ospreys respond to cutthroats returning. We have a lot of work to do.”

Park visitors can help with that work, too. Chefs at Lake Hotel and Lake Lodge will cook lake trout caught, cleaned, and kept on ice by angler for $14.95. Those heading to Lake Hotel need to drop their catch off by 3:30 p.m., or one-hour before their dinner reservation at Lake Lodge.