Fish woes linked to oysters' decline Shellfish remove Pfiesteria from water

September 27, 1997|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Due to an editing error, an article Saturday about the role of oysters in controlling harmful algae such as Pfiesteria microorganisms misstated the view of three scientists. The three had spoken in another context, agreeing that Chesapeake Bay seafood appeared to be safe. They were Dr. Sandra E. Shumway of Southhampton College at Long Island University, Dr. Katherine Richardson of the Danish Institute for Fisheries Research and Dr. Patrick Gentien of the French Institute for Sea Research.

The Sun regrets the error.

Decimation of the Chesapeake Bay's once-abundant oyster population over the past century may have removed one of the bay's natural mechanisms for controlling organisms such as Pfiesteria piscicida, scientists say.

Percy Donaghay of the University of Rhode Island Graduate School of Oceanography said yesterday that oysters are a kind of marine "grazer." They remove algae and other microscopic phytoplankton such as Pfiesteria -- and the nitrogen they contain -- from the water.

FOR THE RECORD - CORRECTION

The oysters' feces are then processed by bacteria in bottom sediments. The bacteria convert the nitrogen to a gas and remove it from the ecosystem.

"They're the equivalent of sewage-treatment plants," said Roger Newell, a professor at the University of Maryland's Center for Environmental Science at Horn Point.

By nearly wiping out the Chesapeake's oysters, Donaghay said, humans -- and more recently oyster parasites -- have created a bay in which thenutrients stored in one-celled algae are no longer removed from the ecosystem when the algae die. Instead, they are left to be consumed by the next well-fed generation of algae.

Chesapeake is an Algonquian Indian word meaning "great shellfish bay." The bay was once studded with oyster reefs so large that they rose above the surface in places.

In the past century, over-harvesting has reduced the bay's oyster population by 99 percent.

The 1996-1997 harvest was about 130,000 bushels, down from annual harvests of 2 million bushels two decades ago.

Newell said oysters once were abundant enough to filter the bay's water in three to five days. Today it takes them 300 to 400 days.

The bay's waters are rich in nutrients, making them ripe for the growth and frequent "blooms" of a variety of phytoplankton. Some of those organisms can be expected to be harmful or toxic.

Scientists investigating the recent Pfiesteria outbreak don't know whether high nutrient levels in the Pocomoke River and two other bay tributaries contributed to the Pfiesteria-related fish kills and human illnesses there, but that is possible.

Harmful algae blooms occur when a variety of factors converge to produce a favorable combination of environmental conditions.

Scientists think the one-celled Pfiesteria organism usually spends its time eating other algae. Something caused it to release toxins that stun fish and then dissolve the fish flesh so that it can be eaten by the Pfiesteria.

Increased inflows of nutrients from farm runoff, chicken manure and human waste might be part of the reason. The absence of oysters to remove the nutrients from the water also might have played a role.

"The point you need to make to readers is that lots of changes in the Chesapeake Bay have occurred because of man's activities," Newell said.

"One of the most marked has been the removal of oysters, and inputs of nutrients generally that are thought to allow algal blooms to reach levels they didn't before."

If scientists determine that the Pfiesteria problem could be addressed by reducing the flow of man-made nutrients into the bay, they might also find they have another reason to try to restore the Chesapeake's shellfish.

There are no known cures for the parasites MSX and Dermo, which have destroyed many of the bay's oyster beds. Efforts to restore productive oyster reefs, and to raise native oysters that are resistant to Dermo and MSX, have been disappointing, Newell said.

If those efforts fail, the answer could lie in the introduction of foreign oyster species that are more resistant to disease.

"It's a really contentious issue, because history is littered with examples where some do-gooder scientists like me have brought something in and made the problem much worse," Newell said. But with oysters playing such a key role -- in filtering nutrients from the bay and in providing fish habitat around their shell reefs -- "bringing in a resistant oyster species is perhaps beneficial from both a food and commercial, and an ecological point of view," he said.

In agreement yesterday were a number of other experts in the study of harmful algal blooms such as Maryland's Pfiesteria outbreak: Dr. Sandra E. Shumway of Southhampton College at Long Island University, Dr. Katherine Richardson of the Danish Institute for Fisheries Research and Dr. Patrick Gentien of the French Institute for Sea Research.

Pub Date: 9/27/97

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