The Issue

• Thousands of species are being transported around the world in ships' ballast water and introduced when the ballast is discharged into environments where they have not previously lived.
• If they become established, such non-indigenous or "alien" species can cause destructive and irreversible changes in the structure of biological communities, including serious population declines in, or extinctions of, native species. (See briefing paper on Marine Invaders)

The Causes

• Ballast water is used by ships worldwide to operate safely and successfully. Among other uses, ballast aids propulsion and maneuverability by providing weight to submerge the rudder and reduce the amount of exposed hull surface. It also provides for greater stability and reduces stresses on the hull.
• When a ship takes on ballast-normally in coastal waters outside a port to make up for weight lost after unloading cargo-it also takes on thousands of microscopic organisms, including plankton species, the planktonic life stages of other marine species, and pathogens. These organisms are then transported in the ship's ballast tank and released when the ballast is discharged when the ship arrives at another port of call.
• An estimated 11,507,000,000 gallons of ballast water are discharged annually in US coastal environments. In one series of studies, 367 different plants and animals were found in the ballast water of 154 ships arriving in Oregon from Japan.
• Among the species introduced into the United States as a result of ballast water discharge since the 1970s are two species of jellyfish in Chesapeake Bay and San Francisco Bay; the Japanese shore crab, Hemigrapsus sanguineus, along the coast from Massachusetts to Virginia; the Asian clam, Potamorobula amurensis, in San Francisco Bay; the Philippine goby, Mugiligobius parvus,in Hawaii; and the zebra mussel, Dreissena polymorpha, in Eastern North America and especially the Great Lakes.
• Worldwide, examples include the dinoflagellate, Gymnodinium catenatum, introduced from Japan to Australia; the comb jelly, Mnemiopsis leidyi, from North America to the Black and Azov Seas; and the Indo-Pacific swimming crab, Charybdis helleri, from the Mediterranean to Colombia, Venezuela, Cuba and the United States.

The Context

• Ships have always required ballast for smooth and safe operation. Water began to be the predominant form of ballast from around the 1880s onward, replacing rocks, sand and other heavy, solid materials.
• It has been estimated that ballast water may transport over 3,000 species of animals and planes at any one time, and that one introduced species is becoming established every day.
• More than 40 species have appeared in the Great Lakes since 1960; more than 50 have appeared in San Francisco Bay since 1970.
• The use of methods to control the unintentional introduction of alien species via ballast water discharge is limited by requirements for safety, environmental acceptability, technical feasibility, practicability and cost effectiveness. Nonetheless, some techniques are available, and are used to varying degrees.
• The favored method for reducing the risk of introducing alien species into aquatic environments is changing ballast far out at sea. In this technique, ballast water loaded in port or taken on board while transiting inshore waters is changed with open ocean water during passage between ports of call. This method is usually effective because most freshwater, estuarine and inshore coastal organisms cannot survive when discharged into the ocean environment. Similarly, freshwater, estuarine or coastal environments are generally inhospitable to oceanic organisms.
• However, because one of the main functions of ballast is to insure the stability of ships at sea, exchanging ballast while under way may threaten the vessel's safety. Accordingly, changing ballast at sea is not always a practical solution.
• Other possibilities include devising ways to eliminating organisms from ballast water as it is taken on board or discharged. On the Great Lakes, for example, there have been experiments with different filtration techniques. In other regions, researchers are experimenting with the use of heat and ultraviolet irradiation to destroy unwanted organisms, particularly pathogens.

Further Reading

Bax, N. et al.  2003.  Marine invasive alien species: a threat to global biodiversity.  Marine Policy 27(4): 313-323.

Burkholder, J.M. et al.  2007.  Phytoplankton and bacterial assemblages in ballast water of US military ships as a function of port of origin, voyage time, and ocean exchange practices.  Harmful Algae 6(4): 486-518.

Carlton, J.M., and J.B. Geller. 1993. Ecological roulette: the global transport and invasion of non-indigenous marine organisms. Science 261:78-82.

David, M. et al.  2007.  Results from the first ballast water sampling study in the Mediterranean Sea - the Port of Koper study.  Marine Pollution Bulletin 54(1): 53-65.

Dobbs, F.C. and Rogerson, A.  2005.  Ridding ships' ballast water of microorganisms.  Environmental Science and Technology 39(12): 259A-264A.

Tsimplis, M.  2004.  Alien species stay home: The International Convention for the Control and Management of Ships' Ballast Water and Sediments 2004.  International Journal of Marine and Coastal Law 19(4): 411-482.

Wasson, K. et al.  2001.  Biological invasions of estuaries without international shipping: the importance of intraregional transport.  Biological Conservation 102(2): 143-153.