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In a bid to slow the introduction of foreign nuisance species into the Great Lakes, the United States is planning to have all ocean-going ships that enter the U.S. sector of the St. Lawrence Seaway flush their ballast tanks in the open Atlantic with salt water.

The new measure is expected to be in force when the seaway reopens after its winter closing, typically about the end of March.

The saltwater treatment, which has been mandatory since 2006 on ships travelling the Canadian portion of the 3,700-kilometre seaway, is designed to kill organisms adapted to fresh or brackish water that are hitching a ride in the ballast tanks vessels use to maintain their stability.

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Ocean shipping is considered the main route for foreign introductions on the Great Lakes, the world's largest system of fresh water. Although the lakes stretch nearly to the centre of North America, the five bodies and their connecting channels are starting to have an ecosystem populated with species of water fleas, mussels and fish, among other exotic creatures, from places as distant as the Black Sea and China.

Because the lakes are open water, species released in either Canada or the United States quickly spread to both sides of the border.

Environmentalists and seaway officials alike hailed the measure.

"These new, tougher ballast water regulations will protect the Great Lakes-St. Lawrence Seaway ecosystem," said Collister Johnson, administrator of the St. Lawrence Seaway Development Corp., which oversees the U.S. part of the waterway.

Although adding salty ocean water to ballast tanks won't completely eliminate the risk of invaders, Mr. Johnson said it "will kill the vast majority of them." About 500 ocean-going ships, known as "salties," travel the seaway each year.

Environmentalists have been pushing for more than a decade for the new rule, which will come into force nearly 20 years after the discovery in the late 1980s that a dangerous new species - zebra mussels - had spread to the lakes.

"We applaud this action," said Jennifer Nalbone, a spokeswoman for Great Lakes United, an environmental group based in Montreal and Buffalo. "We know that this is going to reduce the risk of invasions."

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Following the zebra-mussel invasion, the Canadian and U.S. governments set up a mixed system of voluntary and mandatory measures to have ocean ships flush their ballast tanks with seawater. But the rules had a major loophole that exempted more than 80 per cent of sea-going vessels, because they were carrying cargo and consequently didn't contain ballast water.

After foreign invaders continued to be found following these initial measures, the shipping industry discovered that even the sludge and residual water at the bottom of empty ballast tanks were teeming with aquatic life.

Mr. Johnson said the new rule will be put in place "fairly rapidly ... by government standards." He said authorities can't do anything "about the horses that are out of the barn," referring to foreign invaders that became established during the period of lax regulations.

The ballast-water provision is unlikely to end the controversy over ocean shipping on the lakes.

There are currently about 180 foreign species in the lakes, and new ones have recently been arriving at the rate of one every 28 weeks on average.

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