It's been almost a decade since the Exxon Valdez spilled 11 million gallons of crude oil into the clear waters of Alaska's Prince William Sound, but it's still hard to shake the images of oil-drenched birds gasping for air. It may be even harder to imagine that foreign marine life invading new waters could cause more damage than that oil spill.
Foreign species eat native species or their food sources, destroy habitats, and block undersea pipes at power stations or factories. The problem of marine invasions is worldwide, largely a byproduct of the shipping trade.
Many marine species enter waters in the ballast water used to stabilize ships. The water is picked up in one or more ports and then discharged into others. The United States port of Baltimore alone receives more than 12 million metric tons of foreign ballast each year, originating in 48 different foreign ports. Ninety percent of that carries live organisms, according to the US Department of the Interior, which is charged with safeguarding the environment. Other bioinvaders travel in imported bait and via the aquarium trade.
In the US, it costs more than $5 billion a year to control marine bioinvaders, according to Cornell University ecologist David Pimentel. The zebra mussels in the Great Lakes alone cost $3 billion.
Last week, President Clinton signed an executive order expanding government efforts to combat foreign predators on land and sea. The order creates an interagency Invasive Species Council that will have 18 months to devise a plan to contain the spread of bioinvaders (see related story page 12).
Zooplankton eat phytoplankton
About 1 percent of the hundreds of sea immigrants in American waters breed and cause a major, damaging invasion. For example, about a year ago, the US Fish and Wildlife Service discovered four new species of tiny zooplankton in Alaska's Prince William Sound brought in ballast water from East Asia via San Francisco. Those zooplankton eat phytoplankton, a food for the Dungeness crab. "These four species of zooplankton may in the long run prove to be infinitely more devastating than the Exxon Valdez oil spill itself, because the species may insert themselves into the food chain," Secretary of the Interior Bruce Babbitt told a recent conference on marine bioinvasions.
One of the more recently discovered predators, an Asian snail found in Virginia's lower Chesapeake Bay, threatens to deplete stocks of oysters and clams on which it preys. The snail, Rapana venosa, was identified about five months ago by Roger Mann, professor of marine science at the Virginia Institute of Technology in Gloucester Point, and his colleagues. But they suspect it may have invaded the Chesapeake's waters as long as four years ago. They worry the hardy snail, which can live in wide temperature and salinity ranges, might also infest the even richer oyster and clam beds of Long Island Sound.
"They can eat one clam every two or three days for the 10 years of their life. That's a lot of clams," Professor Mann says. "I think they are a real cause for worry."
Mann and colleague Juliana Harding, senior marine scientist at the institute, started a project in August 1998 to study the Rapana snail. They say the animal's eggs likely were transported in ballast water around 1940 from its native Sea of Japan to the Black Sea, the eastern Mediterranean, and then via coal ships to Hampton Roads, a major coal-exporting channel in southeast Virginia linked to the Chesapeake Bay.
So far, the snails are not known to have spread outside the lower Chesapeake, Mann says. But ships thought to transport snail eggs in their ballast have regular routes from the bay through Baltimore, Philadelphia, New York, and Boston. "We have a problem in Virginia that I can guarantee will be a big problem in Connecticut in 20 years," Mann says.
Although the Chesapeake Bay is home to about 160 foreign species, the Rapana snail is the only one proving to be particularly hardy and destructive.
Chesapeake Bay once was a premier oyster bed in the US, supplying about 46 million bushels a year in the 1940s and 1950s, Mann says. But disease, pollution, and overfishing reduced that to less than 1 million bushels today. And just as Virginia and Maryland fisheries experts began to rebuild the population, the snail moved in to pose a new threat.
The Rapana burrows about a foot beneath hard sand on the sea floor and either eats the clams that also burrow into the sand or the oysters that rest atop the sea bed. The snails are about the size of a softball and weigh about two pounds, including their elaborate conch-like shell. The snail itself is about the size of a quarter-pound hamburger.
Mann says it is difficult to determine the extent of damage caused by the Rapana so far, partly because it was discovered only recently, and partly because of its stealth-like eating habits.
The snails eat by latching onto an oyster or clam until the shell opens, then slurping out the prey and chopping it to bits with a chain saw-like tongue. They can eat oysters and clams about half their own size in less than an hour. Unlike other predators, which leave a bore hole in their prey, the snails leave no trace. Ms. Harding says this is particularly worrisome because the snails may have invaded local waters before they were detected.
The Rapana has few natural predators in the US. Large, striped hermit crabs and sea turtles eat small Rapana, and bottom-dwelling fish such as striped bass eat Rapana eggs. But large-size Rapana seem to have no predators in the Chesapeake. In their native Sea of Japan, they make a good meal for an octopus.
Scientists are now testing the snails for their nutritional value and taste to see if they could become a commercial fishery, which is the silver lining of the invasion cloud.
How to handle invaders?
There is much debate among scientists, policymakers, environmentalists, and fisheries businesses about how to handle invaders. Preventing the influx and spread of foreign species is costly and complicated. Fitting ships with equipment or creating large enough shore areas to treat ballast water is expensive. And it is difficult to predict which foreign species might prove to be a problem in their new environment.
For example, West Coast fishermen are worried about the impact on local shellfish fisheries of the European green crab. "But the green crab isn't a pest in Japan, where they put it in miso soup," says Armand Kuris, a zoologist at the University of California in Santa Barbara.
Kathy Metcalf, director of maritime affairs for the Chamber of Shipping in Washington, says scientists need to decide whether to try to kill most of the incoming species or focus on those that could cause the biggest problems. She says international and national agreements are needed that define what standards for foreign species containment should be enforced.
"We need to focus on ballast-water management," Ms. Metcalf says, "and we can't focus on just one technical solution to the problem." She says ultraviolet or thermal treatments and filtration of water must be considered in addition to where ships are allowed to exchange ballast water.
Bella Galil, a scientist at the National Institute of Oceanography in Haifa, Israel, advocates keeping all foreign species from entering local waters. In the 130 years since the Suez Canal was built, the eastern Mediterranean has seen large influxes of marine invaders.
That news isn't necessarily all bad. More than half of the fish trawl yield along Israel's coast is immigrant fish, she says, and 95 percent of all shrimp caught along the coast of Egypt and Israel comprise two prawns that came from the South Pacific.
"Most of the things that come in are innocent in their own environment," Ms. Galil says. "You don't know they'll become economic pests or health hazards until they're in your own backyard."