Thieves wiped out three oysters reefs created for scientists to study how the shellfish clean the Chesapeake Bay, prompting public fishery managers to post signs warning people to stay out of the reserves.

The poachers used tongs and dredges to take the healthy, 2-year-old oysters from the sites in the Choptank River. The reefs were built three years ago at a cost of about $87,000. The three are part of 16 publicly financed oyster reefs in Maryland waters, all of which are off-limits to oystering. The reefs were built by the U.S. Army Corps of Engineers, with help from the Maryland Department of Natural Resources.

“What’s much greater than the value of the oysters is the worth of the knowledge that was lost,” said Ken Paynter, a biologist with the University of Maryland. “We had a lot more to learn, both about the oysters and about the community that was growing up around them.”

Oyster beds have practically disappeared in the Bay because of years of overharvesting and diseases, such as Dermo and MSX. However, divers who checked the study reefs three times a year had been finding encouraging signs: The hatchery oysters had grown to market size free of disease, and had attracted an abundance of marine life. Then, during an inspection at one reef six months ago, the divers returned to the surface and said, “They’re all gone,” Paynter said.

The only oysters that remained at that reef were smashed into the mud of the river bottom, a sign that the missing oysters had been taken by a dredge. Only the Maryland skipjack fleet is legally allowed to dredge oysters.

“We have no idea how many times it was hit, whether it was one or 100 times. All we can say is that the oysters are gone,” Paynter said. At two other sites on the Choptank — at British Harbor and Goose Point — 90 percent of the oysters had been tonged off the beds. State officials said whoever poached the oysters probably knew the area was off-limits.

Eric Schwab, director of DNR’s fisheries division, said that the poaching reversed the agency’s thinking on whether to mark the beds, which had not been marked to avoid advertising their location. The agency now realizes that leaving the reserves unmarked makes it harder to catch thieves.

The maximum penalty for poaching oysters is $500 for the first offense; repeat offenders can lose their fishing license. Proving poaching, however, is tough. Lt. Col. Tammy Broll of the DNR Natural Resources Police said county judges are reluctant to convict a poacher unless a police officer has witnessed the poaching and can verify that it took place inside a reserve using navigation gear.