Cow-nosed rays gobbled up most of the 775,000 oysters in an artificial reef days after they were planted in the Piankatank River in late May to help revive the Chesapeake Bay’s declining shellfish population.

The rays ate about 90 percent of the oysters after the Chesapeake Bay Foundation and The Nature Conservancy placed the shellfish on a man-made reef in the Piankatank, the foundation’s Chuck Epes said.

Environmentalists were shocked by the loss, which was discovered by divers.

The foundation placed all of the oysters it had grown this year at its Gloucester Point oyster farm in the Piankatank. A $60,000 federal grant helped support the project.

The rays showed up weeks ahead of schedule, thwarting the foundation and conservancy’s efforts to keep them away from the oysters. There wasn’t time for the oysters to hide beneath a bed of 30,000 bushels of empty oyster shells, foundation oyster grower Tommy Leggett said.

“Going on past histories and stuff, (the oysters) should have been in the clear,” said Bob Fisher, who is studying rays at the Virginia Institute of Marine Science. The rays may have entered the Bay earlier because of warmer-than-normal water temperatures.

The loss of oysters illustrates the difficulty in restoring the shellfish population in the Chesapeake Bay. The conservancy and the foundation are using a strain of oysters that have been specifically bred for their tolerance against disease-causing parasites that have killed off much of the Bay’s oysters. The Bay’s oyster population is now about 1 percent of what it was a century ago.

This is not the first time hungry rays have ruined attempts to boost the declining oyster population. A couple years ago, the Corps of Engineers planted a large amount of oysters in the Great Wicomico River in the Northern Neck, only to have rays swoop in and help themselves to a feast. Rays also regularly raid oyster grounds stocked by private planters.

Ray populations appear to be increasing because their natural predators—sharks—are decreasing in number along the coast because of fishing.

Despite the setback, environmentalists vowed to continue their restoration work.

“We’re not going to stop,” said David Dadurka, a spokesman for The Nature Conservancy.