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Lampreys look like the stuff of horror films: a slithering, tubular body topped with a suction-cup mouth ringed with row upon row of hooked yellow teeth. With this mouth, a sea lamprey anchors to its fish prey and uses its rasping tongue to drill into the victim’s flesh. It remains there for up to a month, feeding on blood and body fluids. Even if a fish survives the attack, the gaping wound left behind often results in death.

In their natural ranges, lampreys are important components of food webs. The problems begin only when they shift from native to invader.

Sea lampreys slipped into Lake Ontario through the Erie Canal in the mid-19th century, and then made it past Niagara Falls around 1919 with the renovation of the Welland Canal. In the lakes, lampreys found a utopia: no predators, and bountiful prey that had no natural defenses against their voracious appetites.

Biological disaster ensued. By 1940, lampreys had colonized the entire Great Lakes system. Harvests of lake trout fell from around 18 million pounds a year to less than 300,000 pounds; other species, especially whitefish, also took significant blows. Unchecked, lampreys killed an estimated 110 million pounds of fish each year.

Desperate to find solutions, Canada and the United States in 1955 established the Great Lakes Fishery Commission. Its researchers tested nearly 7,000 chemicals on lamprey larvae and fish to identify one that would kill the invaders, with minimal effect on other fishes. Eventually, they settled on 3-trifluoromethyl 4-nitrophenol, or TFM, a metabolism-targeting poison applied to larvae-infested streams. At the same time, the commission built about 70 lamprey barriers to limit migration.