In 1919, during construction work on the Welland Canal in Ontario, engineers unknowingly helped sea lampreys — a parasitic fish from the Atlantic Ocean — spread into the Great Lakes. This accidental move had devastating consequences for the region’s ecosystem, particularly for the lake trout fishery. Sea lampreys had been in Lake Ontario as early as 1835, likely entering through the Erie Canal, which was completed in 1825. Before this, Niagara Falls had acted as a natural barrier, keeping the lampreys from reaching the other Great Lakes. However, when the Welland Canal was deepened in 1919, it allowed the lampreys to bypass the falls and move into the rest of the Great Lakes. Sea lampreys are unique creatures that lack jaws but have a powerful, suction-like mouth lined with teeth. They attach themselves to fish and feed on their blood and body fluids, often killing their host in the process. Native fish in the Great Lakes, such as lake trout, had never encountered this predator before and had no natural defenses against it. The impact was severe — only about one in seven fish attacked by lampreys survived. By the late 1950s, lake trout had vanished from Lakes Ontario, Erie, Huron, and Michigan, and their numbers were severely reduced in Lake Superior. This decline happened so quickly that it could not be fully explained by overfishing or pollution alone. The collapse of the lake trout fishery had wide-reaching effects on both the environment and the economy. With lake trout gone, another invasive species — the alewife — thrived in their absence, causing additional ecological problems. In response, the Great Lakes Fishery Commission was created in the mid-1950s to manage the crisis. This binational effort between the United States and Canada included using chemical treatments called lampricides to kill lamprey larvae, installing barriers and traps to stop their movement, and releasing sterilized male lampreys to reduce reproduction. These strategies have been largely successful, reducing sea lamprey populations by about 90% in most parts of the Great Lakes. However, complete eradication has not been achieved, and monitoring will likely continue for many years. Lake Superior’s lake trout population has shown signs of recovery, with numbers now comparable to those before the lamprey invasion. While the damage caused by the sea lampreys was immense, the coordinated response by scientists and policymakers has helped restore some balance to the Great Lakes ecosystem. The story of the sea lamprey serves as a reminder of how human engineering can sometimes have unintended environmental consequences, but also of the power of sustained, collaborative conservation efforts.