USFWS

Warming rivers threaten iconic Michigan fish.

A beloved, cold-loving state fish is in danger of overheating.

GRAYLING, Mich.—If the word “tricos” means anything to you, the Au Sable River is the place to be in late summer.


Tricos are tiny mayflies, hatching abundantly in August. The imitations tediously tied by fly anglers are, of course, just as tiny. On a clear Sunday earlier this month, I was lobbing these pinky fingernail sized flies on particularly productive stretch of the river known locally as the “Holy Waters.”

For some the name rings true, as this is akin to church. The speckled, cosmic colored brook trout, Michigan’s native river trout and official state fish, pop all along the watershed at tricos—real and fake.

The river is cold—Michigan’s brook trout, a sensitive species, prefer water between about 43 degrees and 53 degrees F and can get stressed in water in the high 60s. The Au Sable, Michigan’s most fabled trout stream, has strong groundwater recharge, clear water and all kinds of springs and seeps that provide the cold-water brook trout need to survive.

While the Au Sable might be buffered against rising air temperatures, that’s not true across the state. And, in some streams, climate change could spell big trouble for the little fish.

“Look, there are a lot of streams that are very cold and we’re going to have brook trout for a long time,” said Bryan Burroughs, the executive director of Michigan Trout Unlimited. “But we’re looking at losing places without strong populations in the future. And we’re probably not going to gain much ground at all.”

"We're looking at losing places without strong populations in the future." -Bryan Burroughs, Michigan Trout Unlimited

Not all Michigan rivers—there are more than 300 named rivers and many more streams and creeks crisscrossing the state—have brook trout. Those holding brook trout are, for the most part, concentrated in water in the northern part of the Lower Peninsula, and throughout the Upper Peninsula.

And some are set to change, perhaps too much for the beloved brookies.

A recent study led by Michigan State University found water temperatures in those rivers that are mostly fed by groundwater are set to increase by 0.2 to 6.8 degrees F by the year 2056. Water temperatures in rivers fed by surface runoff are set to warm by 0.4 degrees F to 12 degrees F over that time.

“Any places marginally cold now will fall off,” Burroughs said.

The trend is disturbing—from 1976 to 2006, the average temperature of Michigan streams that hold brook trout increased 1.3 degrees Fahrenheit. Air temperature increased about 4 degrees F over that time.

“As temperatures increase the potential for a river to support large populations of brook trout declines,” said Troy Zorn, a research biology specialist with the Michigan Department of Natural Resources. “Up to 69 (degrees F) or so you could have survival but not a lot of reproduction.

“But around 70 [degrees] you’re pretty much done with brook trout,” he added.

Zorn said there isn’t a lot of a long term temperature monitoring for many streams so it’s difficult to say how many brook trout have been impacted by warming to this point.

But “certainly some systems are likely to become too warm,” said Andrew Carlson, a doctoral student at Michigan State University who led the study.

In a previous study of select rivers, Carlson and colleagues reported that rising temperatures (using various climate models) would render habitat “suboptimal” in the Carp and Escanaba rivers and Bryan Creek in the Upper Peninsula, and the Canada, Martin, Cedar and Duke creeks in the Lower Peninsula.

Carlson said brook trout are most likely to experience stress in summer months—June, July, August—due to the warm air. Warmer water is lower in oxygen and can stress fish out. If it gets too high, it can kill them and impact populations. And it can give an upper hand to parasites like gill lice.

“Every fish has a certain thermal range where metabolism operates efficiently,” Carlson said. “They have a scope for growth, metabolism and energy expenditure, and for brook trout they operate under much cooler temperatures than say bass, or sunfish.”

With warmer temperatures, there can be a delay in development of embryos as well, said Cindy Chu, a freshwater ecologist researcher at the Ontario Ministry of Natural Resources and Forestry. “There’s also an increase in predators and the ability of fungus to establish on eggs … all can lead to a decrease in reproduction.”

Michigan is not alone. When researchers examined eastern U.S. brook trout streams from Georgia to Maine they reported that only 5 percent of the watersheds had populations of brook trout occupying more than 90 percent of historical habitat.

“Wild stream populations of brook trout have vanished or are greatly reduced in nearly half of the watersheds,” the authors wrote in the 2011 report.

Even out West, researchers fear the end of some species as they cross-breed with invaders thriving in the warmer water.

A 2016 study on Ontario's Lake Simcoe watershed found that brook trout-friendly cold water habitat has reduced 27 percent over the past century and is estimated to reduce another 59 percent by 2065 due to a warming climate.

When Minnesota researchers estimated climate change impacts to three streams in the Lake Superior watershed they reported a 2.6 degrees F increase in July average water temperatures over the next 50 years, according to the 2015 study.

While climate change is a global problem, Shannon White, a Ph.D. candidate at Penn State University, said there are local actions that can make a difference. “If I had to pick one battle, it’d be to improve population connectivity,” said White, who studies brook trout and runs an approachable blog documenting her research.

“As temperatures increase the potential for a river to support large populations of brook trout declines."-Troy Zorn, Michigan DNR

Bridges and dams isolate populations of brook trout. By reconnecting populations, it makes them more resilient to warming waters and other climate-related changes such as increased flooding, White said.

“When it’s feasible, we should remove dams and similar structures, it allows the fish to access colder habitat,” Zorn said.

Burroughs said Trout Unlimited, along with “too many partners to name”, are aiming efforts at reconnecting streams and removing some of the more than 3,000 dams in the state.

He said some dam removals have raised downstream water temperatures by as much as 12 degrees F in some cases.

Zorn admits his agency is spread thin and it’s hard to monitor every stream.But he thinks the state should have brookies for a long time—just maybe not in the same spots.

“The Holy Waters? It’ll be just fine,” he said. “But some spots they may vanish and we wouldn’t know for 10 years.”

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