Chesapeake Bay oysters -- ravaged by drought and disease -- are becoming so scarce that state biologists expect this year's harvest to be the smallest since Maryland began keeping records in 1870.

Biologists completing a survey of the oyster population this week blame the decline on a three-year drought that has created a saltier bay and a favorable habitat for the diseases that have been killing oysters for decades.

"We're seeing diseases literally robbing the bay of the oysters," said Christopher Judy, director of the state Department of Natural Resources' shellfish division.

Oysters are being killed by two parasitic diseases, MSX and Dermo, which thrive in salty water, scientists say. The diseases first appeared in the bay in the 1960s and have been killing significant numbers of oysters since the mid-1980s, they said.

But Judy and other biologists say the drought has spread the diseases' deadly effects -- bringing saltier water to the upper bay, where fresh water from rainfall has traditionally allowed oysters to thrive.

"You would expect to see higher mortality because we're in a third year of drought and you have elevated salinity over a long period," said Roy Scott, a senior DNR biologist who works with Judy and is supervising the annual oyster survey.

DNR crews spend 17 days each fall aboard the Miss Kay, a state-owned 46-foot workboat, dredging up oysters, opening their shells and counting them to gauge their population. They also fill gunnysacks with oysters to test them for diseases at the Sarbanes Cooperative Oxford Laboratory operated by state and federal scientists. Results of those tests should be available late next month, they said.

As part of this year's survey, DNR crews brought up shellfish from two dozen oyster bars Friday along the Chester River -- an Eastern Shore tributary near Rock Hall that in the past had sufficient fresh water to prevent the diseases from spreading.

But Judy said the oyster count this year showed that the diseases are spreading north into the Chester and other upper bay tributaries.

DNR crews found that half of the oysters dredged on the Chester were dead -- up from the 35 percent mortality rate found in the river last year and the 18 percent the year before, state biologists said.

"When you get at a 50 percent mortality rate, that's bad -- there's no question," Scott said.

Judy said that the diseases also have spread into the state's oyster sanctuaries, spawning areas established two years ago that are off-limits to watermen.

But Jill Stevenson, DNR deputy director of fisheries, said that the presence of diseased oysters in sanctuaries doesn't diminish their potential for saving oysters. She said that it's too early to determine the effectiveness of the sanctuaries.

"If you have 85 percent of the oysters in a sanctuary die, you still have 15 percent left and among those survivors, you have some trait that is allowing them to survive, and they're worth studying," she said. "If they're living, they're living for a reason."

Oysters help keep the bay healthy, filtering pollutants, improving water clarity and allowing the light crucial to underwater vegetation to penetrate. Their reefs are habitats for a variety of aquatic life.

DNR's oyster survey is expected to be completed by the end of the week. But state biologists say preliminary reports indicate that the harvest -- which began Oct. 1 and ends March 31 -- should drop lower than the 79,618 bushels caught in 1993- 1994, the lowest on record. The harvests have recovered slightly since 1994, with the help of government-funded oyster recovery programs, totaling 148,155 bushels last year.

Oyster harvests have plunged steadily since the 3 million- bushel levels of the mid-1960s, because of disease and overharvesting, biologists say.

But Judy said this year's decline cannot be blamed on overharvesting.

"It isn't a question of overfishing," he said. "The oysters are being killed off by diseases even before watermen have a chance at catching them."

Wilford Kale, a spokesman for the Virginia Marine Resources Commission, said Virginia has yet to complete its oyster survey. But he said that Virginia's bay waters have been devastated by the same diseases.

Many watermen say they are not surprised by the decline of oysters. They also question why the state is waiting to introduce an Asian oyster to the bay that has been found to be more resistant to the parasites.

"With these diseases, it's the worst anyone's ever seen it," said Russell Dize, a waterman from Tilghman Island.

Dize said many watermen are not dredging for oysters this year because so few of them are left. "These diseases are slowly eating their way right up the bay," he said.

Dize and Kenny Keen, a waterman and seafood dealer from Calvert County, said the state should allow Asian oysters to be introduced. "You need a tougher oyster that can handle these diseases," Keen said.

The Asian oyster, Crassostrea ariakensis, looks and tastes a lot like the native bay oyster, and early experiments suggest that it might be resistant to the diseases.

The Virginia Seafood Council, a trade group, hoped to "plant" 1 million baby Asian oysters -- neutered to prevent them from reproducing -- in the bay in the summer as an experiment.

But the plans were shelved in May after federal and state scientists warned that the experiment might have disastrous results. The Asian oyster, which has no known natural enemies, might wreak environmental havoc, they said, bringing some undiscovered disease, crowding out native oysters or causing some unforeseen damage.

The release into a Crofton pond this year of a snakehead, an Asian import with a reputation for devouring all other fish, alarmed biologists and sparked a crackdown on marketing of the species. The fish thrived in the pond because it had no natural enemies.

Maryland and Virginia officials are awaiting the findings of a National Academy of Sciences study of the Asian oyster that will be completed by next summer, said John Surrick, a DNR spokesman.