Written by guest author Heidi Shepard, NW CASC communications graduate research assistant for the winter 2025 quarter
Upper Klamath Lake in south-central Oregon is perhaps the last place one might think of as being trout habitat. It’s shallow and warm, and its high nutrient content fuels summertime algae blooms, which in turn can cause massive fish die-offs. By comparison, the clear, cold rivers and streams that feed the lake are prime real estate for coldwater fish like the redband trout.
Or so you’d think. But Dr. Jonathan Armstrong, an ecology professor at Oregon State University (OSU), is discovering it’s not that simple. Armstrong joined the faculty at OSU in 2016 and quickly partnered with the Oregon Department of Fish & Wildlife (ODFW) to study redband trout. These large salmonids are native to the western United States and are highly migratory, though they never venture into the ocean, instead living their entire lives in freshwater. Over the years, Armstrong has studied their life cycles and the ways they use the diverse habitat throughout the Klamath Basin — and it turns out that these fish unexpectedly spend a lot of time in the warm, eutrophic waters of Upper Klamath Lake. “When you’re in the [cold streams and rivers] they use as refuge, they look glorious…hundreds and hundreds of meters of expansive, clear cold water…exactly what you’d think of as trout habitat,” says Armstrong. “And it’s like, ‘why would you ever leave here if you’re a trout?’”
Why indeed? Why do redband trout choose to spend so much time in a lake that Armstrong describes as “neon green and downright nasty at times”? And why are their invasive counterparts — brook trout and brown trout, the latter of which are typically tolerant to warmer waters — not doing the same? That’s the question Armstrong and his team, in collaboration with partners at ODFW and the Klamath Tribes, are hoping to answer through their current project, which is funded by the Northwest Climate Adaptation Science Center. The Klamath Tribes rely on redband trout for their last remaining subsistence fishery, as other first foods like endemic sucker fish are endangered and salmon have been cut off by dams. His colleague Jordan Ortega, a PhD student at OSU who is currently working as the anadromous fish specialist for the Klamath Tribes, visited the cold, spring-fed tributaries where redband trout are known to spawn. Ortega found that these places were teeming with invasive brown and brook trout — and they were preying on juvenile redband. “It’s nature, you know, everything eats each other,” says Armstrong. But the scientists got the impression that the redband trout’s ability to rear their young in the areas where they’re born is greatly diminished by the presence of these invasive trout species.
Armstrong and his team, which also includes Oregon Cooperative Research Unit assistant professor Dr. Melanie Davis, were also puzzled by the brown trout’s behavior. “In a lot of places, you have these mixed assemblages of native and nonnative salmonids, and the way they tend to distribute themselves is that brown trout usually go the furthest downstream, to the warmer water,” he explains. “We usually think of brown trout as one of the salmonids that are more tolerant to warm habitats. What was interesting in the Klamath Basin is that this wasn’t the case: we only found brown trout in spring-fed or snow-fed habitats.”
Researchers suspect that at certain water temperatures, a freshwater parasite may kill invasive trout but not redband trout
Parasites and water temperature, Armstrong explains, are two pieces of this puzzle. Salmonids infected with Ceratonova shasta (C. shasta), a freshwater parasite native to the Northwest, experience higher mortality rates in warmer water temperatures, which does not bode well for their future on a rapidly warming planet. But Armstrong and his colleague, OSU disease ecologist Dr. Julie Alexander, are looking on the bright side. In collaboration with the Klamath Tribes, they suspect that at certain water temperatures, C. shasta appears to kill invasive trout, but not native redband trout. Armstrong and his team have hypothesized that since redband trout have co-evolved with local pathogens, parasites, and warm temperatures, they’re more tolerant to these stressors than invasive brown and brook trout. This allows redband trout to take advantage of warmer habitat like Upper Klamath Lake, while the invasive trout are limited to the colder tributaries. However, at higher temperatures, C. shasta is lethal to native fish too, so Armstrong and his team are researching the optimal temperature range where C. shasta could benefit redband trout by killing their invasive competitors while allowing them to survive — essentially acting as the enemy of their enemy.
Ultimately, Armstrong’s goal is to improve the ability of his project partners to anticipate the future locations where redband trout may thrive, but invasive trout won’t — and to concentrate their conservation efforts accordingly. To do this, he and his team are working to provide Ambodat (formerly Klamath Tribes Aquatic Resources) and the Oregon Department of Fish & Wildlife with maps of the distribution of invasive species and diseases in the Klamath Basin, as well as models that can account for the effects of disease on fish distribution.
Summer season aside, warmer habitats like Upper Klamath Lake can act as an all-you-can eat buffet for redband trout
Coldwater fish research and conservation efforts tend to focus on the coldest habitats and disregard those that Armstrong has called “loser habitats” — places that are already warm and projected to continue warming as the climate continues to change. According to Armstrong, this preferential treatment neglects key habitat that highly migratory species like redband trout not only tolerate, but thrive in. Armstrong and his team knew that redband trout leave Upper Klamath Lake for a few months every summer when it gets too hot and eutrophic. What they didn’t expect, during their fieldwork, was seeing redband trout in the lake again as early as late August, when the water was still bright green with algae growth. “It was barely [tolerable] and they were already back in there,” Armstrong recounts. “I remember that was really striking because it showed us how valuable that habitat was.”
But just what is it about Upper Klamath Lake that makes it so valuable to redband trout? To tackle this question, Armstrong and his team sampled the trouts’ stomach contents. “Fish that eat other fish often have empty stomachs,” he tells me. “Around one out of every three has something in its stomach, because they have these really patchy encounters with prey. So what was astounding when we started sampling these redband trout is every time we got our hands on one, it would cough up multiple fish.”
It makes sense, Armstrong says, because that lake is so productive: “When redband trout are in the lake, they’re pretty much just feeding at their physiological capacity, like every day is Thanksgiving.” That’s how these trout get as large as salmon without ever venturing into the ocean. And they aren’t the only freshwater salmonids that utilize warm habitat like this: so do Siberian taimen, the largest members of the salmonid family, native to northern Asia. Taimen, Dr. Armstrong explains, are big river obligates. “A taimen can’t be a taimen if it has to live in a tiny little snowmelt stream. They’ve got to get big and eat things and use large rivers,” he says, referencing the fact that conservation efforts not only focus on colder areas over warmer ones, but also on resident species over migratory ones like taimen and redband trout.
At Wild Salmon Center’s invitation, Armstrong shares redband trout research with visiting salmonid experts from Mongolia
In October 2024, Dr. Armstrong met with a delegation of Mongolian scientists who study taimen. They traveled to the U.S. at the invitation of the Wild Salmon Center to compare U.S. research and conservation efforts on redband trout with their own work on taimen, since the two are so similar in their biology and their habitat needs. Armstrong serves on the Center’s Science Advisory Board and has co-authored papers on redband trout with Matt Sloat, their Director of Science, so he was the logical choice to lead the delegation on a trip to see how Klamath River ecosystems are bouncing back after several recent mainstem dam removal projects.
The Mongolian scientists “wanted to learn about the work on redband trout, see these habitats firsthand, and see the different technologies and research approaches they could think about doing,” Armstrong explains. Matt Sloat is doing research on taimen in collaboration with the Mongolian scientists, including studies to understand their physiological tolerances to dissolved oxygen levels in warming waters and radio telemetry to understand how they move across the landscape. The group is also looking at whether taimen are moving to find refuge during summer, as well as where they’re spawning and what habitats they’re getting their nutrients and growth from. By comparing their work to the state of redband trout research, the Mongolian scientists could gain a better sense of how to support these apex predators.
Similarly, the approaches used by Armstrong and his research partners include physiology studies and temperature-transmitting radio tags. Once attached to the trout, these tags allow the researchers to not only see where the fish are, but to know the temperature of the water they’re in. “You can get in a plane and fly over the lake with a little receiver, and even if you can’t take the time to pinpoint the fish, it’ll tell you that it’s in 15° Celsius water, whereas the lake’s at 25° Celsius,” Armstrong explains. This data is significant because when combined with the results of the physiology studies, it will help Armstrong and his team better understand what habitats redband trout use at different times of the year — and why.
Climate-informed management can help migratory fish persist in a warming world
What does the future hold for redband trout? While there are many uncertainties, one thing is clear: Dr. Armstrong’s research will help his project partners make informed decisions about redband trout habitat conservation. The maps and models he’s developing on disease and temperature will, he hopes, help focus conservation efforts on areas where redband trout may thrive, but invasive brown and brook trout will not. The changes taking place due to the recent mainstem dam removals along the Klamath are also on Armstrong’s mind. “Maybe soon [redband trout] will be eating juvenile chinook salmon,” he says, grinning. “That’s an exciting possibility, to think about them eating marine-derived prey.”
As for Armstrong’s future? He’s partnering with Trout Unlimited on a new NW CASC-supported project to quantify the benefits to migratory fish of removing migration barriers like culverts. “You take a barrier out, and then try to quantify the benefits of the habitat downstream. But that can be difficult sometimes because if that place is seasonally warm, like Upper Klamath Lake is, it doesn’t even register as trout habitat,” Armstrong explains. So this project will examine the ways migratory fish use habitat systems, eventually translating that research into decisions about barrier placement or removal. Armstrong sees this project as an extension of his research on redband trout — shining a spotlight on the needs of migratory fish in a warming world.
This story was written by Heidi Shepard. Heidi was the NW CASC communications graduate research assistant for the winter 2025 quarter. She is currently pursuing her master’s in library and information science at UW, after spending the past several years as an environmental educator and river guide on the Colorado Plateau. Heidi can be reached at hes84532@uw.edu.