It is well-known that sea-going vessels are significant sources of imported, invasive pests. When cargo ships flush their ballast water in port, they also flush out any hitchhikers that may have come along for the ride.

Such invaders have caused serious problems in U.S. waters. For example, zebra mussels — thought to have been introduced by a cargo ship in the Great Lakes region — are spreading rapidly through U.S. waterways, clogging pipes and outcompeting native aquatic species.

A marine scientist from the Monterey Bay Aquarium (Monterey, Calif.) may be onto a solution, however. The researcher, Mario Tamburri, has found that a method used to reduce corrosion in ships' ballast tanks may also be deadly to invasive pests. To prevent ballast-tank corrosion, some vessels use nitrogen gas to eliminate oxygen in ballast water. This may also prove lethal to unwanted organisms.

Existing methods to eliminate ballast-water hitchhikers include filtration, heat treatment and pesticidal materials. However, cost and environmental effects have proven to be significant drawbacks. Deoxygenation, on the other hand, is cost-effective and appears to be benign to the environment.

Tamburri notes that while deoxygenation isn't cheap, its dual benefits of reducing corrosion and stopping invasive pests makes it a desireable tactic.

It was Japanese scientists who first developed the deoxygenation technique for fighting corrosion. By bubbling nitrogen gas through the ballast water, oxygen is removed and rust is thereby inhibited. Currently, most ships simply paint the surfaces of the ballast tanks. The constant maintenance required for painted surfaces is costly, however. Deoxygenation may save as much as $100,000 a year in maintenance costs.

In Tamburri's tests, deoxygenation killed water-borne invaders in two to three days. This gives ocean-going vessels — which often require weeks to complete their journeys — ample time to eliminate pests. Tamburri concedes that the method may not kill all marine organisms present, but it may be the best compromise between effectiveness and affordability currenlty available.

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