2. Focus on IMO(英文)
International Maritime Organization, 4 Albert Embankment, London SE1 7SR, United Kingdom
Tel: +44 (0)20 7735 7611 Fax: +44 (0)20 7587 3210
E-mail: rkohn@imo.org or nbrown@imo.org Web site:www.imo.org
October 1998
Alien invaders - putting a stop to the ballast water hitch-hikers
Introduction
Alien life forms that hitch a ride across the oceans in the ballast water of ships have been creating significant problems for the marine environment, public property and human health.
Unlike oil spills and other marine pollution caused by shipping, exotic organisms and marine species cannot be cleaned up or absorbed into the oceans. Once introduced, they can be virtually impossible to eliminate and in the meantime may cause havoc.
Specific examples include the introduction of the European zebra mussel (Dreissena polymorpha) in the North American Great Lakes, resulting in expenses of billions of dollars for pollution control and cleaning of fouled underwater structures and waterpipes and the introduction of the American comb jelly (Mnemiopsis leidyi) to the Black and Azov Seas, causing the near extinction of anchovy and sprat fisheries.
The bacterium Vibrio cholerae (cholera) has been transported from Asia to Latin American coastal waters, probably through discharges of ballast water, and South-East Asian dinoflagellates of the genera Gymnodinium and Alexandrium, which cause paralytic shellfish poisoning, have been dumped in Australian waters, harming local shellfish industries.
Ships are designed and built to move through water carrying a cargo, such as oil or grains. So if the ship is travelling empty to pick up cargo, or has discharged some cargo in one port and is on route to its next port of call, ballast must be taken on board to achieve the required safe operating conditions. This includes keeping the ship deep enough in the water to ensure efficient propeller and rudder operation and to avoid the bow emerging from the water, especially in heavy seas.
Ballast is thus defined as any solid or liquid placed in a ship to increase the draft, to change the trim, to regulate the stability or to maintain stress loads within acceptable limits. Water has been used as ballast from the 1880s onwards, thereby avoiding time-consuming loading of solid materials and the potential dangerous vessel instability resulting from the ballast shifting during a voyage.
Some types of ships require large amounts of ballast water, primarily for journeys when the ship is unladen, including dry bulk carriers, ore carriers, tankers, liquefied gas carriers, oil bulk ore carriers.
Other ships require smaller quantities of ballast in almost all loading conditions, to control stability, trim and heel. They include container ships, ferries, general cargo vessels, passenger vessels, roll-on,roll-off ferries, fishing vessels, fish factory vessels, military vessels.
Globally, it is estimated that about 10 billion tonnes of ballast water are transferred each year. Each ship may carry from several hundred litres to more than 100,000 tons of ballast water, depending on the size and purpose of the vessel. That ballast water, probably scooped up and pumped to the ballast tanks in or near the port where the cargo has been delivered, may contain all life stages of aquatic organisms. It has been estimated that ballast water may be transporting 3,000 species of animals and plants a day around the world.
