Habitat expected to shrink by 50 percent in coming decades
By Bob Berwyn
SUMMIT COUNTY — A combination of rising temperatures and changes in the timing of runoff and streamflows could reduce trout habitat in the western U.S. by about 50 percent during the next 70 years, with some populations disappearing completely within just a few decades.
The mechanisms for temperature-driven extirpations are complex, but the bottom line is that an ever-warming world isn’t going to leave much room for cold-blooded fish.
“They operate within a very narrow temperature range,” said U.S. Geological Survey biologist Andrew Todd. Variations in temperature can affect spawning, even if the temperatures don’t reach levels that are directly lethal to the fish, Todd said.
And the hydrology is also important, he explained. Even small changes in the amount and timing of precipitation can have a big impact on smaller headwaters streams. And unlike birds or mammals, trout don’t have the ability to move freely if conditions become unsuitable.
“They can’t just go to Montana,” Todd said.
The research looked at data from nearly 10,000 sites throughout about 400,000 square miles of the Western United States and looked at the relationships between growth rates, incubation times, competitive ability relative to other species, and asynchrony with prey, which can cause negative population growth rates even if temperatures never reach the lethal range for individual trout.
“The study advances our understanding of climate change impacts by looking beyond temperature increases to the role of flooding and interactions between species,” said Seth Wenger, lead author for a team of scientists from Colorado State University, Trout Unlimited, the U.S. Forest Service Rocky Mountain Research Station, the U.S. Geological Survey and the University of Washington Climate Impacts Group.
Increases in regional air temperatures and dramatic changes in stream-flow patterns have been well-documented during the past 30 years, said co-author Kurt Fausch, professor in CSU’s Department of Fish, Wildlife and Conservation Biology and an expert on trout ecology and management in the West.
Accurate water temperature data for the streams themselves are harder to come by, since many smaller streams aren’t consistently monitored for temperatures. But anecdotally, anglers and fishery biologists have all been observing changes that can be linked to temperature, like brown trout, which are more tolerant of warmer water, moving higher up into mountain drainages.
“It’s exciting to see these ideas being used,” Fausch said of the study, “but the impending loss of trout habitat is both startling and depressing. The West is iconic for trout fishing, but much of this is projected to go away,” Fausch said.
While some trout are likely to disappear simply because the water in the streams and rivers gets too warm, the impacts from climate change go beyond that direct effect, he said.
As snowmelt and runoff patterns change, some streams that currently flow year-round are likely to become seasonal streams, drying up completely or dropping to flow levels that can’t sustain fish, especially in late summer and fall, critical times for some species, he explained.
And those same changes are causing a “de-coupling” of ecosystem functions, as the timing of prey and predators just don’t match up anymore.
The hatching of trout eggs has evolved over millennia to coincide with the emergence of insect larvae that provide food for the newborn trout. The insect hatch, in turn, is timed to coincide with specific stream flows. When that delicate balance is disrupted, the food chain breaks down.
The study was published this week in the peer-reviewed science journal, “Proceedings of the National Academy of Sciences.” It predicts native cutthroat throughout the West could decline by as much as 58 percent and introduced brook trout could decline by as much as 77 percent.
Rainbow and brown trout populations would also decline by an estimated 35 percent and 48 percent, respectively. These losses would have major impacts on trout fishing, which generates hundreds of millions of dollars in recreation annually in the United States and is a major factor drawing anglers to Colorado and the West.
The study notes that the decline of cutthroat trout is of particular significance because cutthroats are the only trout native to much of the West and a keystone species in the Rocky Mountain ecosystem.
“This research also builds on 15 years of work with graduate students at CSU to find ways to prevent our native cutthroat trout from going extinct in the face of declining habitat and non-native trout invasions,” Fausch said.
The study will help resource managers decide where to focus their efforts in the coming decades.
“Where will the strongholds be 30 to 70 years from now?” Fausch said. “Where would you put your time and energy?”
The researchers used an ensemble of climate models to arrive at the study’s findings. Some models predicted more warming than others, but under even the most optimistic model, cutthroat trout populations in the West could decline by 33 percent. Scientists note that most of the 14 unique forms (subspecies) of cutthroat trout are already in trouble—two are extinct, and most of the rest now occupy less than 15 percent of their historic native range with several of these listed under the Endangered Species Act. Declines from a changing climate would impact native cutthroat trout beyond the impacts they’ve already suffered.
The study can be read in its entirety online at the Proceedings of the National Academy of Sciences website.
The research was funded by the National Fish and Wildlife Foundation, the U.S. Geological Survey and the U.S. Forest Service Rocky Mountain Research Station.