ALONG THE FRENCH RIVER — Josh Dumke was squatting in the grass, just out of the stream, carefully holding a tiny brook trout in one hand and a syringe in the other.
It looked like the little fish, sleepy and calm after a quick bath in water spiked with clove oil, was about to get an insulin shot, or maybe a tetanus booster.
Instead, Dumke was injecting a dab of paint just under the fish’s scales, a marker that would identify the trout as being caught below beaver dam No. 3 in this stretch of river and above beaver dam No. 4.
After it woke up, the little trout was set back in the river to swim away, sporting its temporary tattoo.
Dumke, a senior research scientist at the Natural Resources Research Institute at the University of Minnesota Duluth, is marking fish up and down the French River just outside Duluth this month. He has four colors and, by making them in different locations on the fish, he has a different variation for fish in each of 11 different stretches of stream he’s studying.
When the fish are recaptured after three weeks — Dumke and crew use electro-shocking to temporarily stun the fish just long enough to be scooped off the surface — researchers will be able to tell how far up and downstream they went, and how many beaver dams they passed over, under or through.
“We want to know what impact beaver dams are having on fish movement. Are the dams really the impenetrable barriers that some people assumed? Or are fish getting past them in high-flow events after rainfalls?” Dumke said. “It’s pretty certain they can get down through the shorter, leakier dams. But the higher, tighter dams? We don’t know yet.”
This time of year, many North Shore streams near Duluth are generally low-flow because they don’t have cold-water springs or a lake at their headwaters. Most start in swamps. But they can flash to higher flows after moderate or heavy rains. Even a quarter inch of rain can make a big difference. The study also is using flow meters, dozens of in-stream thermometers and trail cameras near the dams to gather data,
“Hopefully, we’ll get a good rain between now and then (retrieving the fish) so we can see how much movement there is,” Dumke said.
He’s confident he’ll recapture some fish downstream of spots where they were first captured, pushed down by rain and flow. He’s not as sure how or if fish can get through or over dams going upstream against the current.
I don’t know,” he said. “We’ll see.”
For as long as anglers have been throwing worms and feathers and spinners into trout streams, there’s been one word that always raised anglers’ wrath: Beavers. Actually two words: Beaver dams.
Common wisdom is that beaver dams slow fast-running streams, making them warmer and less hospitable to cold-water-loving trout. The ponds dams create also allow sediment to build up where there is no current, covering important gravel spawning habitat. And, maybe worst of all, beaver dams can block fish from swimming up and down streams, isolating populations that should be mingling, mating and moving to where conditions are best, not just where beavers allow them.
But emerging science, especially in western U.S. trout streams, seems to show beavers are less of a nemesis to trout than believed, and maybe even a benefit.
“Out west, they are reintroducing beavers as part of stream restoration efforts on trout streams. It’s basically the exact opposite of what is done here,” said UMD Prof. Karen Gran, who heads the school’s Earth & Environmental Sciences department and who is an expert on river hydrology.
Gran is leading the beaver dam study that’s funded by Minnesota Sea Grant. While Dumke is looking at dam impacts on fish movement, Gran is looking at dam impacts on water flow.
Already, Gran noted, scientists know some of the old beliefs on beaver dams may not be true. Beaver ponds may have warmer surface water than upstream riffles, but those same pools can actually hold colder water at deeper levels. Those ponds can serve as cold water refuges for trout during warm, dry periods.
In other states, Gran noted, stream experts are mimicking some beaver dam characteristics in rivers to help prevent erosion, slowing high flow events to keep the river more clear for trout.
Gran is especially looking at the Knife River, where extensive beaver trapping has occurred since the 1990s as part of a DNR effort to keep the stream open in the name of steelhead trout. But some residents along the Knife said that, since beavers have been trapped and dams removed, stream flow in summer months has actually declined. This month, between rains, some upper reaches of the Knife have entirely run dry between isolated pools of water.
“There is some thought that beaver dams may actually help store water, recharge the water table, the shallow groundwater, that’s released more slowly and helps fill in during these low-flow months, like we’re seeing right now,” Gran said. “The (old school of) thought had been that beaver dams reduce flow. But that may not be the case.”
Gran is looking at several stretches of the upper Knife and its larger tributaries, some of which have had beavers trapped and other where beavers are being left to do their thing, at least during the study. The comparison should net some interesting results.
“We’ll have two years of data to see if we can sort it out,” she said.
TROUT PASSING SOME DAMS
As for how beaver dams impact trout movement, so far the answer is: It depends. In the stretch of stream we visited there are dams every 40 or 50 yards, some old, some new. In some stretches, researchers found dozens of fish. In others, just a few.
“Every dam is different,” Dumke said.
“The older, inactive dams tend to be leaky and have more places where fish can get through. They are slowly losing their effects over time,” he noted. “Some of the 6-foot-high active dams, where there is a lot of mud being applied constantly, are probably more effective at stopping most of the fish … except maybe when they are breached during high-water events.”
On one morning, the NRRI crew found more than 100 small trout, both brookies and steelhead rainbow trout — as well as native creek chubs, black nose dace and pearl dace minnows — in just one 100-foot stretch between dams. The brookies are native. The rainbows were stocked the last two springs in hopes they would “imprint” on the French River while getting bigger during their first two years of life and then somehow make it downstream past the beaver dams and migrate into Lake Superior. That’s where they’d grow big before returning to the mouth of the French to spawn. And that’s where they would be caught, by anglers on either the lake or in the lower river.
Over the past two years, more than 36,000 tiny rainbows have been stocked in this part of the French River, near Pioneer Road just upstream of the spot we were at. So the fact they are getting past beaver dams and downstream this far is good news for the Minnesota Department of Natural Resources’ new North Shore fishing program. The DNR is counting on these rainbows to eventually make it down to the lake, where they are hoped to make up for the discontinued Kamloops rainbow trout stocking program.
“I’m really glad you guys (NRRI) are finding the steelhead in here, because we really didn’t know if they would make it down this far,” said Nick Peterson, DNR migratory fish specialist, as he watched the electro-shocking effort. “I was a little worried they wouldn’t be in here.”
Peterson said staying in one place is not usually a good plan for trout in any stream, especially those that run low in late summer, and that easy movement up and down streams usually produces the best chance for survival and growth. The beaver study might help guide where and how many trout the DNR stocks in streams where beaver dams are present.
“Beaver activity in our streams often results in higher water temperatures in those areas (where dams make streams wider and deeper) sometimes where water temperatures exceed stressful and lethal temperatures for trout,” Peterson said. “That said, all beaver dams are not created equal. It’s also important to consider the characteristics of each beaver dam when determining the overall impacts of beavers on stream fish.”