Losing a Colorado Native: The Case of the Greenback Cutthroat Trout
by Charles Trowbridge
The greenback cutthroat trout, Colorado’s state fish and one of its last remaining native trout species, is once again in danger of extinction. The iconic fish was actually declared extinct once, in the late 1930s, but in 1973 the U.S. Fish and Wildlife Service discovered small pockets of genetically pure greenback trout, allowing it to be placed on the endangered species list. The FWS later downgraded the trout’s status to “threatened,” which is how it remains today. Recent studies, however, suggest extinction could come sooner than previously thought. Climate change, invasive nonnative species that prey on the trout and a lethal disease all threaten the long- and short-term existence of the greenback cutthroat trout.
If things continue at their current clip, the species could potentially be exterminated within five years, according to Kurt Fausch, a biologist at Colorado State University who is an authority on the imperiled fish.
“The greenback cutthroat trout now occurs in only about 0.5 percent of its native range,” saidFaush. “Both brook and brown trout can exterminate cutthroat trout within about five years, if conditions are suitable to the nonnatives.”
These conditions are numerous and different, but they have one common thread: a warming climate.
When the temperature warms, the snowpack at high elevations melts and causes debris, such as silt and rocks, to be pushed through mountain streams at a higher rate, eroding banks, destroying habitat, and causing in-stream flow disruptions. If streams have an increase in debris flow from erosion, the trout’s eggs are in danger of being scoured from the gravel bed, decreasing the likelihood of maintaining a steady population.
Drought or flooding, both potentially linked to climate change, can also harm or even destroy habitat for native trout. Unfortunately, Colorado is especially susceptible to drought and other water-related problems. Almost 70 percent of the water available for consumption each year comes from snowmelt, according to a 2002 report by Trout Unlimited’s Colorado Water Project. The majority of this runoff occurs over a three month period, from May to July, which means that if the runoff is low, the streams will not have enough water for trout survival, and if the runoff is high, bank erosion and habitat destruction could wipe out the existing populations.
Impacts to native trout are not limited simply to climate change. Humans have destroyed parts of the trout’s habitat and wildfires have caused riverbanks to erode. These impacts, coupled with changes in water flow, have forced cutthroat trout high into the mountains — areas most affected by adverse water flow conditions.
Earlier this year, Fausch co-authored a report highlighting the importance of taking other factors besides water temperature shifts into account when assessing threats to the greenback.
“One paradox is that, because nonnatives and habitat destruction have combined to relegate greenback cutthroat trout to high elevation streams, warming temperatures are probably not as much a problem as other risks,” Fausch said.
Faush said competition with other species, like the brown trout, is also considered an immediate threat.
The brown trout is one of the most pervasive nonnative trout species, and it has proven especially adaptable to warm water temperatures, unlike the cutthroat. Due to the timing of the brown trout’s lifecycle, and its particularly vigorous predatory habits, it poses the greatest danger to the greenback species, according to Dana Winkelman, leader of the Colorado Cooperative Fish and Wildlife Research Unit.
“All these trout compete at some level with each other,” said Winkelman. “Brown trout are a particularly vociferous predator.”
The brown trout lifecycle differs from the greenback’s in that baby brown trout hatch in the fall, giving them the entire fall and winter seasons to feed and grow. By the time greenback eggs hatch in the spring, the brown trout have grown to the perfect size to feed on these hatchlings.
But, there is an even more important reason brown trout have fared so well: whirling disease. Borne by a parasite called Myxobolus cerebralis, the devastating disease threatens almost all trout species, save for brown trout. Infected juvenile trout experience neurological damage, which causes them to swim in a “whirling” motion instead of straight, as the name suggests.
The widespread disease has altered population numbers. Areas formerly home to native trout populations are now dominated by brown trout, explained Winkelman. “Brown trout co-evolved in Europe and Asia, so they get the disease, but not the effect. When it came it wiped out all the other species,” he said.
Another reason brown trout are seemingly impervious to the disease is directly related to their lifecycle. The parasite thrives in warmer water temperatures, so when brown trout hatch in the fall and grow through the winter, the cold temperature of the water prevents juveniles from becoming infected. The greenback, hatching in spring, is far more susceptible because the water is warmer at that point, creating ideal conditions for the proliferation of the parasite.
A 2001 report from Colorado State University suggests that warmer water temperatures related to climate change could actually be beneficial to the greenback cutthroat trout. And even though much more research has been done in this field since 2001, there are still a few possible scenarios in which warmer water temperatures could benefit the greenback trout, said Winkelman.
For instance, since greenback trout are relegated to small streams at high elevations, the environment naturally limits the growth of juveniles. If these juveniles do not grow big enough to protect themselves from frigid winter water temperatures, they will die, explained Winkelman. Food is scarce in the winter, and the ability to feed and grow is almost impossible. If water temperatures warmed, there is a possibility that the greenback trout would have more time to prepare to overwinter, he said.
“But, temperature is a double-edged sword,” said Winkelman. “Warmer temperatures might help the [whirling] parasite, even at higher elevations.”
As brown trout populations continue to grow due to environmental adaptability, greenback populations get pushed to higher and higher elevations. The greenback trout, by nature, is competitively inferior to brown trout, so in the rare instances the two species overlap, it will not fight for habitat space with the far more aggressive brown trout. As a result, the already-diminutive habitat range of the greenback has continued to shrink considerably.
Generally, most greenback trout populations are isolated from each other. Each population typically inhabits small sections of stream, about two miles. Having small, isolated populations means that if one group begins to die off for some reason, the opportunity to mate decreases, and eventually, the entire group could die off, explained Fausch.
“So, working to extend these [areas] is some of the best insurance we can have against losing them,” he said.
Looking Out for the Greenback
The Colorado Division of Wildlife, in conjunction with the U.S. Fish and Wildlife Service, as well as other agencies, has taken considerable measures to try safeguard the existence of native fish species such as the greenback trout. These include making certain areas that contain native greenback cutthroat trout populations “off-limits” for stocking other trout species.
Over the last year, the Poudre River State Fish Hatchery, a facility that collects and fertilizes eggs from mature trout, sent more than 1.5 million greenback eggs to production facilities, that in turn developed them to be stocked at high elevations and ponds across the state, said hatchery manager James Ingram.
The agency is also especially careful when stocking streams with nonnative species, explained Ingram.
“Anytime you start to change the arrangement of species, you change the ecosystem from the way it used to operate,” he said. “You don’t know what effect that’s going to have.”
For instance, if the Colorado Division of Wildlife only stocked brook trout, an aggressive, carnivorous species, in a certain location, the fish would wipe out the food source, creating an imbalanced ecosystem. Eventually, the brook trout would die off, and the prey base would explode, he explained
Today, although efforts to maintain existing populations have seen some past success, the multitude of threats facing the greenback trout only seem to be growing.
At the moment, most of the areas where greenback trout still remain are relatively safe from predators, but that could change in the near future, cautioned Fausch.
“Brook trout are better at moving upstream into the colder waters where cutthroat have been relegated,” he said. “Brown trout don’t typically occupy such small or cold streams, but I have seen them moving upstream in places they never occurred in Colorado, and others are seeing this around the West.”
Charles Trowbridge is a CU-Boulder graduate student studying print and broadcast journalism with an emphasis on environmental reporting. Before coming to CU, he studied music and English in Oregon, where he became interested in environmental issues related to preserving wild fish populations. Born and raised in Alaska, he plans to return upon completion of his degree to examine the effect of climate change on commercial fishing.
Correction: Updated March 6 at 4:05 p.m.
*The original version of this caption mistakenly identified the fish shown as a greenback cutthroat trout fingerling.