Cultivation of non-indigenous crops and livestock is the foundation of US agriculture. Introduction and cultivation of aquatic species is now believed to be economically profitable, also. There are numerous ways in which exotic marine and freshwater species are introduced, generally associated with human activity. Intentional introductions were also made for sport or food, as well. The two more recent sources of introductions are for the purposes of aquaculture, and via ballast water.
All cargo ships contain huge ballast tanks.
These tanks are filled at ports to help steady cargo-less ships as they travel and emptied once cargo is loaded. As ships travel faster and world trade grows, species are better able to survive the journey, and the threat of invasive species from ballast water increases. The US alone receives at least 21 billion gallons of ballast water each year from around the world, leading to problems like that of the well-known zebra mussels . Able to thrive, the mussels spread throughout the Lakes, as well as the Mississippi and Hudson River. They have displaced native freshwater mussels of the area, and drastically altered the food web.