In November 1991 a strain of cholera was discovered in the ballast water of three ships at ports on the east coast of the United States. The vessels had stopped at ports in South America. In June 1992 a further two ships - also intercepted at eastern US ports - were found to be carrying cholera in their ballast water.
In Australia, scientists examining the ballast water of a Norwegian vessel calling at a Queensland port from Singapore, discovered an organism that could cause botulism in animals and humans. Incidents such as these have prompted the International Maritime Organisation (IMO) to introduce tougher guidelines on ballast water discharges.
All ships use ballast water for stability when sailing unladen. The water is taken aboard at the port of origin and emptied into the harbour when the ship arrives to load its cargo.
However, because ballast is commonly taken on in shallow water, or may occur near a dredging site, a large quantity of silt can also be loaded. This, or the water itself, may harbour aquatic organisms which could be toxic, carry diseases, or assume more dangerous characteristics in alien surroundings.
According to Dr Bill Farnham, of Portsmouth University's marine laboratory, the characteristics of some species in alien environments are often unpredictable. He says: 'A normally innocuous organism can behave quite differently in a new environment.'
In the US, awareness of the problem has been heightened by an infestation of zebra mussels brought to the Great Lakes by European cargo ships. This was 'a disaster', says Steve Thorp, environment officer of the Great Lakes Commission. Since the mollusc's discovery in 1985 it has reproduced rapidly, clogging the intake and discharge pipes of irrigation systems and power plants. The mollusc also harms fish stocks by filtering out essential nutrients in the water. A survey at the National Fisheries Research Centre, Great Lakes, shows it has caused damage costing dollars 5bn (pounds 3.4bn).
In Australia, an outbreak of a toxic alga on the south coast triggered legislation to control ballast water discharges. The dinoflagellate, thought to have been carried aboard Japanese vessels offloading woodchips at Hobart, poisoned shellfish stocks in southern Australia and Tasmania. The algae became so dense in some areas that they caused 'red tides'.
Neil Chambers, marine adviser to the International Chamber of Shipping, says the high volume of coal and iron ore exports from Australian ports makes areas of the coastline vulnerable to contamination. He says 58 million tons of foreign water is discharged annually into Australian waters.
In the UK the Marine Conservation Society is seeking funds to study the hazards of ballast water discharges. It says: 'The introduction of shellfish parasites in ballast water could have a significant impact both upon the native populations and shellfish farmers.'
Scientists at Southampton University discovered a foreign species of tubular worm on pontoons at the city's docks. Other species in the area include the Australasian barnacle and the Far Eastern sea squirt.
The IMO's spokesman, Roger Kohn, says the new measures to deal with ballast water discharges are not mandatory, but he hopes that they will become so. Short-term recommendations include ships exchanging old ballast water for new in mid-ocean, ideally where the depth is 2,000 metres or more. The IMO also suggests that water should be checked against a list of potentially dangerous organisms before a ship leaves port.
For the longer term, it proposes measures such as chemical treatment, oxygen deprivation and ultraviolet light disinfection. Changes to ship design to reduce the volume of ballast required or improve the way it is taken aboard are also under consideration.
Mr Chambers says: 'At this stage it is sensible that the directives should be voluntary because the remedies available are not necessarily the best or most effective.'
He says chemical treatment of ballast water may cause as much environmental damage as some of the organisms themselves. Another solution - heating the water to kill micro-organisms - would be expensive, impractical and time-consuming.
In the short term, Mr Chambers says the most practical solution is to control the problem at source by providing information on the best areas for ships to take on ballast water. This will require the co-operation of port authorities. The issue is being considered by Associated British Ports (ABP) and the European Harbour Masters' Association (EHMA). It is also being monitored by the International Association of Ports and Harbours and the American Association of Port Authorities.
Captain Malcolm Ridge, vice-president of the EHMA and marine adviser to ABP, says ports cannot afford to ignore the issue. He believes that water contamination can cause more long-term problems than oil pollution, because it is not immediately obvious.
It is 'too early' for ports to know exactly what preventive measures they should take, says Capt Ridge. The matter will be raised for the first time at an ABP general meeting this month, and a working group will present its findings to the annual EHMA congress in Marseilles in May.
The author is deputy editor of 'Port Development International'.