Archived Outdoors

Putting the squeeze on trout: Study says that acid and warming waters could make life harder for trout

out frFrom habitat destruction to competition with non-native trout, Southern Appalachian brook trout have met their share of challenges over the past century. A new study illuminates another issue that trout — and not just brookies — might have to contend with in the years ahead.

Actually, a pair of issues — acidity and warming water temperatures. Neither of these are newly identified problems, but the study looks at their combined effect. The verdict?

“You’re forced into a much narrower range,” said Andrew Dolloff, a fisheries biologist with the U.S. Forest Service and one of the study’s lead authors. Others helping with the study include representatives of E&S Environmental Chemistry Inc. and Oregon State University.  

Both acidity and warming temperatures have the effect of reducing suitable habitat for coldwater fish like trout, but they work at opposite ends of the elevation profile. At the lower elevations, warming temperatures make the water too toasty for trout. Meanwhile, acidity is a bigger problem high in the watershed.

“As water comes down lower in the watershed, it’s able to dissolve minerals and stuff from the soil and buffer the pH from the streams,” Dolloff said. “The higher you go, the less of that there is.”

The net effect, the study says, could be shrinking habitat as downstream areas warm and headwaters continue to hold acidity.

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Dolloff has been involved in freshwater stream research throughout his 28 years at the Forest Service’s station in Blacksburg, Virginia. In the 1990s, he worked on an acid rain study that showed acid deposition to be a real problem for the ecology of freshwater streams.

Since then, things have been looking up where acid is concerned — cleaner technology and stricter air quality regulations have greatly reduced the rate of deposition — but acidic compounds still linger in the streams, especially in the headwaters where there’s less material available to counteract the acid.

“It’s going to take a long time for streams to recover,” Dolloff said.

More recently, though, he’s been working on the temperature side of things, getting funding together to look at 200 coldwater streams in the Southern Appalachians deemed representative of the tens of thousands of such streams in the region. The team installed two temperature probes in each stream — one in the water and one in the air — with those probes taking readings every 20 minutes. From those readings, they took the temperatures from the month of July and developed an average temperature for that month for each of the 200 streams. Those results were paired with the expected temperature increase climate change models suggest. That allowed the team to project which streams are likely to become too warm to accommodate trout.

“Some places that are blue (color representing adequate coolness on the study map) they’re going to be blue no matter what,” Dolloff said. “But there are others where that’s not true, where things aren’t ideal.”

Dolloff’s team paired its findings on temperature with data on acidity in the Southern Appalachians adapted from a 2013 study published in Water Resources Research.

Between acidity and temperature, some places in the study area are projected to have extremely limited habitat for trout going forward. The North River in Virginia’s George Washington National Forest, for example, is projected to have only 90 kilometers, or 2.3 percent, of its length suitable for trout. According to the study, 93.5 percent of the river will be too warm for trout and 12.3 percent will be too acidic.

Things in Western North Carolina, however, are looking a lot better than that. Overall, 42.7 percent and 37.6 percent of studied streams in the Pisgah and Nantahala national forests, respectively, are projected to remain suitable, with projections going as high as 68.4 percent suitable for the Highlands district of the Nantahala National Forest.

Elevation plays a big part in those results, Dolloff said. In terms of percentages, warming is a bigger culprit than acidity in limiting trout habitat, and WNC has plenty of streams with high elevation to support low stream temperatures.

But WNC is in good shape for another reason, as well — its large tracts of intact forests. That undisturbed acreage shades the streams, reducing the amount of warmth they absorb from the sun. It also means that connections with other waterways in the watershed are more present than in developed land, giving trout and other aquatic creatures more options when a heat wave hits or a landslide buries a stream section.

“If you force all the animals up to isolated streams that don’t allow the pieces to connect, that leaves them pretty vulnerable,” Dolloff said. “With birds or salamanders or whatever, in the case of birds they can fly and salamanders, they can walk to get to that drainage where they can have genetic mixing and have some measure of protection, but a fish, you can go up or you can go down, and that’s it.”

Dolloff doesn’t necessarily fear that warming temperatures and acidity will combine to eliminate trout species from coldwater streams. At least not directly.

“If you’re squeezed from above and squeezed from below and there’s some other catastrophic events and you wipe out these isolated populations, there’s no way for them to repopulate without human intervention,” Dolloff said, adding that reintroductions are fraught with all sorts of complications regarding genetic variation within a species and a specific specimen’s fitness to survive in a specific environment.  

Dolloff is the first to say that his study is not the be-all and end-all of research on this topic. For starters, the study used just one year’s worth of data, and ideally the research should be replicated across years to get more solid results. But, he said, it’s indicative of the issue, and climatic effects on coldwater fisheries is a topic he intends to study further.

For instance, research on the aquatic communities of 20 of the 200 streams in the temperature study is underway, with early results indicating that some streams labeled “coldwater” already look more like warmwater systems. There are plenty of fish in those streams — they just aren’t trout. They’re species that are more at home with warmer temperatures.

“There’s all kind of ways that increasing temperatures can turn things topsy-turvy in ways you might not expect,” Dolloff said.

The study is just one piece of the larger puzzle of the whys, whens and whats of climate change. But Dolloff hopes the results will give people who live near to these mountain streams “a sense of awareness of the fragility of some of these areas.”

“Even if you don’t like to eat trout, they’re really good canaries in the coalmine,” he said. “They’re really strong indicators. If a trout can’t live there, something is wrong.”



Read it yourself

The study, “Downstream Warming and Headwater Acidity May Diminish Coldwater Habitat in Southern Appalachian Mountain Streams” is available online at

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