Dozens of Baltimore schoolchildren joined watermen and scientists in mid-July to begin placing seed oysters in two protected Choptank River nurseries in the hope they can help the Chesapeake's oyster population recover from two diseases.

About 900 bags of the tiny oyster spat, attached to clean oyster shells recovered from Maryland shucking houses, were placed in nurseries as the first stage of a project to recover Maryland's oyster population.

The project is the first major effort for the Maryland Oyster Recovery Partnership, a cooperative venture among watermen, environmentalists, University of Maryland Center for Environmental and Estuarine Studies scientists, and the state Department of Natural Resources.

Don Meritt, who oversees the university's Horn Point hatchery, said about 2.5 million seed oysters would be planted in the two Choptank River nurseries.

Youths from the Living Classrooms Foundation in Baltimore spent a week at Horn Point planting shells harboring spat in nurseries near Jamaica Point and Cabin Creek.

"We're giving them oysters in exchange for their labor," said Robert M. Pfeiffer, the partnership's executive director.

The students will return to Baltimore with their own bags of seed oysters, which are to be planted in the Patapsco River.

Jessica Munitz, who will be entering the ninth grade this year, said she joined the Living Classroom group because she likes science. "This is fun," she said, as she splashed in the Choptank River, placing bags of seed oysters on wooden pallets.

The Choptank is one of six major rivers designated as Oyster Recovery Areas. The others are the Chester, Nanticoke, Patuxent, Severn and Magothy.

Pfeiffer said the oysters are being placed in areas of lower salinity, where the two oyster parasites, Dermo and MSX, are less prevalent.

Pfeiffer said the oyster shells are packed in mesh bags to protect the young oysters from predators, such as crabs and rockfish.

"There's safety in numbers," Pfeiffer said, explaining that oysters on the outside shells may be eaten, while spat on shells inside the packed bags will remain safe.

The bags of young oysters will grow in the nursery areas for two to three months and then be moved to a specially constructed reef in a sanctuary area farther upriver.

The disease-free oysters will then be monitored for three years so scientists can study water quality, oyster growth, survival and disease.

Scientists will be especially interested in studying the effects of Dermo and MSX if the parasites strike the sites, Pfeiffer said.

He said Dermo typically strikes oysters between the second and third years of growth, hitting oyster populations just before they reach market size at age three years.

While there is no known cure for either disease, scientists hope the research will enable them to "look at how to manage around the diseases" and produce oysters that are disease-free, Pfeiffer said.

In the past 30 years, Maryland oyster harvests have ranged from 3.2 million bushels in 1973 to 80,000 bushels in 1994. Preliminary data for 1995 indicate 148,000 bushels were harvested in the 1995 season, a significant upswing after years of decline.

State officials believe two successive years of high freshwater runoff lessened the distribution and intensity of MSX and Dermo, according to a shellfish program summary report by DNR's Tidal Fisheries Division.

"This increase does not indicate a long- term turn around for the fishery," according to the report. ÒMSX and Dermo are expected to return in force when salinities increase again.