Prospects

for

Disease-Free

Seed

Spawning uninfected oysters in hatcheries may be one way to manage around disease

Is it possible to plant disease-free oysters in low salinity waters -- where disease may be present but is generally less virulent than higher and bring them harvest before Dermo or MSX kills them?

In principle, says Don Meritt of the University of Maryland Center for Environmental Science (UMCES), the answer is yes. "The problem," he says, "is in knowing for sure that we are beginning with disease-free seed."

Because most productive grounds in the Bay are infected with Dermo and are subject to getting MSX early in their growth, the best way to produce oysters that are without disease, says Meritt, is in hatcheries from stocks that are themselves free of disease.

Meritt is doing just that at the UMCES Horn Point Laboratory hatchery, working together with researcher Ken Paynter of the University of Maryland at College Park, the state Department of Natural Resources, and the Oyster Recovery Partnership.

The Partnership, which grew out of a unique consensus agreement in Maryland among watermen, aquaculturists, resource managers, legislators, scientists and environmentalists, has developed a long-range plan for restoring oyster populations and thus improving Chesapeake Bay habitat. The agreement calls for the use of disease-free hatchery seed in the reconstruction of oyster bars in the Bay system, and also sets limitations on transporting, planting and harvesting oysters.

In 1998, Meritt and colleagues produced more than 20 million oyster spat at the Horn Point hatchery – while this is a major achievement for a research hatchery, it is still only enough to plant some 20-30 acres of bottom ground. Considering that Maryland alone has some 270,000 acres of designated public oyster grounds (though most no longer produce harvestable oysters), long-term repletion efforts face an enormous task.

Nevertheless, important inroads are being made: hatchery-reared seed planted in the Choptank and Severn rivers have been assisted by low salinities these past two years, the result of high precipitation which has helped moderate the intensity of disease. Meritt is working with Paynter to measure the success of this project, which also employs hundreds of volunteer students each summer season whose efforts include unloading thousands of bags of oyster seed from setting tanks and transporting them to boats where they are off-loaded into the river.

After a month or so, when oysters have grown hardier, students transport the bagged oysters upriver, where they release the spat-laden shells for planting on oyster bars.

"We've been tracking the oysters planted in the Choptank for three years now," says Paynter. "They haven't grown very fast, which is due to the low salinity; the up side, though, is that they've been essentially disease free for that time." So far, so good, he says. "The oysters are about two inches long now on average and mortality has been quite low, even though the salinity dipped to one part per thousand in 1996."

While commercial culture of oysters has been a risky proposition in the mid-Atlantic region, Meritt is finding commercial growers in the Bay interested in the potential for managing around Dermo with disease-free seed. Economics is the key – if growers have leased grounds to grow their oysters and can get a high enough price for their labors, new ventures will be getting underway.