Climate Change Could Crash Colorado Cutthroat Populations
Climate Science Indicates That Cutthroat Trout Face An Uphill Battle For Their Survival
Text and photography by Forrest Czarnecki
A flash of gold strikes through the crystal-clear lake water before a soft ripple breaks the otherwise placid lake surface. A few yards away a black-speckled, olive-colored fin breaks the surface before dipping back into the emerald abyss below. On the other side of the lake several more rings slowly dissipate one after the other.
Glaciers stand sentinel across the ridgeline that protects the lake, and spruce trees blanket the mountainside below. Sunlight dances across the water as another ripple forms near the grassy shoreline.
The high alpine lake teems with dozens of greenback cutthroat trout, the state fish of Colorado.
But that may not be the case in the future, as climate change could spell disaster for these native fish and the cold water that they call home, according to a study done by the United States Geological Survey and Colorado State University.
As these fish face an uphill battle to survive a warming climate, they are caught in a net made of climate science and changing living conditions.
Currently, the three subspecies of cutthroat trout that are found in Colorado occupy only a fraction of their historic range, according to Colorado Parks and Wildlife.
Although there have been conservation efforts from both state and federal agencies to expand the trout’s range, one of the biggest challenges that the cutthroat faces is climate change, according to the study, which was commissioned by CPW.
How it Kills
A warming climate may seem like an unlikely culprit for killing fish that normally swim in lakes and streams where the water temperature is below 60 degrees Fahrenheit, but the proof is in the water.
The ideal water temperature for cutthroat trout is around 55 to 60 degrees Fahrenheit. This temperature range allows the water to hold ample dissolved oxygen, and is the optimum range for cutthroats to stay active, according to Trout Unlimited.
Water naturally contains dissolved oxygen, which is where many underwater life forms get the air that they need to breathe. According to the USGS, as water temperature increases, the ability of the water to hold dissolved oxygen decreases.
As the dissolved oxygen levels within a body of water decrease, more physiological stress is put on the fish that live there. These physiological stressors can include stress on cardiac and respiratory systems, as well as individual organs like the heart, gills and brain, according to an article published in the Journal of Fish Biology.
If the water in a lake or stream gets too warm and the stress on the fish gets too high, it can result in a physiological systems shut down in the fish — which results in death.
Compounding Problems for the Cutthroats
Warming climate conditions and changing weather patterns only compound other problems that the cutthroat trout is currently facing, according to Dave Lawrence, an ecologist with the National Park Service’s Climate Change Response Program.
Some of these other problems range from shifting habitats and less-than-ideal water conditions, to the potential for genetic extinction of certain populations, according to an article published in Nature.
Seeking Colder Waters
As some high alpine waters warm up and experience shifting temperature gradients, some trout species will seek out colder waters, according to research done by the United States Forest Service. Suitable cutthroat trout habitat will continue to dwindle as the climate continues to warm, forcing some fish to either find a new cold-water home if possible, or go belly-up.
“As stream temperatures warm these [fish] will shift their distribution, and they can move into cooler upstream areas to a certain extent,” said Lawrence, who also has several years of experience studying the effects of climate change on certain trout species in the western United States.
“The issue with regard to climate change is that it may warm certain waters to a such an extent that there is no more habitat or space [for the trout] to occupy, so there’s only a limited extent to where the can move. At [some] point they reach a mountain top, and they can’t cross a watershed divide, so they essentially get to a point where they can no longer get out of the warm water,” Lawrence said in a phone interview.
While the problem of streams and rivers warming to the the point that cutthroat trout may no longer be able to survive in them is a distant threat, that same study done for CPW points out that larger bodies of water such as lakes and ponds may provide suitable habitat for the fish.
The study states that while some streams or rivers may increase in temperature, bigger bodies of water, like deep lakes and ponds can provide a thermal refuge, or colder living space, for the fish.
“That’s plausible, it depends on the [lake],” Lawrence said, “If there’s a system that’s stratified enough to maintain cold water temperatures, I could see that as a refuge.”
“Most of those [lakes] are also occupied by non-native trout, so they are not necessarily resolving that particular problem, unless those [lakes] are non-native-free, which is pretty rare,” added Lawrence.
Non-native Problems
Warming waters are just one of the problems that cutthroat trout are facing across the west, including Colorado. Along with warming waters comes a slew of other problems, such as some not-so-friendly neighbors. A study published in Nature in 2014 discusses how climate change can compound pressures that native cutthroat trout are facing. Some of these pressures include increased resource competition from non-natives and the potential for hybridization between non-native fish and native fish.
The study found that as some streams get warmer, there were more cases of hybridization between non-native fish like rainbow trout, and native cutthroat trout.
“The worry there, of course, is that [it] can ultimately cause a genomic extinction of a native species, where it basically gets hybridized out of existence,” Lawrence said.
Another concern for native cutthroats includes direct resource competition from those non-native species like rainbow trout and brown trout.
Certain trout species, like rainbow and brown trout, have a higher tolerance for variable water temperatures, which means they can inhabit waters that are at or above a cutthroat’s range. This can lead to competition for food resources, spawning grounds and living space.
“For cutthroat trout [one of the biggest problems] is competition with other sport fish like brown trout, rainbow trout and brook trout,” said Justin Reeves, an assistant professor with the Department of Fish, Wildlife and Conservation Biology at CSU.
“Rainbow trout are just bigger, they’re meaner, they’re more aggressive,” Reeves added.
Variable Precipitation, Changing Rivers and Wildfire
As the climate warms and weather patterns shift across the western United States, glaciers and high altitude snowfields are beginning to disappear, according to research from the University of Washington.
The effects of receding glaciers and disappearing snowfields will only get worse as the climate continues to change, and those effects will be felt downstream, according to Lawrence.
“If you look at places that have [glaciers] and snow, both a reduction in the snowpack and an earlier melting of snowpack will have a really strong influence on trout,” Lawrence said.
Lower seasonal stream flows can hurt trout by decreasing available habitat, and by increasing stress on them during a time when there’s more competition for food sources and water temperatures are naturally higher.
“Many of these snowmelt-dominated systems have a low-flow period, which is a natural period of low flow in the summer, but that period becomes extended or exacerbated as you lose snow,” Lawrence said.
“When you combine that with potential drought situations, that’s naturally a period of time where the trout are really eking it out, and they don’t grow a lot. But they need to survive those summer low-flow periods, and ultimately if that’s extended it’s going to have [a negative] influence on them as well,” Lawrence added.
Not only do shifting weather patterns affect the temperature of the waterways that cutthroat trout are found in, they can also affect the physical characteristics of those streams and rivers.
Those physical changes can come from hotter weather patterns, changing precipitation and changing wildfire cycles, according to Lawrence.
Warmer climate conditions and arguably improper forest management practices have helped increase the intensity and frequency of wildfires across the west. Those wildfires can have deadly effects on cutthroat trout populations.
“Another indirect impact [of climate change] is the severity and extent of wildfires in the west. That’s a combination of land management as well as the influence of climate change. When you get broader, more intense fires that’s ultimately is going to have a pretty direct impact on both the streams as well as the animals that live in those systems,” said Lawrence.
When a wildfire moves through an area, it oftentimes leaves the soil exposed to the elements — including rain and hail — which can cause erosion and flooding within the downstream waterways. This creates the perfect recipe for sedimentation in those waterways.
“When you combine a fire with a big precipitation event, you get a lot of sediment added into those rivers,” Lawrence said.
Sedimentation in smaller waterways decreases habitat availability for cutthroat trout, while also leading to warmer water temperatures. As waterways become shallower through sedimentation, the water will heat up faster.
In some cases, an extreme wildfire can even lead to extinction of a species, which was almost the case with the Gila trout in New Mexico during a 2012 fire.
“In some systems the fires can completely extirpate trout populations, particularly for really, really range-restricted species,” Lawrence said.
Last year, Colorado may have had a less destructive wildfire season than some other western states like California or Montana, but that does not make the state’s cutthroat trout immune to the wildfire problem.
Although lakes or ponds may be less susceptible to warming temperatures, their chemistry and water quality can be easily affected by wildfires.
Trapper’s Lake, the location credited as “the cradle of Wilderness” in northwest Colorado, was ravaged during a 2002 fire that burned 17,000 acres. Trapper’s Lake has the highest concentration of Colorado River cutthroats in the world.
The Rio Grande cutthroat trout is one of the most at-risk subspecies of cutthroat trout in the state when it comes to danger from wildfires. Currently, the Rio Grande subspecies only occupies 12 percent of its historic range, and is only found in the headwaters region of the Rio Grande River in south-central Colorado.
Ongoing Research and Future Projections
Current climate science compiled by CSU, the USGS and CPW predicts that the indirect effects of climate change may not be felt by cutthroat trout for years, or even decades in some of the further projections.
