MISSISSIPPI STATE -- Mississippi River flooding and an open spillway northwest of New Orleans may spell disaster for the 1997-98 oyster harvest.

Oysters grow in the brackish (part salt) waters of the Sound, the waters of the Gulf of Mexico along the coastline. As the water is diluted to become freshwater, they die, said Dr. David Veal, director of the Mississippi State University Sea Grant Advisory Service in Biloxi.

"If the Bonnett Carre Spillway stays open long enough, it will cause some oyster deaths, and it certainly will influence the oyster harvest for the rest of this season and maybe some of next year," Veal said.

Oyster season typically lasts from September to April, and this year has been poor due to red tide. To date, the oyster harvest is valued at $2.3 million with 179,547 sacks landed, down from 326,037 sacks landed in the 1995-96 season.

The Bonnett Carre Spillway on the Mississippi River in Louisiana was opened March 17. It was done to prevent flooding in New Orleans and to protect the levees, said Jim Addison, New Orleans District Army Corps of Engineers spokesman.

The spillway diverts about 14 percent of the river into Lake Pontchartrain at a rate of 240,000 cubic feet a second. Lake Pontchartrain flows into Lake Borgne, which empties into the Gulf of Mexico.

Through this path, the fresh waters of the flooding Mississippi River are diluting the brackish waters of the Mississippi and Louisiana Sound. Given proper conditions and enough time, part of the Gulf of Mexico from the mouth of the Mississippi River eastward will become freshwater.

Oysters can survive in freshwater for about a week before they start dying. While this would create a short-term disaster, the diluted saltwater should mean good news later.

"The oyster harvest has been declining for more than 100 years in the Gulf because of the lack of freshwater," Veal said.

With the spillway closed, freshwater cannot flow into the marshes east of the Mississippi River as it did before levees were built in the 1930s. Instead, brackish areas where oysters and young fish grow become too salty.

"That decreases the productivity of the oysters and increases the productivity of oyster pests," Veal said.

The added freshwater should restore the natural balance.

"Historically, when we have opened the spillway we have seen immediate loss of production followed by three to five years of record oyster production," Veal said.

The spillway has been opened just eight times in its 66 years. Bays control the amount of water that passes. The last time it was opened, May 20, 1983, all 350 bays were opened. As of March 27, the Corps had opened 298 bays at a rate of 15 to 40 a day, Addison said.

The spillway will probably stay open for a month, Addison said. By staying open, it is reducing the Mississippi River flood crest, currently at 16.8 feet, by a foot and a half to two feet.

It is impossible now to predict what affect the open spillway will have on oyster production, Veal said. The Sound also receives freshwater from the Pearl, Biloxi, Wolf and other rivers.

Addison said the Corps is conducting a $500,000 monitoring program of the open spillway's effects. The Corps will monitor the productivity of shellfish and other fish in affected areas.