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Rising water temperatures threaten fish populations

Drought, lower flows pose serious challenges to aquatic life in Southwest Colorado
Ryan Votta, assistant manage at the Durango Fish Hatchery, checks the water temperature on the Animas River on Wednesday. During the summer, temperatures can rise to the upper 60s, putting aquatic life at risk.

With rising temperatures and prolonged drought, Southwest Colorado’s rivers and streams are increasingly running lower and warmer, creating an uncertain future for fish and other aquatic life.

“Fish don’t sweat,” said Jim White, an aquatic biologist with Colorado Parks and Wildlife. “So if you’re a cold water fish, and the water temperature keeps rising on you, you don’t have a lot of options to cool off.”

Ryan Votta, assistant manager at the Durango Fish Hatchery, holds a net of Gunnison River rainbows to be stocked in the Animas River. Colorado Parks and Wildlife chooses to stock fish that are more tolerant to water quality issues.

Droughts, of course, spell lower water in rivers and streams, which means water temperatures can easily rise in summer months, sometimes to levels that can stress and kill fish populations.

Trout, for instance, are cold-water fish best suited for temperatures between 50 and 60 degrees. When temperatures rise past 70 degrees, trout can stop feeding and are more easily at risk of disease.

“And it’s not just a Southwest problem,” White said. “It’s a problem we’re seeing across the continental U.S.”

But with Southwest Colorado and the Four Corners the epicenter of extreme drought during the past few years, refuges for trout in smaller streams and tributaries are becoming perilously at risk.

Species shift

Fish in the Animas River battle urban runoff, leaching of heavy metals from abandoned mines around Silverton and sediment loading. And they’re certainly no strangers to above-average temperatures and low-water years.

More than 100 years of data show water levels in the Animas River are on a downward trend. And though temperature tracking dates back only three years, it’s well-known the river can jump to the high 60s on a daily basis during the summer.

Animas graph
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All these pressures cause fish in the Animas to not be able to reproduce.

Instead, the Animas’ robust trout population through Durango, which carries a Gold Medal designation from the state intended to highlight rivers and streams with outstanding angling opportunities, relies on CPW stocking trout.

As a result of water-quality issues, CPW tends to stock more hardy species that have a better chance at surviving. White said throughout local waterways, there seems to be a species composition shift as more tolerant fish endure.

“We’re seeing a lot more brown trout where there used to be rainbow and cutthroats because browns are more tolerant of warmer water,” White said. “And we’re seeing this across the state.”

Trouble in tributaries

Perhaps most concerning to researchers are the high country streams and creeks that serve as important habitat when main stems of rivers become unlivable.

A recent study of the upper Dolores River basin looked at what would happen to 40 or so trout streams under different climate scenarios, taking into account faster than normal temperature rises driven by climate change.

“And it’s not pretty,” said Duncan Rose, director of the Dolores River Anglers chapter of Trout Unlimited.

Water volumes have been decreasing in the Animas River since the late 1800s. Water temperature data doesn’t date back that far, but the river does rise in temperature to nearly 70 degrees in the summer.

If severe drought conditions continue, the upper basin could lose 44% of its stream flow over the next 50 years. Water temperatures would rise, habitat would be reduced and certain stretches could undergo ecological collapse.

Already, Rose said, impacts can be felt on the ground. Trout populations have not yet recovered in four streams in the upper Dolores basin that dewatered during the severe drought in 2018, he said.

“There’s a harsh reality of climate change in the upper Dolores,” Rose said.

Don’t forget bugs

Scott Roberts, an aquatic biologist with Mountain Studies Institute, said bugs that live in rivers and streams face the same challenges, too.

Aquatic insects gather in pools or the rapids of shallow rocky riffles, but when there’s less water in the river, those sections can become stagnant or run dry, and as a result, bugs can’t survive.

“We see it pretty readily (in the Animas watershed),” Roberts said. “We just hope not to see multiple years of drought in a row.”

In the Sierra Nevada, after a historic drought from 2012 to 2015, researchers saw significant declines in aquatic insects and other macroinvertebrates, according to a March 2019 study.

David Herbst, a research biologist at the University of California-Santa Cruz who co-authored the study, told the college’s newspaper that researchers documented declines in about 40% of common insects like mayflies and stoneflies.

“What we saw was pretty disturbing, with a diverse and rich fauna of stream invertebrates becoming decimated, especially as the smaller streams became intermittent and sections of the channel dried,” Herbst said.

What can be done?

In light of the host of challenges to the region’s waterways, many of which are driven by larger, outside global issues, Rose said the study on the upper Dolores ranked the streams researchers believed were most likely to withstand drought.

Warmer temperatures and lower water in rivers and streams across Southwest Colorado pose a risk to fish and other aquatic life.

That way, preservation and conservation efforts could focus on those streams, so if worst-case-scenario climate models materialize, aquatic life will have some strongholds where it might be able to survive.

“The engines of climate change are massive and way bigger than the resources we have to throw against it,” Rose said. “So with adaptation, we can try to enhance the natural process as much as we can. But we have to accept we’re going to lose streams and habitat as climate change occurs.”

There’s also simpler ways to help trout: don’t fish during the hottest times of the day, when they are most stressed.

“We recommend going up to a high mountain creek, where it’s not an issue,” said Ty Churchwell with Trout Unlimited. “Or if that’s not an option ... we recommend fishing either early morning or late in the evening.”

And this year, more than ever, the advice should be heeded.

Joe Lewandowski, spokesman for CPW, said fishing license sales are up nearly 25% compared with last year.


Animas graph (PDF)

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