By Tim Hirsch BBC environment correspondent

Ships could have to filter their ballast tanks

It is estimated that 3,000 different species of animals and plants are dumped in foreign ports every day from the ballast tanks of cargo ships.

The unwanted hitch-hikers can create havoc in their new environment, and the International Maritime Organisation (IMO) has been pressed for years to impose binding rules to prevent this major threat to marine and freshwater life.

Human health

The problem comes when unladen vessels fill their tanks with seawater to provide stability, then empty it into the dock at their destination as they load up their cargo - along with tiny organisms, crustaceans and fish larvae thousands of kilometres from their proper habitats.

Some of the more aggressive and opportunist stowaways have become so successful that they have crowded out native species and even threaten entire industries.

For example, when the American comb jelly arrived in the Black Sea it feasted on fish eggs and plankton, helping to put an end to anchovy and sprat fishing in the region.

The European zebra mussel travelled in the opposite direction, arriving in the American Great Lakes where it has clogged intake pipes for power plants and factories, and also threatens native fish populations.

Disease organisms can also be transported in the tanks, infecting seafood and posing a risk to human health.

Close to home

The best-known example of the problem in the UK is the Chinese mitten crab, whose numbers have exploded in the River Thames over the past decade. They burrow into the banks, causing serious erosion and even undermining flood defences.

The crabs are flourishing just yards away from the London headquarters of the IMO, which has called this week's special conference to finalise a new convention regulating ballast water, 15 years after the problem first appeared on the agenda of the UN organisation.

The eventual aim is to ensure that all water carried by ships is treated or filtered before being discharged. But some environmental groups fear that some governments will weaken the agreement under pressure from their shipping industries.

Andreas Tveteraas, of the Worldwide Fund for Nature, said: "Anything short of the immediate adoption of a convention that is sufficiently strong in reducing the threats posed by invasive species is unacceptable, as it will result in continued risks to both global biodiversity and human health."

1. European Zebra Mussel One of 130 species which have invaded the Great Lakes through the St Lawrence Seaway, these shellfish clog water pipes and compete with native fish for plankton. 2. American Ctenophore These voracious jellyfish-like predators are thought to be the main cause of the collapse of sprat and anchovy fishing in the Black Sea. 3. Northern Pacific Seastar Scientists are warning that Australia's shellfish industry may be under threat from a plague of these highly fertile starfish.