4 | How do invasive species get here?

Ballast transport
Ballast water is fresh or saltwater (sometimes containing sediments) held in tanks and cargo holds of ships. Ballast water is pumped onto ships at different ports of call to add weight to ships that may be carrying little or no cargo. These ships are considered to have "ballast on board" (BOB). The added weight of the ballast water causes the vessel to sit lower in the lakes or ocean, thus providing stability and maneuverability during voyage and clearance under structures, such as bridges, in waterways. Even ships that have "no ballast on board" (NOBOB) may have water and sediments that could not be completely pumped out of the ballast tanks.

Once NOBOB and BOB ships arrive in another port of call, the water held in ballast tanks is discharged, exchanged or water is added. This ballast exchange may occur at multiple ports of call during a vessel's long journey. Larger vessels often carry millions of gallons of ballast water and, with more than 80 percent of the world's commodities carried on ships, that is a lot of water traveling around the globe every year!

But it is not only water or sediments that goes into ballast tanks. When ballast water is pumped into the ship at a port, it includes organisms found in that port. These organisms range in size from microscopic phytoplankton and zooplankton to crabs and fish. According to MIT Sea Grant, it is estimated that as many as 3,000 exotic species are carried in ships' ballast every day around the globe.

Those organisms that survive the trip are ultimately released when the ballast water is discharged at the final port destination. In many cases, the nonindigenous species are not kept in check by the indigenous predators and competitors they might find in their native waters. Therefore, they have a very good chance of increasing their population and out-competing native organisms for food and space. Though not all species brought in ballast water have disastrous effects on the Great Lakes (or even survive the trip), zebra mussels are a good example of the ecological and economic damage ballast-borne invasive species can cause.

Other routes of invasion
Although ballast water is considered the primary pathway of invasion into the Great Lakes, nonindigenous species have been introduced to the lakes in the following ways:

• removal of physical barriers during the building of canals (such as the Chicago Sanitary and Ship Canal)
• accidental introduction or unintential release of organisms through aquaculture ("fish farming"), the aquarium trade, or the use of exotic bait for fishing
• through the stocking of sport fish

Graphic: Ballast water release. Courtesy L. David Smith/MIT Sea Grant.

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