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Bioinvasion: Foreign Waters

National Geographic Today producer Chad Cohen spent more than two months investigating the biological threats posed by non-native species in the United States. His three-part series on the subject airs on the U.S. National Geographic Channel on January 23, 24, and 25 at 7 p.m. EST. The following is a transcript of part two of Cohen’s series.


Although not native to the United States, the hydrilla has clogged waterways from Florida to Washington State.
Photograph by Kevin Fleming/CORBIS

VCRs from Japan, sofa beds from Sweden, fine china from the Netherlands. The 900-foot cargo ship Texas, bound for Baltimore, Maryland, carries a virtual smorgasbord of goods from five continents. It also holds millions of gallons of water in the ship’s ballast to keep the boat stable.

It’s the policy of the Texas to dump its ballast out at sea. But every day thousands of ships like it suck up water in one port and spit it out in another—sometimes halfway around the world.

Forty thousand gallons (151,000 liters) of foreign water get pumped into U.S. waters every minute. With them, thousands of species of marine life from foreign shores—fish, shellfish, plants, and microorganisms.

“You think about it and you don’t know what’s in there, you know? For sure it’s something,” says the ship’s captain, Gunnar Johansen. Something that under the right circumstances could thrive there.

Zebra Mussels: an Asian intruder

In 1986, a ship from the Black Sea emptied its ballast water into Lake Erie and released a single stowaway, the zebra mussel. These thumbnail-sized mollusks are prolific colonizers that attach to almost any surface. Divers have spotted dense mats with as many as 60,000 mussels in a square foot (900 square centimers).

“Wall to wall,” says one diver. “Just wall to wall mussels. Just a carpet of mussels on the bottom there.”

The mussels clog pipes and intakes throughout the Great Lakes and have rapidly spread to waterways from the Hudson River to the Mississippi to the Gulf of Mexico.

Zebra mussels threaten water that millions of consumers depend on for drinking and electricity and cost U.S. $30 million each year to monitor and control in the Great Lakes alone.

New regulations now require that ships entering the Great Lakes dump their ballasts 200 miles (300 kilometers) from shore. But it’s business as usual in other ports.

More Than Just Mussels

Biologist Greg Ruiz and his group from the Smithsonian Environmental Research Center have sampled more than 200 ballasts from ships entering Baltimore Harbor looking for alien invaders. He says there are more exotic species in U.S. waters outside the Great Lakes.

“It’s difficult to say what ships are doing,” explains Ruiz, “because we only have about a 20 to 25 percent compliance with reporting, not particularly good.”

Since Columbus’ time, ships have been moving species around the world. But as our economy becomes more and more global, the rate of shipping, as well as the size and speed of ships, has increased—and so has the rate of invasions.

The parasitic sea lamprey decimated native fish populations in the Great Lakes. The hairy Chinese mitten crab now in San Francisco Bay cuts off the water supply to homes and farms, and European green crabs in New England threaten precious native oysters, crabs and clams.

“We know there are hundreds, if not thousands, of species being delivered that are not presently established in recipient ports,” says Ruiz. “What proportion of those are becoming established, or could become established, is sort of an elusive question.”

It’s not just 500-ton (450,000-kilogram) freighters that move invasive species around. The aquatic weed hydrilla sticks to the engines and bottoms of recreational boats.

“[Hydrilla] end up clogging canals,” explains Jonathan Phinney, executive director of the American Society for Limnology and Oceanography. “They destroy fish habitat by essentially forming a canopy over the surface of the water and limiting the amount of movement for fish to go through, as well as lowering the amount of dissolved oxygen in the water.”

Phinney says the hydrilla’s hardiness was the reason why it was brought to the United States from Asia in the first place to line the bottoms of aquariums.

“People will buy this plant in a shop with a goldfish; the goldfish dies; they dump the plant; the plant can get dumped into a river or a pond and just start establishing itself very quickly,” says Phinney.

Hydrilla is now choking waterways from Florida to Connecticut to Washington State, costing millions. And there is little hope of eradicating it.

Back in Ruiz’s lab, the most threatening invader isn’t a plant or an animal. He recently found the deadly cholera bacterium in ballast water that was released into Chesapeake Bay. While it’s unlikely that conditions will be suitable to incubate an outbreak of the disease, the find shows us that people are moving organisms around in ways we could never expect.