Joshua McKerrow - The Capital Non-native oysters such as these were let loose in Carr Creek on the Severn River by a careless boater who broke their cage and anchor.

Asian' oysters escape into Severn River

By PAMELA WOOD, Staff Writer

Published September 05, 2007

Some "Asian" oysters are on the loose in the Severn River, but scientists don't believe there is much cause for alarm. For two years, the sterile oysters have been held in a cage made of chain-link fencing at the bottom of Carr Creek. They're part of an ongoing study testing the viability of the nonnative oysters in the Chesapeake Bay.

Because the oysters aren't native to the bay, scientists tried to control the experiment to minimize the chance of escape. The cage was set down in a no-anchorage area and buoys marked the site.

But a boater still managed to thwart their efforts, officials said.

When divers checked the site July 24, they found the cage had been moved 6 feet along the bottom and a boat anchor was embedded in the cage, said Dr. Donald F. Boesch, president of the University of Maryland Center for Environmental Science.

He said a boater tried to anchor in the area and got stuck on the cage, dragging it before the anchor broke off. The cage remained intact, but some oysters slipped out the bottom.

For the past several weeks, researchers have tried to account for all the oysters, Dr. Boesch said.

"They've done a very repeated, thorough search around the cage to find the oysters," he said.

Divers found 557 oysters. There's still a possibility that up to 110 more could still be in the river somewhere, according to a report on the incident prepared by scientists Kennedy Paynter, of the University of Maryland, and the Virginia Institute of Marine Science's Mark Luckenbach. They said most of the remaining oysters either were destroyed or buried in the creek floor.

The oysters that were recovered were tested in the lab to verify that they were indeed the "Asian" oysters, Crassostrea ariakensis, and that they still were sterile.

The sterile "triploid" oysters have been known to occasionally revert to their reproducing "diploid" status.

Given the number of remaining oysters and the chance of reversion to reproducing status, Dr. Boesch said the chance of the remaining oysters mating is less than 1 in 3,000.

"That's probably an overestimate of the risk," Dr. Boesch said.

Just to be safe, the oysters will be pulled out of the river as soon as next week, about a month before the experiment was supposed to end.

Scientists were looking at the rates of growth, survival and disease in the ariakensis oyster compared to the native oyster, Crassostrea virginica. The experiment is part of the ongoing "Oyster Environmental Impact Statement" being developed jointly by Maryland, Virginia and the U.S. Army Corps of Engineers.

The statement looks at various factors for replenishing the flagging oyster population, including introducing the ariakensis oyster to the bay because it is believed to be hardier and more disease-resistant.

The report has been delayed several times and is now expected to be released in the spring.

Because the Carr Creek oysters have been in place since October 2005, scientists have enough data to work with, even though the experiment is ending early, Dr. Boesch said.

University of Maryland Center for Environmental Science scientists worked with the Maryland Department of Natural Resources and the Corps of Engineers on their plan following the discovery of the escaped oysters.

Eric Schwaab, the deputy secretary of DNR, said he thought university officials handled the incident well.

"They've operated in good faith." Mr. Schwaab said. "They did everything they could to minimize risk up front and post-event. We're happy with the steps they've taken."

(Revised Mar 2008)