Crews remove invasive lake trout from Lake Yellowstone. Photo: Trout Unlimited
By Matt Miller, senior science writer
Good news on the invasive species front: populations of invasive lake trout in Yellowstone Lake are declining, thanks to an intensive research and control effort.
The latest results of that effort were announced this week at a media event hosted by Trout Unlimited.
Conservationists often point to Lake Yellowstone – the largest lake in Yellowstone National Park – as a telling example of the pervasiveness of invasive species. Even in a well-protected and beloved national park, invasive species can’t be kept at bay.
Lake trout decimated the ecologically and economically vital Yellowstone cutthroat trout fishery. Huge numbers of lake trout swam in the large and deep lake, a seemingly insurmountable problem.
But now there are strong indications that trend is reversing.
When I was a kid, fishing the Yellowstone River was one of those outdoor experiences I put on my dream list. I had read the glowing articles in outdoor magazines, where anglers caught 50 or more fish a day.
And not just any fish: colorful native trout with the bright red slashes that give them their name.
By the time I first visited Yellowstone National Park in 2001, those 50-fish days were already gone. The Yellowstone cutthroat trout population had crashed.
As Yellowstone fly fishing guide Boots Allen said in yesterday’s media conference, “The cutthroat of Yellowstone were one of those things that you just couldn’t imagine disappearing. There were so many of them. You’d see dozens of fish rising in every riffle.”
Lake trout happened.
An Ecological Crash
Yellowstone cutthroat trout. Photo: Trout Unlimited
There are a lot of stories – some verging on myth – of how lake trout got into Lake Yellowstone. A common story is that an angler with a bucket dumped some into the lake to create new fishing opportunities, but no one can verify that story.
Whatever the case, the first lake trout was documented in 1994. Analysis of fish otoliths – the ear stones that can be aged like tree rings – indicated that lake trout had been in Yellowstone for at least a decade.
In Yellowstone, when lake trout reach four years, their diet is primarily cutthroat trout. An article in the journal Ecological Applications found that a mature lake trout could eat up to 40 cutthroat trout a year.
The decimation of cutthroats is not just about fishing. Far from it. At one point, there were 3.5 to 4 million cutthroats in Lake Yellowstone – likely the largest remaining population in the world.
Many of those fish traveled up rivers and small streams that connected to Lake Yellowstone to spawn – undertaking a migration similar to salmon.
Those spawning fish fed ospreys and bald eagles and river otters and grizzly bears. They were a key foundation of the freshwater ecosystem. (Lake trout don’t offer these benefits because they spawn in the lake itself, and don’t move into streams).
A river otter eats a spawning Yellowstone cutthroat trout in the national park. Photo: Matt Miller/TNC
There were genetic and behavioral differences within the cutthroat population. Some fish spawned in tiny streams. Some stayed in the lake to spawn.
The lake trout population explosion threatened to wipe out that irreplaceable diversity.
“This isn’t about one species of fish,” says Dave Hallac, chief of Yellowstone’s Center for Resources. “This is about an entire ecosystem. The loss of cutthroat trout has cascading impacts for the entire park.”
How bad was it? In one well-studied spawning water, Clear Creek, the population declined from 70,000 cutthroat trout in 1978 to less than 500 fish in 2008.
“The Yellowstone native cutthroats are as integral to Yellowstone’s larger ecosystem as bison and grizzlies,” said Jack Williams, senior scientist for Trout Unlimited in an interview. “And they’re one of the most significant populations of native trout in the world. If we can’t save them here, in our flagship national park, where can we save them?”
But the story of lake trout in Yellowstone Lake follows a familiar and depressing invasive species narrative: by the time the species is documented, it’s too late.
Or is it?
Signs of Lake Trout Decline
Large lake trout like this have decimated native fish populations in Yellowstone. Photo: National Park Service
Two years ago, an effort led by Yellowstone National Park, Trout Unlimited and other partners began using long gill nets and fish traps to capture invasive lake trout – a method that decimated native lake trout in the Great Lakes.
Since then 1.4 million lake trout have been removed. But is it having an effect on the overall population?
An initial study by Montana State University says that the population is in decline. The results will be published later this year.
One way fisheries biologists measure fish populations is by measuring how much effort it takes to catch fish: what they call “catch per unit effort.” In the two years since the control effort began, the catch per unit effort has increased significantly.
The catch in gill nets has decreased from eight fish per 100 meters of net to five fish per 100 meters.
“These are very promising signs,” says Hallac. “We are having to work harder to find lake trout. Our best-available analyses indicate a negative growth trend for lake trout.”
As the lake trout population decreases, gill nets become less effective, according to Trout Unlimited’s Jack Williams. Scientists will implant lake trout with radio tags to pinpoint where fish congregate. They’ll also experiment with an electrified grid to kill lake trout eggs at the bottom of the lake.
Even with all those efforts, it is likely impossible to eliminate lake trout from a 130-square-mile lake.
“This is about suppression, not eradication,” says Hallac. “The goal is to reduce the lake trout population to the point where it is no longer adversely impacting the Yellowstone cutthroat trout population.”
One of the most hopeful signs has emerged among the cutthroat trout population. In the past, researchers had found adult cutthroats persisting in the lake, but very few smaller fish. Recent fish surveys have found young cutthroats, a sign that fish are not only reproducing but surviving.
“I’m very encouraged by some of our recent success,” says Hallac.
National parks are not immune to conservation threats, whether invasive species or climate change. But the parks also offer a great outdoor laboratory to try new techniques. The Lake Yellowstone lake trout removal is an ongoing research effort that also is one of the most hopeful stories for native trout conservation.
Around the West, the various subspecies of cutthroat trout have not fared well in the face on non-native trout, dams, habitat destruction and a litany of other threats.
But thanks to protected areas and good research, the Yellowstone cutthroat trout offers a more hopeful story. I’ve seen the results of protection efforts by The Nature Conservancy and other partners on the South Fork of the Snake River, another important cutthroat trout river.
I’ve seen the incredible change on the Conservancy’s Flat Ranch Preserve (celebrating its twentieth anniversary this summer), where a river restoration project completely changed the habitat for cutthroats. Ten years ago, the Flat Ranch Outlet was a straight, eroded channel where fish were few and far between.
The last time I visited, beautiful cutthroats were rising in the riffles and pools of a willow-lined stream.
Now, we also have hopeful signs from the cutthroat’s greatest stronghold. Using the best fisheries research available, there’s an excellent chance that the cutthroat will be restored to its proper prominence in our first (and to my mind, greatest) national park.
Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.