'Stowaways' threaten world's seas

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More than 100 countries are expected to sign a UN treaty regulating the management of ballast water by vessels around the world.

It aims to halt the spread of aquatic organisms, from jellyfish to crabs, from algae to mussels, which can be devastating in new ecosystems, when discharged with ballast water at a ship's destination.

Ballast is any material used to balance an object. Modern vessels use water, which is easy to pump on and off. It is essential to provide balance and stability to ships when they are unladen.

A ship sailing with an empty hold will have filled its ballast tanks at its source port, and when it reaches its destination and takes on cargo, the ballast water will be discharged. With it may go any number of tiny living creatures picked up at the source.

It is estimated at least 7,000 different species are being carried in ships' ballast tanks around the world at any one time. When new species do survive and are able to breed, they can become serious pests quite rapidly.

For instance, the European zebra mussel is harmless on this side of the Atlantic, but transported in ballast tanks to the Great Lakes between Canada and the US, it causes ecological chaos, fouling underwater structures and pipes and resulting in pollution control costing of billions of dollars.