As with many non-native and invasive species, snakehead fish have no natural predators in the United States. This allows for competition and consumption of native fish species. Additionally, Northern snakehead fish can survive in waters with low oxygen levels, giving them a competitive advantage over native species.
Round gobies originated in the Black and Caspian Seas. They were introduced into the Great Lakes by ballast water discharges from ships and were first discovered in 1990 along the St. Clair River (a Canadian river north of Detroit). Round gobies are a threat because of they are capable of rapid population growth after they reach new areas.
Sea Lamprey are primitive, jawless fish native to the Atlantic Ocean. In 1921, lampreys appeared in Lake Erie for the first time, arriving via the Welland Canal, which was constructed for ships to avoid Niagara Falls on their way up the St. Lawrence Seaway. Shortly thereafter, sea lamprey quickly populated all of the upper Great Lakes.