Greg Ruiz and his colleagues are used to finding all manner of organisms when they board foreign cargo ships arriving in the Chesapeake. Deep in the ballast tanks of these giant freighters, they find algae, worms, crabs and other species that don't belong within thousands of miles of the Bay.

But even these researchers -- who have boarded more than 100 vessels over the past three years as part of their research at the Smithsonian Environmental Research Center -- were surprised last spring when they found an entire school of mullet swimming inside a ship.

"I don't know how many there were," Ruiz said. "We brought back 15 or 20. Those fish were remarkable because of their size. But fish are not uncommon in ships. Usually they're small, like sticklebacks."

Ultimately, Ruiz said, some of the fish -- which came from the Eastern Mediterranean -- were probably dumped into the Chesapeake Bay when the ship pumped out its ballast tanks.

Some call this "biological pollution." Every day, in ports all over the world, cargo ships unwittingly move organisms from one corner of the planet to another.

This happens because cargo ships contain huge ballast tanks that are routinely filled and emptied to help steady the ship as it moves from port to port, loading and unloading cargo.

Ballast tanks are huge - a single large ship can hold millions of gallons. In that water may be thousands of organisms, ranging from microscopic bacteria that pose human health risks to algae that causes red tides, from zebra mussels which have plagued the Great Lakes to the mullet found by Ruiz.

When these organisms are released, they can spread disease, threaten or exterminate native species, and -- as with the zebra mussel -- cause billions of dollars in damage.

U.S. ports receive at least 21 billion gallons of ballast water each year -- an estimate considered low by many. The Chesapeake Bay ports of Norfolk and Baltimore combined receive 3 billion gallons annually, more than any other area on the East Coast.

"We are working with a potentially explosive problem," warned Rep. Wayne Gilchrest, R-Md., a co-sponsor of legislation introduced in Congress last month that seeks to control ballast water discharges. Other sponsors from the Bay states include Connie Morella, R-Md. and Phil English, R-Pa. Co-sponsors of a Senate version of the bill include Sens. Paul Sarbanes, and Barbara Mikulski, both Maryland Democrats.

The National Invasive Species Act of 1996 would require the U.S. Coast Guard to establish voluntary guidelines that encourage ships entering U.S. waters to first exchange ballast water at sea. The idea is that species picked up in harbors around the world would be killed when pumped into high-salinity ocean water. Likewise, organisms taken in from the ocean are generally less likely to survive when they are released in coastal waters.

Under the legislation, the Coast Guard would establish sampling techniques and record-keeping procedures to monitor compliance. If compliance lags, the legislation would allow the secretary of transportation to establish mandatory ballast water exchange programs for specific regions of the country.

The legislation would also establish standards for what constitutes an effective ballast water exchange. As it is, biologists and ship captains may view what constitutes an exchange very differently.

The legislation would also provide $750,000 a year from 1997 through 2002 for aquatic nuisance species prevention and control research in the Chesapeake Bay.

A report last year by the Chesapeake Bay Commission warned that the Bay was a "hot spot" for potential invasions because of its large amount of shipping activity. "If a non-indigenous species takes hold, it could potentially eclipse all the other work that we have done" to restore the Bay, said Ann Swanson, executive director of the commission, an advisory panel consisting of lawmakers and state administration officials from each of the Bay states.

In response to concerns raised by the report, the general assemblies of all three Bay states passed resolutions last year calling for congressional action on the issue. Noting that the zebra mussel has dramatically altered Great Lakes ecology, the resolutions warned that similar invasions in the Bay could have "catastrophic ecological and economic impacts for the region."

In the Chesapeake, Ruiz has found that about 90 percent of roughly 120 foreign ships he has sampled contained living organisms. About 70 percent of the ships have dinoflagellates, a type of algae, some forms of which cause toxic blooms.

Laboratory tests show that many of the organisms are able to live in simulated Chesapeake Bay conditions. "Not only do these survive, but we're able to raise larvae up to maturity in the laboratory, and across multiple generations," Ruiz said.

In recent years, Ruiz said the Japanese shore crab and at least two kinds of sea slugs from other parts of the world have invaded the Bay through ballast water.

Besides such visible invaders, Ruiz said ballast water can carry bacteria and microscopic organisms that may spread diseases. For example, a strain of cholera imported to the Gulf Coast from Latin America via ballast water in 1991 caused the temporary closure of shellfish beds there.

"We haven't looked in any great detail at the transfer of bacterial viruses and other microorganisms in ballast," Ruiz said. "We know that such organisms are probably there in great abundance because those are common organisms in estuaries around the world."

Because of their small size, though, such invaders are not easily monitored.

Scientists have identified more than 100 aquatic species in the Chesapeake that are not native. While it is unclear how many of those were brought into the Bay in ballast water, it does indicate that the Bay is susceptible to outside invasions.

Scientists believe the number of aquatic invasions in coastal areas has been increasing in recent years as world trade has grown and ships have become faster, making it more likely that organisms on board will survive the voyage.

Much of the ballast water attention has focused on the Great Lakes, which has been victimized by a series of invasions in past years. The zebra mussel arrived in the ballast hold of a ship in the mid-1980s, and has since spread throughout the Great Lakes as well as much of the Mississippi and Hudson River drainages.

The rapidly reproducing mussels have clogged water intakes at industries and municipal water supplies. Control efforts are expected to cost $5 billion by the turn of the century. The foreign mussel has also caused ecological damage, wiping out native freshwater mussels in some areas and altering the food chain by filtering out of the water huge amounts of the algae favored by many native fish.

At the same time, recent Great Lakes studies suggest that zebra mussels encourage the growth of harmful blue-green algae by rejecting them as food. Further, the wastes produced by zebra mussels appear to encourage blue-green algae growth. Blooms of the blue green algae, Microcystis, which is toxic to fish and can make humans sick, have recently been reported in parts of the Great Lakes.

"Many now consider the changes caused by the zebra mussel to be more significant than the changes caused by nutrient and toxic loadings," said Rep. Steve LaTourette, R-Ohio, a sponsor of the legislation, at a National Forum on Nonindigenous Species Invasions of United States Waters, a gathering of scientists and policy-makers that took place at the Capitol March 22 to discuss the issue.

Two other fish invaders of the Great Lakes, the ruffe and the goby, have also raised concerns because of their competition with native fish species. Because ecosystems are so complex, it is difficult to anticipate what impact any particular species -- or several species -- may have.

It's a kaleidoscopic world," said Jim Carlton, a professor of marine science at Williams College -- Mystic Seaport, and a leading expert on invasions stemming from ballast water. "Every time we move the glass lenses a little bit, a different picture appears."

For example, while the zebra mussel has caused problems in the Great Lakes, the situation was made even worse by the introduction of a non-native fish, the goby, around 1990. The goby is a predator of the zebra mussel, which might seem like good news.

But the zebra mussel, being a filter feeder, concentrates toxics in its body.

"It's a kaleidoscopic world. Every time we move the glass lenses a little bit, a different picture appears."

Gobies that feed on zebra mussels therefore concentrate even larger amounts of toxics in their systems. Gobies are then eaten by the native smallmouth bass - a popular sport fish. As a result, the presence of two exotic species has created a food chain which has turned a recreational species into a potential health risk.

Even under the new law, scientists at the forum agreed, invasions would continue. The amount of ballast water that can be exchanged varies based on ship design and sea conditions -- exchanges under some conditions cannot be safely made.

Also, the law would only apply to ships entering the United States from other countries. Ships moving along the coast would not have to exchange ballast water as they go from one port to another. As a result, they could easily move species up and down the coast.

Nonetheless, proponents say the legislation would buy time for coastal areas until better technologies and ship designs are found to help reduce the risk of invasions with unpredictable impacts.

"We can't guarantee that a particularly invasive species might not be released today which would have a tremendous impact on Chesapeake Bay blue crab fisheries, Puget Sound oysters, or any of a number of other resources around the country," Carlton said. "Whenever we introduce a new species, it's an ecological roulette with nature É We'd rather not play that game."