It looks like a giant leech. About 18 inches long, an inch in diameter, with a circular mouth lined with rows of teeth that suction to the sides of fish. And it has a needle-sharp, rasping tongue that bores a hole through the scales, so it can suck out the blood and fluids.
Meet the sea lamprey, fittingly known as the "vampire of the Great Lakes." It was one of the earliest aquatic nonnative species to invade the U.S., long before zebra mussels and Asian carp garnered headlines.
And it's been one of the most destructive invasives. It first swam into Lake Erie in the 1920s through the Welland Canal around Niagara Falls and quickly killed millions of fish.
"It's caused tremendous damage," said Doug Jensen, Aquatic Invasive Species Program Coordinator at Minnesota Sea Grant. "It's called the 'Vampire of the Great Lakes' for good reason. It made lake trout go extinct in all the Great Lakes except for Lake Superior."
But in one of the few invasive species success stories, scientists have devised a way to keep sea lamprey largely in check in the Great Lakes, which has allowed the lake trout population to rebound.
But in the 1950s scientists developed a chemical called TFM that kills sea lamprey when they're still small larvae in streams — before they grow their suction mouth and rasping tongue and swim out into the lakes.
The Great Lakes Fishery Commission now treats about 200 rivers around the Great Lakes. This lampricide, combined with other control efforts, including barriers built across streams to prevent lamprey from accessing spawning territory, has helped cut the lamprey population in the Great Lakes by 90 percent.
In Lake Superior, state fisheries managers recently ended a longstanding stocking program, concluding hatchery-raised fish were no longer necessary to build the population. And for the first time in decades, they opened a small commercial lake trout fishing season along the North Shore near Duluth this year.
"It's kind of a culmination of 50-60 plus years of rehabilitation coming full circle," said Cory Goldsworthy, Lake Superior Area Fisheries Manager for the Minnesota Department of Natural Resources.
Last week, Goldsworthy and two other biologists set a 250-foot-long net off the shore of Lake Superior in Duluth to catch lake trout as they came into shore to spawn so they could assess the health of the population.
They slowly pulled the net on to the boat, every so often stopping to untangle a glistening, big lake trout from the gill net.
Most of the fish looked fat and healthy. Several had old lamprey wounds on their sides that had healed over. Only one had a gaping wound where the hole bored by a lamprey's rasping tongue was still visible. "So what we're seeing is good. Really good," Goldsworthy said. Close to Duluth, only about five out of every 100 lake trout they survey show lamprey wounds, he said, which is right around the target goal established by the Great Lakes Fishery Commission.
But farther up the shore, Goldsworthy said, the rate is higher, about 10 percent. And it's even higher at the tip of Minnesota, off the shore of the reservation of the Grand Portage Band of Lake Superior Chippewa.
"What we saw in 2017, was closer to 15 or so wounds per 100 fish," said Seth Moore, director of biology and environment for the band. He said they've seen a four-fold increase since 2015.
Over the past decade, he said the band has seen a very slight decline in the abundance of lake trout band members are catching.
"We're concerned about lake trout mortality, both lakewide and locally," Moore said. "This is a subsistence community, and tribal members harvest lake trout both for individual subsistence, and also they sell them commercially."
While lamprey numbers are stable in lakes Michigan, Huron and Ontario, they've increased in lakes Superior and Erie, the Great Lakes Fishery Commission recently reported. The comission isn't sure why.
Moore suspects there could be more lamprey spawning in rivers near Thunder Bay, Canada, that aren't treated with chemicals.
Warmer water temperatures could also be helping, the Fishery Commission said, along with the rebounding lake trout population, which gives the lamprey a larger base of prey to feed on.
Biologists like Moore aren't losing sleep over the spike in lamprey numbers, at least not yet.
But the control program costs about $20 million a year, and those costs could rise soon. One big concern is the St. Louis River. State and federal agencies have spent tens of millions of dollars to clean up contaminated sediment in the river.
Biologists say that could make the river an excellent producer of lamprey in the near future.
"The amount of available rearing habitat in the St. Louis river estuary right here is unbelievable," said Goldsworthy. "And if it does produce, it's huge, so the amount of money to treat that is incredible."
The scariest thing of all about sea lamprey is they're here to stay. Even though the control program has slashed lamprey numbers by 90 percent in the Great Lakes, there are still about 800,000 at any one time.
And those lamprey still do a lot of damage, said Jensen with Minnesota Sea Grant.
"There's still more lake trout killed by sea lamprey predation in Lake Superior than sport and commercial fishing combined."