An oil spill of unknown origin sullied the pristine water of the outer Edgartown harbor yesterday, ruining an entire crop of juvenile shellfish at a hatchery owned by the Martha's Vineyard Shellfish Group and posing a possible threat to the rich bay scallop beds off the north shore of Chappaquiddick.

"We've got about a million oysters that are probably now toast," declared Rick Karney, the longtime director of the shellfish group. Mr. Karney said a smaller, experimental scallop crop was also wiped out by the contaminated water. About 11 million seed quahaugs removed from the hatchery last week had a narrow escape.

"It's a shame," said Edgartown harbor master Charlie Blair. "It's black, it's nasty and it isn't gasoline or diesel because it's droplets suspended in the water. It looks like bilge oil."

Investigators from the U.S. Coast Guard Marine Safety Office in Providence, R.I. were dispatched from a field station on Cape Cod to survey the outer harbor yesterday. Biologists and environmental scientists at the Department of Environmental Protection and Division of Marine Fisheries were also standing by waiting for reports from the Coast Guard. When Coast Guard investigators arrived on the scene to tour the area with the harbor master late in the afternoon, they reported finding no oil in the water.

"They didn't see any oil out there as of now, but you're talking about hours after the report came in, so it must have dissipated," said Coast Guard Petty Officer Michael Schober late yesterday afternoon. "It's kind of late in the game to find a responsible party," he added.

The first reports of a problem came in early yesterday morning after Amandine Surier, Mr. Karney's assistant, came to Chappaquiddick to rinse the shellfish at the hatchery located on the Chappy Point Beach, a public beach owned by the Martha's Vineyard Land Bank.

The hatchery uses water pumped in directly from the outer harbor. When Ms. Surier rinsed the shellfish, she said she noticed a strong odor of fuel.

"The fumes were really strong; the more I rinsed the stronger the smell. There was no multicolored film on top of the water - it was transparent, but it felt a little bit oily. The smell of gasoline was so strong I had to open all the windows, and I got light-headed from the fumes," Ms. Surier said yesterday.

Ms. Surier called Mr. Karney at the main hatchery on the Lagoon Pond in Vineyard Haven. He told her to collect water samples, and he then called Mr. Blair. Edgartown shellfish constable Paul Bagnall was also called.

"They were right on it," Mr. Karney said. He said one of the water samples taken by Ms. Surier was redolent with petroleum fumes.

"You can't even put your nose in it," he said.

Meanwhile, Mr. Blair said he had received a call from Ann Mechur, a veteran Vineyard swimming instructor who was giving lessons at a private beach about a half-mile east of the shellfish hatchery. She reported a black oily substance in the water and on the beach.

"After the calls from Rick and Ann, I put two and two together," Mr. Blair said.

Mr. Blair contacted the Coast Guard, the state DEP, the state DMF and the environmental police.

The Coast Guard and the DEP are the official investigating and enforcement agencies for oil spills. The DMF monitors shellfish damage from spills.

Once the problem moved into the state environmental bureaucracy, the response slowed somewhat. The Vineyard environmental police officer was away. Two Coast Guard spill investigators were dispatched from the field office at Otis Air Force base on Upper Cape Cod, but it was late in the afternoon by the time they reached the Island.

Mr. Blair took a water sample from the private beach beyond the hatchery, and he said it contained black droplets of oil. He said the oil was suspended in the water, not floating on top.

"Charlie thinks it may be crankcase oil, which means it is on the bottom, which from the standpoint of shellfish is as bad as you can get," said Mr. Karney later.

"It definitely sounds like it's mixed in with the water. . . . The reports I am getting are that it's very, very fine droplets of a black oil substance mixed into the water column. To me what that says is that somebody tried to clean a bilge and tried to emulsify the oil. Spilled oil is not fine droplets," said Michael Hickey, the chief biologist for the shellfish program for the state DMF.

A series of productive bay scallop beds are located in the outer harbor just off Chappaquiddick. The entrance to Cape Pogue Pond, another pristine bay scallop ground, is north and east of the area where the oil turned up in the water yesterday.

"We've got Cape Pogue right out there, and I don't like this one bit," said Mr. Blair, who worked as a commercial shellfisherman for many years.

Mr. Hickey said it was too early to say how much damage might be caused by the spill, in part because the scope of the spill was unknown.

"It depends on the kinds of oil and also how much oil. Thick heavy oils can smother shellfish; thinner oils can be toxic particularly to juvenile shellfish, so there's all sorts of things that come into play. At this point we don't know how much; we don't even know what kind - but this sounds like bilge oil," he said.

"If you look at the surface water you don't know it's there," said Mr. Karney. "This wipes us out for what's on Chappaquiddick," he added. Mr. Karney said in addition to about a million seed oysters, the Chappaquiddick hatchery was the nurturing place for an experimental crop of genetically altered scallops. The trial work with the scallops was underwritten with funding from the Great Pond Foundation and the Southeastern Massachusetts Aquaculture Center. "It took a good two months to get the trial going, and now that's history," Mr. Karney said.

The shellfish group has operated a hatchery on the Lagoon Pond in Vineyard Haven since 1978. The hatchery on Chappaquiddick has operated since 1995.

Mr. Karney said this marks the third oil spill the hatchery has had to contend with in the Edgartown harbor. "In Edgartown we don't have the nutrient problems we have in the Lagoon, but this is the third oil spill, and it gets us to the question of whether it makes sense to be there - and what does this say for the shellfish in Edgartown?" he said.

"The problem is the time frame between when it was reported and now, and right now we are not dealing with any oil any more," said Petty Officer Schober.

He said any private group that is harmed by an oil spill can make a claim to a national pollution fund center. "It's the one that pays for mystery cleanups," he said.