A new technique to render oysters sterile may allow Virginia Institute of Marine Science researchers to begin testing foreign oysters in the York River as early as next spring.

The field testing is part of a multiyear VIMS study to determine whether nonnative oysters are resistant to the diseases that have devastated native oysters, especially in Virginia.

If any of the four Asian strains being tested are able to survive under Bay conditions, it could spur a policy debate over whether foreign species should be a part of rebuilding the Chesapeake's population of oysters, which historically played important ecological and economic roles.

Such an introduction would be controversial, and even the use of foreign species in open-water experiments has proven contentious in the past. Critics argue that nonnative species might quickly replace the native oysters, or that they may trigger other unknown impacts to the ecosystem.

To minimize the risk of an accidental introduction during the experiment, researchers plan to only use "triploid" oysters, which have three sets of genes and cannot reproduce.

In 1993, VIMS scientists attempted a similar field study with triploid oysters. During the experiment, though, some oysters began reverting to normal "diploids" - with two sets of genes - capable of reproduction.

After the appearance of these "mosaics" - oysters in which some cells have three sets of genes while others have two - the experiment was cut short and the oysters removed from the river.

This year, a new technique will be used to produce the triploid oysters for the study.

The triploids used in 1993 were created through a chemical process. This time, using a technique developed by Standish Allen of Rutgers University's Haskins Shellfish Research Laboratory, oysters chemically treated to become tetraploids - with four sets of genes - were crossed with normal diploid oysters to create non-reproducing triploids. Because the oysters were created through breeding, rather than a purely chemical treatment, the triploid condition is thought to be more stable.

"We're testing that right now," said Eugene Burreson, an oyster disease specialist at VIMS. "If we start seeing mosaics developing in the chemically treated ones and not in the mated ones, and we can follow that for a number of months, then we'll be sure that they're not going to revert. And then we'll be able to accelerate our studies. And we'll know that by the fall, I think."

The field studies are critical because scientists cannot test the oysters with MSX - one of the two diseases plaguing native oysters - in the laboratory. Instead, the oysters are placed on trays in the river and routinely checked to see how they are surviving compared with native oysters.

MSX and another disease, Dermo, are major contributors to the dramatic decline in the Bay's oyster populations. Virginia's oyster harvest in 1959 was 4 million bushels; in 1995, it was less than 50,000 bushels. While Maryland oysters have also suffered from the diseases, the impact has been greater in Virginia because the diseases thrive in saltier water.

While the 1993 study was cut short, it had indicated that Japanese oysters were resistant to the diseases that killed almost all of the native oysters used in the study, though the foreign oysters grew slowly.

The new study will test three strains of the Japanese oyster, Crassostrea gigas, and another Asian species, Crassostrea rivularis. The species were selected for the study based on their close resemblance to the native Eastern oyster. They are reef-forming species and are tolerant of subtropical latitudes and high stress environments.

The research, outlined in a plan submitted to the governor and the General Assembly earlier this year, was to take four years. But with the new technique to produce triploids, the research could be completed within three years.

"The ultimate result of this outcome is the ability to perform in-field testing at virtually no risk of reproductive potential," said Robert Byrne, VIMS director of research and advisory services. "These emerging results justify the re-examination of our original, more conservative plan."

The change in schedule was first announced by Secretary of Natural Resources Becky Norton Dunlop in a statement from the governor's office.

"This is good news, indeed," Dunlop said. "The scientific community has maintained that a flourishing oyster resource once played a critical role in preserving water quality in the Chesapeake Bay. These great populations of oysters, in addition to supporting large fisheries, were able to filter the waters of the Bay in just a few days. All true friends of the Bay will want to be supportive of VIMS as it carries out these important tests throughout the upcoming months."

As a result of the study, scientists hope to determine which species or strains are most likely to survive in the Chesapeake, and what geographic range the nonnative species should be allowed to cover.

Interest in using foreign oysters in the Bay has grown as the native species has declined, showing no evidence of being able to adapt to the diseases, which have been in the Chesapeake for 30 years.

Besides their economic value, research in recent years has suggested the oyster decline has taken an environmental toll as well. The reefs built by the oysters provide important habitat for many other Bay species. In addition, the huge oyster population that historically inhabited the Bay is thought to have helped maintain water quality by filtering large amounts of water as they fed.