Are our trout streams cooling because of climate change or will warmer weather evict trout from some streams?
It turns out, there are no easy answers. Like so many things dealing with climate change, it’s complicated, even for experts.
Cooler water with warmer air seems contradictory. But here’s the hidden side: Climate change in this region means much more precipitation. That means more water into our complex karst geology with so many layers of rocks. When it comes out of those rocks, it’s often around 48 degrees; that cooler water would enhance existing streams and maybe add more miles of trout water.
Brown trout have been found in a few branches of the Root River -- a tributary of the Upper Mississippi River in southeastern Minnesota -- where water was much too warm for cool-water-loving trout. Finding trout there once, maybe, but not three or four times over the past several years. So, why are they there?
Lonnie Hebl, 51, said his stepson, Dylan Hoot, 13, has been catching brown trout in the North Branch Root River. Hebl said he caught a few trout there back in the 1970s or 80s, but nothing after that.
Then Hoot caught a nice trout last year.
“I really didn’t know what it was,” he said. “I’d never caught a brown trout before.”
“I thought holy smokes, I haven’t seem a brown trout caught out here in years,” Hebl said. In the past year or so, in fact, Dylan has caught three trout, and he said a neighbor caught one. That stretch of the Root River is maybe 15 miles from any designated trout water.
As he fished recently for whatever was biting, Hoot said the neighbor also reported seeing a much larger trout in the river. He looked down and said he saw an 8-inch brown at the head of a riffle.
The experts' overall answer to this is “it’s complicated.”
Here is what some of them said:
- Jeff Green, regional Department of Natural Resources hydrologist, said we are getting much more rain so yes, it’s indeed possible that water could be cooler. He added another twist: Conservation tillage or retiring land from production. That would further let more water seep into the ground instead of running off. “It does make some sense,” he said.
- Tony Runkel, chief geologist with the Minnesota Geological Survey, agreed “it’s conceivable.” But he said looking decades down the road, water could warm because ground water is the average annual temperature for an area and it would go up with warming. Also, some springs are closely connected to the surface so water pours out soon after it rains, leaving less time for it to cool, he said. “That is not adding a whole bunch of cold water,” he said.
- Dr. Dylan Blumentritt, a Winona State University geoscience assistant professor, like Runkel, said how change affects the streams depends on how deep ground water is before it enters the stream. Not all ground water is 48 degrees, he said. “This is a really complicated system,” he said. “It takes a lot of smart people to try to figure this stuff out.”
He added that he also worries that heavy rains are washing more and more sediment into streams and rivers, also damaging or destroying habitat.
A 2010 study of fish distribution in Wisconsin, predicted a dire future for cold-water trout in that state over the next 50 years. “Overall, declining species lost substantially more stream length than increasing species gained. All three cold‐water and 16 cool‐water fishes and four of 31 warm‐water fishes were predicted to decline, four warm‐water fishes to remain the same and 23 warm‐water fishes to increase in distribution. … Results of this study suggest that even small increases in summer air and water temperatures owing to climate warming will have major effects on the distribution of stream fishes in Wisconsin.”
A 2013 University of Minnesota study of trout streams in the region also said the whole thing is quite complex but in general, it doesn’t look good for trout because change will mean streams will be “thermally unsuitable. … Generally, under such conditions summer cold water habitat within these streams will be restricted to headwater and progressively smaller areas downstream of groundwater-fed springs during the summer.”
Climate change is also going to affect the Mississippi River in ways many experts never thought of. Megan Moore, DNR team leader for the Long Term Resource Monitoring of the Mississippi, said the river has been able to adjust to all the high water that has been plaguing it for years. But it can flush zooplankton out of the system so fast that many fish that depend on it can’t get it, she said. Just how bad this is is unknown, she said.
In other words, it’s complicated.