When people sail the sea, marine organisms tag along. If carried long distances, these hitchhikers can invade and disrupt ecosystems far from their natural homes, pushing out the local species. Some invaders catch a ride by attaching themselves to the sides or bottoms of boats. But many more stow away in ships' "ballast tanks."
When a ship leaves port without cargo, it fills large ballast tanks with water to help keep its balance. At the next port, the ship pumps out the water so that it can load up with cargo. Tiny animals in the water get a free ride from one port to another. And with roughly 50 million gallons being emptied into U.S. waters every day, ballast water is one of the biggest transporters of non-native marine species.
In many cases, the newcomers don't take hold. But some species like the veined rapa whelk (Rapana venosa), wreak havoc on their newfound homes. The rapa whelk is native to Asia but was transported to the Chesapeake Bay—probably as larvae in a ship's ballast water. These predatory mollusks now pose a serious threat to the Chesapeake clam fishery.
Sometimes small, simple steps can go a long way toward solving big problems. Ship captains can help prevent many stow-away species from invading new areas simply by flushing and refilling ballast tanks with water from the open ocean before they arrive in port. In deep water far from the coast, animals flushed out of the ship's tanks are not likely to survive. Similarly, any deep-water organisms brought on board in the new batch of water, are not likely to make a home in shallow coastal waters at the ship's next port.
Scientists have found that this technique can remove more than 90 percent of the tiny animals in ballast tanks. The U.S. and others countries have begun requiring ships to either keep their ballast water on board or flush it thoroughly in deep waters far from the coast. Meanwhile, scientists at the Smithsonian Environmental Research Center and elsewhere are busy studying other techniques, such filtering or treating ballast water with UV light or heat. They hope that down the line, these techniques will put an end to high-seas hitchhiking altogether.
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