ALONG THE FRENCH RIVER, Minn. _ 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."
