MIT Sea Grant Center for Coastal Resources

Marine Bioinvasions Fact Sheet: Ballast Water

What is Ballast Water?

Ballast water is fresh or saltwater held in the ballast tanks and cargo holds of ships. It is used to provide stability and maneuverability during a voyage when ships are not carrying cargo, not carrying heavy enough cargo, or when more stability is required due to rough seas. Ballast water may also be used to add weight so that a ship sinks low enough in the water to pass under bridges and other structures.

Ballast Water Vocabulary:
  • In Ballast: A ship carrying ballast water and no cargo.

  • With Ballast: A ship with cargo and ballast water.

  • No Ballast On Board (NOBOB): A ship that is technically carrying no ballast water. A NOBOB ship may be carrying residual ballast water and sediments that could not be pumped out of the tanks.

  • Ballast Exchange: The process of releasing ballast water then taking on new ballast water.

Usually ballast water is pumped into ballast tanks when a ship has delivered cargo to a port and is departing with less cargo or no cargo. Ballast water is then transported and released at the next port-of-call where the ship picks up more cargo. If a ship is receiving or delivering cargo to a number of ports, it may release or take on a portion of ballast water at each port. In such cases, the shipís ballast water contains a mix of waters from multiple ports.

Why should we be concerned about Ballast Water?

Organisms living in coastal waters may be pumped into ballast tanks along with the water. If a ship takes on ballast water in a shallow area, sediments and any associated organisms may be pumped into ballast tanks. When ballast water is released, these organisms may also be released.

The release of ballast water may introduce non-native organisms into the port of discharge. These introduced species, or bioinvaders, are also referred to as exotic species, alien species and nonindigenous species. Typically, very few organisms are able to survive in new surroundings because temperature, food, and salinity are less than optimal; however, the few that do survive and establish a population have the potential to cause ecological and economic harm. Populations of bioinvaders may grow very quickly in the absence of natural predators. In turn bioinvaders may displace native organisms by preying on them or outcompeting native species for food and habitat space. Economic damage may occur when a bioinvader displaces species that are harvested for food or other goods, or when bioinvaders damage structures.

Mnemiopsis leidyi, a comb jelly (similar to a jellyfish) feeds on anything smaller than itself that gets stuck to the sticky lobes near its mouth, especially zooplankton. It was probably introduced via ballast water from New England (USA) into the Black Sea. In its new environment, Mnemiopsis has no natural predators and has outcompeted native species for food. As a consequence, the once profitable anchovy fisheries in Russia and Turkey have almost disappeared.

One of the most widely known invasions in the United States is that of zebra mussels into the Great Lakes. These organisms grow on almost any structure, forming large clumps of mussels which can clog water intake pipes and damage or impair other structures. Periodically these organisms need to be removed from pipes and other structures which requires time, money, and possibly specialized equipment.

Another example is Teredo navilis, a ship worm that is actually a bivalve (a relative of clams), that was introduced to the United States during colonial times on wooden ships. Ship worms are the termites of the sea, they bore holes into wood and damage wooden docks, piers, and seawalls. This damage has prompted the treatment of wood with chemicals that discourage Teredo from attacking the wood, however these chemicals pollute the surrounding water as they leach from the wood.

Not all invasions have disastrous effects. Populations of bioinvaders may become economically profitable if they are harvested for food or commercial goods. Other exotic species may fill in a niche that was previously vacant in an ecosystem.

How do organisms survive in ballast tanks?

At present, we can not predict which organisms will die during a long journey in a ballast tank or why some are still alive when ballast water is released. Larger organisms often survive the journey by eating smaller ones. When faced with unfavorable conditions, some microorganisms and plankton species will form spores or other tough outer coverings for protection. As a spore, an organism may survive for a long time without food or in a different salinity or temperature than its natural environment. Once the environment becomes favorable again, such as when they are discharged into a port, the organism may change back to its active form.

Organisms may establish semipermanent or permanent communities in the layer of water and sediment that often exists in the bottom of ballast tanks. In these situations, adult organisms may reproduce and release larvae into ballast water while adults remain in the sediment. This pathway leads to the release of the same nonindigenous species into multiple ports.

What can be done about this problem?

There are two basic approaches to dealing with bioinvaders: stop them from invading in the first place, or eliminate organisms that have invaded. Getting rid of established invaders is practically impossible; keeping established invaders from causing damage is very difficult and expensive. Stopping invasions before they occur is the more practical and economical solution in the long term.

In order to stop an invasion, organisms must not be discharged from ballast tanks. This can be achieved by not taking organisms into ballast tanks, killing organisms during the voyage, or not discharging organisms when ballast water is released. More research needs to be conducted on methods to prevent introductions of unwanted species. Unfortunately, no ballast water treatment method can completely eliminate the risk of introducing exotic species. The goal of managing ballast water is to minimize the risk, possibly by targeting species that are known to have the potential to cause ecological and economic harm.


The United States requires all ships traveling in the Great Lakes or the Hudson River above the George Washington Bridge to exchange ballast water in the open ocean prior to entering these waterways. With the exception of these two waterways, no mandatory regulations exist for minimizing the risk of exotic species introductions in the ocean or other waterways in the United States.

The International Maritime Organization (IMO), an United Nations organization, recommends that ships exchange ballast water in the open ocean to minimize the risk of introducing nonindigenous organisms to coastal waters. The organization is working on adding ballast water regulations to the International Convention for the Prevention of Pollution from Ships, 1973 (MARPOL) which all member countries must follow.

Because the transfer of nonindigenous species via ballast water is an international issue, regulations for the management of ballast water to prevent introductions will be most effective if applied internationally.


Ballast water is one of the major pathways for the introduction of nonindigenous marine species. Because of the potential for ecological and economic damage posed by these organisms, ballast water should be managed to minimize the risk of species introduction.


Carlton, James T. and Bridget A. Holohan, eds. March 1998. USA Ballast Book 1998-1999: Ballast Research in the United States of America.

National Research Council. 1996. Stemming the Tide: Controlling Introductions of Nonindigenous Species by Shipsí Ballast Water. 141 pages.

On international environmental agenda: garbage, ballast water and emissions. Marine Log 75(896): pp15-19. March 1995.

The Introduction of Nonindigenous Species to the Chesapeake Bay via Ballast Water. A Chesapeake Bay Commission Report. January 5, 1995.


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  this page last updated on: 4 December, 2002