Just a year after the BP oil spill crippled Louisiana's oyster industry, the fishermen face a new problem. Freshwater is set to be diverted from the mighty Mississippi River into the salty waters where the shellfish grow, potentially killing them.

To protect people, homes and businesses along the big river, the Army Corps of Engineers plans to open at least one spillway, sending water out of the river. The tactic may ease the pressure on levees, but it will almost certainly kill the shellfish, too.

Fourth-generation oysterman Shane Bagala spent months skimming oil to make money. Earlier this week, though, he embarked on his first oyster run, returning with a healthy catch. But he became worried when he heard the corps was considering opening a spillway.

"I'm very concerned because I'm just getting back to work now for the first time since the oil spill. Now it looks like something else might be threatening us," said Bagala, who has fished for oysters for 22 years.

The corps plans on Monday to open the Bonnet Carre Spillway, built about 30 miles northwest New Orleans in response to the great flood of 1927. The spillway diverts river water to brackish Lake Pontchartrain, and from there east into the fertile fishing and oyster grounds of Lake Borgne and the Mississippi Sound, and ultimately the Gulf.

It's been opened nine times since 1937, most recently in spring 2008, when the river was swollen by heavy rains in the Mississippi Valley.

The corps is also considering opening the Morganza Spillway, about 35 miles northwest of Baton Rouge. It diverts river water into the Atchafalaya Basin. It hasn't been opened since 1973.

Mike Voisin, an owner of an oyster processing and sales business southwest of New Orleans, said Morganza's opening would devastate oyster harvesting grounds that largely avoided damage from the oil spill.

"If Morganza opens, I assure you there will be significant (oyster) mortalities," he said.

Freshwater kills oysters because it wreaks havoc on their metabolism, preventing them from keeping a saltwater balance. Large amounts of oysters died to the west and east of the Mississippi River after the oil spill when officials used freshwater diversions in an effort to push back the oil so it wouldn't creep inland, Melancon said. Oil that spewed for months after the well blowout a year ago also contaminated oyster beds.

Roughly half as many oysters were harvested last year compared to 2009.

Nicholls State University biology professor Earl Melancon Jr. was concerned opening the spillways could set the oyster recovery process back a year.

"It's Mother Nature giving us another blow after what BP did last year," he said. "That's dramatic for these oystermen."

Al Sunseri, co-owner of P&J Oyster Co. in New Orleans, said the areas that would be harmed by Morganza's opening are some of the most productive in the state.

"It's pretty discouraging," he said. "It seems like there's never any end in sight. We've been in survival mode for months now."

Harry Blanchet, a biologist for the Louisiana Department of Wildlife and Fisheries, said the state didn't record any oyster deaths after the corps opened the Bonnet Carre Spillway in 2008, but large areas of oysters died in 1983 when it was opened to curb severe river flooding.

Mortality rates vary according to how long a spillway remains open and how much water passes through its gates, Blanchet said.

Corps spokeswoman Rachel Rodi said the Bonnet Carre Spillway may be open for two to four weeks.

Voisin, the owner of a processing business, said he understands why the spillways need to be open.

"I think it's the right thing to do," he said. "We live in this area, too."