Northwest Wisconsin's Brule River is revered by anglers for its steelhead and trout fishing. But the picturesque stream that tumbles northward into Lake Superior provides great habitat for more than just sport fish.
Each spring, thousands of sea lamprey head for the Brule to spawn. Many of them are trapped on their way upstream by a low concrete dam that spans the river.
"It was put here just for keeping the lamprey out of the upper reaches of the river," said Bill Mattes, who has helped trap and control sea lamprey for 17 years.
The sea lamprey was one of the first non-native species to invade the Great Lakes, and it's been one of the most destructive. The eel-like parasite nearly wiped out the entire lake trout population. But in one of the rare success stories about exotic species, scientists have devised ways to keep the blood-sucking fish in check. That has allowed lake trout to make a historic comeback.
Next to the dam, Great Lakes Indian Fish and Wildlife Commission fisheries aid Acorn Armagost lifts a grate from the top of a big concrete pen next to the dam. Water flowing down a series of steps allows jumping fish to swim upstream, but lamprey can't make the climb and are trapped.
DEADLY TO NATIVE FISH
Mattes, Great Lakes Section Leader of the Great Lakes Indian Fish and Wildlife Commission, watches his co-worker descends a ladder, scoops out a netful of the slithering, snake-like creatures, dumps them in a bucket, and grabs one.
"Go ahead and turn that baby over," Mattes said. "See that tongue down in there?"
A pointy tongue sticks out from the middle of a round mouth, about the size of a half dollar, filled with concentric rings of yellow hook-like teeth.
"That's the rasping tongue," Mattes said. "These other teeth, circle teeth are just for holding on, and that one is actually the one that breaks through the skin and then they suck the fluids out."
Tom Davies, a U.S. Fish and Wildlife Service technician, takes the lamprey and gently presses its mouth to the palm of his hand. He lets go and it dangles upside-down, its mouth suction-cupped to his hand.
"You can see that the teeth, they're a work of art as far as being able to latch on to something and really just stick," Davies said.
If you were to press the lamprey's mouth to your hand, it wouldn't hurt. But it's weird, and unnerving. And, worse, the thing doesn't let go, so you'd have to shake the lamprey off. The sea lamprey is not a danger to humans, as it only preys on cold-blooded creatures.
Great Lakes Indian and Wildlife Commission section leader Bill Mattes shows off a sea lamprey.
It is, however, a danger to native species. A single adult lamprey can grow as big as five pounds and literally suck the life out of 40 pounds of Lake Superior fish in a single year.
Lamprey were first discovered in Lake Superior in 1939. A new canal bypassing Niagara Falls allowed them to migrate all the way from the Atlantic Ocean, said Marc Gaden, communications director for the Great Lakes Fisheries Commission, which oversees the lamprey control program.
Gaden said sea lamprey quickly decimated the lake trout fishery.
"You read the literature, and you hear the commercial fishing operations and the people who made a living watched the fishery dissolve before their eyes," he said.
Over about two decades, the lamprey wiped out the lake trout in the lower four Great Lakes. In Lake Superior, only a few remained.
"And you think about it, this is the top species in the world's largest freshwater lake, and it got the stuffing knocked out of it by lamprey," Gaden said.
A LONG FIGHT
Scientists began frantically searching for a way to control lamprey. They struck gold in the 1950s when they discovered a chemical called TFM kills lamprey larvae while they're still the size of earthworms, before they swim into the Great Lakes and begin preying on fish.
Thanks largely to TFM and traps and barriers like the one on the Brule River, 90 percent of the lamprey in Lake Superior and the rivers that feed it are killed, Gaden said.
"That's amazing," he said. "That's a 90 percent reduction of a species that before that control was wreaking untold havoc on the fishery."
Three Minnesota DNR workers are among those trying to control the species. In a 25 foot Boston Whaler, they leave the shore of Lake Superior north of Duluth and head toward a 750-foot-long net they set deep in the water the day before. Every spring the DNR catches lake trout along the North Shore to count how many fish are suffering lamprey wounds.
A small motor slowly pulls in the long net. Every so often a gleaming lake trout comes over the edge of the boat in the net. DNR Fisheries Biologist Corey Goldsworthy carefully untangles each one.
"Another clean fish, no marks, no scars," Goldsworthy said. "It's kind of been the story of this assessment so far is a lot of native fish with no lamprey marks, which is great, it's what we want to see."
The lake trout they find are about five pounds and two feet long, silvery with a soft pinkish hue. Only one fish, a fat, deep-swimming species of lake trout called a siscowet, has a minor lamprey wound.
The fish they catch average about 1.6 fresh marks per one hundred fish, down considerably from the 4.9 marks per 100 fish last year.
"Over the last four or so years we've seen a steady decline in the number of fresh lamprey wounds on fish," Goldsworthy said.
The annual surveys indicate the lamprey control program is working, said Goldsworthy's boss, DNR area fisheries supervisor Don Schreiner.
"What it tells us is we're getting better at finding the source of some of the lamprey that's produced," he said.
When scientists find places where lamprey spawn, they hit those areas with TFM. According to the latest estimates, lamprey numbers have plummeted from nearly 1 million in Lake Superior, to only around 80,000. That and a lake trout stocking program has allowed the native fish to rebound slowly, enough to justify a small commercial fishery north of Two Harbors, Schreiner said.
"It's probably one of the best fisheries success stories certainly in the U.S. and probably in the world," he said. "Not very often do we get a chance to restore a fishery that's down to very minimal numbers."
LAMPREY HERE FOR GOOD
Still, Schreiner said, lamprey kill more Lake Superior trout than sport and commercial anglers combined.
As Mattes and his partner finish a day's lamprey trapping on the Brule River, lamprey trapping, a couple dozen flop around in small coolers. It's a small catch. Two days earlier the trap held 900 lampreys that became farm compost.
Mattes worries people may grow complacent now that lamprey are largely held in check.
"They don't really see it's an issue until you take the thumb off the controls for a couple years, then you would see big changes," he said.
Every year the U.S. and Canadian governments spend about $20 million on the lamprey control program. But the success story has a sober ending. The sea lamprey is the only species out of about 180 exotics to invade the Great Lakes that can even be controlled. Scientists say there's no hope it will ever be completely eradicated.