Over the last few centuries, the Great Lakes have been colonized by many non-native species. One of the first of these species is the sea lamprey, which was discovered in Lake Ontario in the 1800s (it has a contested status as native or non-native in Lake Ontario). They likely moved into the Great Lakes once canals were completed connecting their native range along the Atlantic coast directly to the Great Lakes for commercial shipping lanes, but had possibly been there a long time. Once another canal connecting Lake Ontario to Lake Erie was completed around 1900, sea lamprey were able to get around Niagara Falls and into the rest of the upper Great Lakes, where they were not native.
Sea lamprey are anadromous, like salmon, they are born in freshwater and live there a short while before migrating to the ocean to grow to adulthood. They then swim upstream and return to their birthplaces to spawn the next generation. This is how they were able to colonize the Great Lakes once canals were built. They have been known to travel as far as 320 km upstream from the ocean.
The Great Lakes populations have evolved such that they no longer have direct contact with the ocean. Their ocean equivalent is the Great Lakes. Because sea lamprey feed off of large fish as parasites, they have been a detriment to all those that make their living from Great Lakes fisheries for the last 100 years. The fisheries in the Great Lakes are valued at around 7 billion US dollars, so there are many programs that try to reduce sea lamprey populations. Some programs use selective pesticides known as lampricides, to specifically target lampreys, and some use barriers like dams to prevent movement and reduce spawning opportunities upstream. New technology has allowed new kinds of barriers that are temporary and not physical. Electrical barriers have been tested for the last 70 years, but new devices are portable and can be deployed over a shorter time frame when lamprey migration is at its peak, making a difference against the lamprey, while also reducing impacts on non-target species. There are still concerns over non-target species, but it is an improvement over past technology.