U.S. researchers say they have developed a technique to kill harmful marine life that hitches a ride on cargo ships from other parts of the world.

The invasive species found in ballast tanks, which are used to steady ships as they load and unload cargo, could be eliminated by microwave emitters fitted to the exit valves on the tanks, according to researchers from Louisiana State University's Agricultural Center, whose work will appear in the journal Environmental Science and Technology.

The microwaves would generate enough heat to kill the organisms living in the ballast water, which the United Nations lists as one of the four main threats to the world's marine ecosystems.

Large cargo ships usually have internal tanks that draw in water when cargo is being unloaded in order to avoid capsizing. The tanks are pumped out when the ship is reloaded, often after the ship has travelled to a different part of the world. As a result, species from one part of the world hitch a ride to waters in a different part, where they can harm the native marine life.

According to the UN, more than 80 per cent of the world's commodities are moved through shipping, resulting in more than five billion tonnes of ballast water being dumped each year.

Marine species have shifted habitats naturally for centuries, but land barriers and differences in water temperatures have limited their dispersal. With the growth in shipping fleets, the natural barriers have broken down, and many ecosystems have been jeopardized by the introduction of foreign species, the UN said.

Invasive species have contributed to a deterioration of the fishing business in the Black Sea and required large expenditures to clean up U.S. waterways.

The Great Lakes were found last year to harbour more than 300 invasive species brought by cargo ships. They include the zebra mussel, first detected in 1980, which upsets the natural balance by competing with zooplankton for food and suffocating existing mussel populations. The round goby and the Asian carp are two particularly destructive species of fish that arrived in the Great Lakes in the 1990s.

The international shipping community in 2004 agreed to find ways to limit the damage caused by the discharge of ballast waters.