About a decade ago I traveled to Yellowstone National Park to report on the discovery of non-native lake trout in Yellowstone Lake. One of the key concerns at the time was that the large, voracious lake trout would overrun the lake's native cutthroat trout, a much smaller species that would prove to be a wonderful food for the lake trout.
At the time, fisheries biologists held out hope that they somehow could blunt the population growth of the lake trout. The one solution that seemed most worthwhile was gill-netting. In recent years that effort has generated a measure of success, with more than 100,000 lake trout netted.
Unfortunately, that apparently is not good enough. In a story that began circulating Saturday in Fisheries, the journal of the American Fisheries Society, National Park Service biologists say the cutthroat trout population in Yellowstone Lake has dropped at least 60 percent in recent years.
Earlier this year I revisited the lake trout issue for Wyoming Wildlife Magazine. I found that biologists are working as hard as ever to find a solution to the lake trout problem. Here's a version of that story:
When it comes to procreation, lake trout are pretty gregarious creatures. Unlike other trout species that head up tributaries to pair off and sow their proverbial oats, lake trout prefer group get-togethers.
“They’re group spawners instead of a paired spawner,” says Patricia Bigelow, a fisheries biologist at Yellowstone National Park. “They kind of all get together and have an orgy, whereas a typical stream-spawning trout would dig a redd and then bury those eggs for extra protection. The lake trout, they are just broadcast spawners.”
And that is part of the problem that the national park’s fisheries staff faces as it continues to beat down a population explosion of lake trout that threatens to overrun the pristine cutthroat trout population of Yellowstone Lake. If lake trout headed into the lake’s 124 tributaries to spawn, it’d probably be relatively easy for biologists to devise a system to kill the non-native fish. Unfortunately, lake trout spawn in the lake’s open waters, making it difficult for biologists to corral them.
Lake trout population estimates, and subsequent kill rates by park crews, show just how difficult the task remains today, more than a decade after lake trout were discovered in Yellowstone Lake. Shortly after the voracious trout, which at maturity can each can eat 40 or more cutthroats annually, were discovered in the lake in 1994, U.S. Fish and Wildlife Service biologists estimated that their population could number as high as 100,000.
Since then, more than 100,000 of the large fish have been killed, largely by gill-netting. While that slaughter would seem to be impressive, it has not convinced Yellowstone officials that the lake trout’s threat to Yellowstone Lake’s native cutthroat trout fishery is over.
Indeed, despite the slaying of more than 26,000 lake trout last year alone, the number of spawning cutthroat continues to drop precipitously, a disturbing trend that has impacts not only on the cutthroat population itself but also aquatic and terrestrial life upstream of the lake in its dozens of tributaries.
“There has been a decline each year, a steady decline,” says Todd Koel, who oversees the park’s fisheries program. “It’s not a straight up-and-down line, but it’s a really steep line.”
In just one year, from 2003 to 2004, the number of cutthroats heading into Clear Creek on the lake’s eastern shore to spawn plummeted from 3,432 to just 1,406. That 2004 tally represents the lowest count since 1959, when just 3,353 spawning cutthroat were spotted in the stream, a low point associated with an over-harvesting of cutthroat eggs by the park’s hatchery operations.
Even worse, cutthroat trout no longer spawn at Pelican Creek on the lake’s north shore where they are victims not only of lake trout but also whirling disease.
In a bit of academic serendipity, the educational paths of three graduate students has spawned a multi-disciplinary effort at the University of Wyoming to both discover a new tool to battle the lake trout invasion and to gauge their impact not only on cutthroats, but also on the surrounding landscape.
For her part, Bigelow is trying to devise a map that not only will outline movements of lake trout in the lake that covers 84,000 surface acres but also pinpoint where they spawn. With that information, fisheries biologists could focus their gill-netting efforts on those areas.
“The bottom line is to try to find any new spawning areas,” says Bigelow, who is applying her mapping work towards a doctorate degree at the university. “But also we want to be able to know which areas have high potential for being pioneered as new spawning areas, so we can keep an eye on them.”
Park officials contend lake trout gained access to Yellowstone Lake through the bucket of an angler who wanted some day to pull a little bigger trophy from the lake than a cutthroat. While that might be so, it did not mark the first time someone tinkered with Yellowstone’s fisheries.
Until 1889, 40 percent of the park’s lakes and streams were considered barren. But then, with the help of the U.S. Fish Commission, an aggressive stocking program brought brook trout to the upper Firehole River, rainbows to the upper Gibbon, and both brown and lake trout to Lewis and Shoshone lakes. Between 1889 and 1955, according to park records, more than 310 million fish were planted in Yellowstone’s waterways.
More recently, park officials have placed a priority on managing lakes and streams for their native species, an emphasis most visible these days in the efforts to spur a collapse of Yellowstone Lake’s lake trout population.
A blue high-country jewel, Yellowstone Lake long has been renowned for its cutthroat fishery. Today, despite the lake trout invasion, it remains the top destination for anglers who enter the park. To keep it so, fisheries biologists during the past decade have tried to muffle the lake trout population by gill-netting and electro-fishing.
A glimmer of hope for their crusade came in 2003 when gill-netting efforts to monitor the cutthroats’ population seemed to indicate an increase, as the “average number of cutthroat trout caught per net increased slightly (from 2002 numbers) from 6.1 to 7.4,” reported the park’s fisheries biologists. But while gill-netting is proving more and more successful in muffling the numbers of mature lake trout, it is ineffective against smaller lake trout that are similar in size to cutthroats, as the nets are designed to allow the cutthroats to pass through the mesh.
To develop her map of lake trout movements and density, Bigelow relies on hydro-acoustic sonar to help find the big fish.
“I’m hoping that by doing really dense transits I can see lake trout movement,” she says. “Because they’re group spawners, they also will stage in an areas before they go in to spawn and I’m hoping I’ll be able to see changes in density as maybe an indicator that there’s a spawning area nearby.”
If Bigelow’s mapping proves successful, the gill-netting attack on lake trout could become even more focused.
“I think it will be easy to narrow the search window. How small a search window we can narrow it to I guess is the question,” she says. “It’s a huge lake. Lake trout, luckily, are a little bit picky in areas that they can successfully spawn in, so that helps.”
Unlike cutthroat trout, which spawn in the lake’s tributaries, lake trout remain in Yellowstone Lake and search out bottoms where cobbles can offer a measure of protection for their eggs and emerging fry. So far, the West Thumb area of the lake seems to be most popular with spawning lake trout, although they also head to the shores off Carrington Island and near Solution Creek as well as in Breeze Channel.
Until the lake trout population shows signs of shrinking, though, the impacts of their existence in Yellowstone Lake could range far beyond the fate of the cutthroats, as 42 bird and animal species rely on cutthroats for at least part of their diet. Already there have been studies that discuss the loss of cutthroat trout on such species as grizzly bears, eagles and osprey.
“The conclusions are that as the lake trout go up and the cutthroat trout go down, the food supply for the terrestrial animals, that are seasonally important, are going down, so there’s going to be some kind of effect,” says Dr. Wayne Hubert, a fish biologist with the Wyoming Cooperative Fish and Wildlife Unit at the University of Wyoming who is overseeing Bigelow’s project. “That energy supply in the cutthroats in the springtime, when everything else in a lot of other places is still covered up with snow, is pretty important. It might only be for three or four weeks, but that’s a source of food.”
Lusha Tronstad, another University of Wyoming doctoral student, is examining the lake trout impact on a much, much smaller scale: How might the reduction in cutthroat spawns impact the chemistry and food web of the lake’s tributaries?
Although she has another field season of work to complete before she can reach any solid conclusions, Tronstad says her studies so far of Clear Creek, Cub Creek, and a third, unnamed tributary indicate that a reduced cutthroat population in the lake could affect the entire food chain within a tributary.
“I’m finding that a lot less nitrogen is taken from Yellowstone Lake up to the tributary streams by the cutthroat trout, just because there are fewer cutthroat trout spawning,” she says.
Reduced nitrogen levels, she explains further, could adversely impact the growth of algae, which feed aquatic macroinvertebrates, which feed cutthroat trout fry.
“Perhaps this could reduce fry recruitment and so it would kind of send the cutthroat trout spiraling downwards. It would inhibit their recovery, perhaps,” she offers. “They call it bottom-up effect, because it starts at the bottom. It starts with nutrients and then it works its way up to the plants in the stream, the algae, and then up to the invertebrates and then up to the fish.”
River otters also stand to suffer from an unbridled invasion of lake trout and that could harm the vegetation surrounding the lake and its namesake river. Jamie Crait, another University of Wyoming graduate student investigating Yellowstone’s lake trout problem, with an emphasis on otters, says cutthroat trout are a dominating portion of the otters’ diet. After gorging themselves on cutthroats, the otters eventually retreat to a favored “latrine site” where they transfer nitrogen and other nutrients from the cutthroat to the land, where plants thrive on this natural fertilizer.
“We have seen evidence that plants on some of these latrine sites, or social scent-marking sites, have increased growth,” says Crait, “so it looks like, potentially, getting this excess nitrogen from otter urine or feces, leads to some increased growth in the plants.”
While otters don’t stray too far from water, “if you go along the landscape and you have enough coverage of these latrine sites, it adds a little point of heterogeneity that you wouldn’t have with the otters,” he adds.
If Bigelow is successful in pinpointing lake trout spawning grounds, both active and potential, settling on a method of disrupting the spawn remains to be done. Not only do countless other aquatic species reside in the lake, but the lake bottom is a enthralling realm of deep canyons, spires, and geothermal founts.
“Not only don’t we want to kill other species, but we don’t want to destroy the substrate or any other potential invertebrate fauna, or even bacterial or microscopic fauna,” says Bigelow. “The substrate on the bottom of the lake is incredible fascinating. You can’t just go around killing all that, or physically destroying it or burying it in something that would suffocate other organisms.”
Will Bigelow’s work give park officials another tool to reduce the lake trout infestation? Koel believes so.
“We’re trying to do the best we can, stay on top of the best science we can to address the problem,” he says. “And a really critical time for us to have a good impact on the lake trout population is when they’re spawning, so one of the keys to doing that is to make sure that we know where all the spawning locations are or where they potentially could be.”
Once all those locations are known, the biologists will seek out methods of killing the lake trout and their eggs. Already the park, which so far has focused its efforts on gill-netting and electro-fishing, has asked the engineering department at Montana State University to investigate possible tools for destroying eggs.
Another question that also remains unanswered is whether the lake trout population has grown too big to cripple.
“That’s a good question,” agrees Hubert. “That’s something that none of us know. There is strong evidence from the lake trout control program that there is an impact from that program on lake trout. The size structure of the fish in the lake, and the catch rates of the fish in the nets are declining. Those are indications that they’re having an effect on the population. The question is, is it enough?”