Experts have found organisms and lethal bacteria are infiltrating from foreign shores. Tim Judah reports.

ALEADING scientist is calling for action to defend Scottish coasts from voracious and aggressive forms of marine life which ''hitchhike'' around the globe by ship. Elspeth Macdonald, a senior marine biologist at the Scottish Office's Marine Laboratory in Aberdeen, has just completed a three-year study into the problem. She says that while Scotland has so far escaped the worst of the ship-borne predators ''you can't be complacent . . . in

this field we are working in the dark''.

In recent years scientists have become increasingly alarmed that plankton, seaweeds, molluscs, and deadly bacteria are being transported across the world in ships' ballast water. With larger vessels travelling ever faster, more exotic and potentially hazardous marine life is surviving long voyages than ever before.

From the Black Sea to Tasmania marine biologists have discovered ship-borne pests can cause environmental havoc and billions of pounds worth of damage.

On discharging cargoes ships must fill up with ballast water to keep their correct levels. This water is then pumped out at the ship's next port of call which may be on the other side of the world.

As ballast water is normally taken on in nutrient-rich coastal waters it inevitably contains many forms of wildlife and microscopic organisms.

Ms Macdonald said that her team's research into a wide range of animal and plant plankton had, so far, proved reassuring. This was because none of the organisms they had identified as travelling to Scottish shores had yet proved to be a dangerous environmental hazard.

This was no reason to let Scotland's guard down, though. Ms Macdonald pointed out that because international shipping, especially massive oil and gas tankers, dump some 25 million tonnes of ballast water into Scottish waters every year more work needed to be done. Ways also had to be found to sterilise ballast water before it was discharged.

Ms Macdonald said: ''This is not a national issue, it's an international one and it's not going to go away. We have to be aware that something may look innocuous but it is very hard to predict its effects.''

Ms Macdonald cited the case of the European zebra mussel which, in its home waters, is completely harmless. Transplanted by ballast water in 1986 to the North American Great Lakes it has turned into a rampaging plague causing billions of dollars worth of damage. Its predilection for clustering around the warm water outflow pipes of power stations means that they frequently have to be shut down while the crustaceans are scraped off.

In the Black Sea a small American jellyfish, believed to have arrived in a cargo vessel in the 1980s, has devastated once-rich fishing grounds. With none of its normal natural predators to keep it at bay it has gorged itself on Black Sea plankton and fish eggs and has been largely responsible for the collapse of the local fishing industry.

Fears are now growing that the Black Sea invader is advancing into the Mediterranean. In recent years indigenous marine wildlife around the coasts of Australia and New Zealand have found themselves under attack. They have been threatened by northern Pacific starfish which arrived by ship from Japan and aggressive giant sea worms which hail from Russian and South East Asian ports.

A form of giant Japanese seaweed is also besieging breeding grounds for edible molluscs and abalone off the Australian coasts. Another form of Japanese seaweed is thriving around British coasts, but, while it is pushing out local forms, it is not believed to have any other adverse affects. In some parts it is known colloquially as ''Japweed''.

Japanese seaweeds have also been spreading in the Mediterranean alongside South African worms.

Three years ago ports in the Americas and Australia were put on high alert following reports of cholera bacteria being spread through ballast water.

Ms Macdonald said that, so far, British coasts have been relatively insulated from the ballast water pests because a large proportion of shipping reaching these shores comes from within northern Europe where marine life tends to be similar. She warned, though, that concerted international and pre-emptive action needed to be taken now.

She said plans were afoot for collaborative work to be done within the framework of the EU.

The ballast water problem is now deemed so serious that last month the UN's International Maritime Organisation meeting in London discussed ways of dealing with the problem.

One idea is that ships exchange their coastal ballast water for water from the relatively sterile high seas. This can be dangerous, though, if storms blow up while the waters are being changed.

Another proposal is for ships' ballast waters to be heated to 40 degrees centigrade to kill the pests by cooking them. They could also be irradiated with ultraviolet light. Treating ballast water with disinfectants can have side effects and could also result in noxious chemicals being flushed into the sea.

Manfred Nauke of the IMO's Maritime Environment Division said the organisation was attempting to come up with a global solution to the problem. Despite the urgency of the situation, he was not optimistic that a full-blown international conference devoted entirely to the maritime ''hitchhikers'' could be held before 2002.