Volunteers release bags full of oysters from the floats yesterday, in an effort to bolster Narragansett Bay’s oyster population.

The Providence Journal / Gretchen Ertl

BRISTOL — A small boat towed a long train of floating cages, each one packed with hundreds of razor-edged oyster shells, toward the rocky shore on the far edge of the Roger Williams University campus.

Steve Patterson waded out a little way and came back with a cage in his hands and a smile on his face.

“Get ready for oohs and ahhs,” he said. “These aren’t going to be the thumb-sized ones you saw last year.”

With that, Patterson opened the cage and dumped a heaping load of oysters into a bin. Some volunteers took them away to carefully measure each one.

The oysters — among some 276,000 grown this year — were not bound for local restaurants yesterday morning. By sunset, Patterson and the burgeoning group of volunteers he coordinates had redistributed thousands of them to new oyster fields taking root in Bristol Harbor and off Prudence Island.

The delivery of oysters to the Bay’s water each December is part of a large-scale effort to replenish the state’s oyster population.

Historically, oysters thrived in Narragansett Bay and supported a significant amount of the Rhode Island shellfish industry.

Contamination associated with the Hurricane of 1938 reduced the population. Then infectious parasitic diseases annihilated a lot of the remaining oysters.

The program at Roger Williams is trying to turn things around.

It turns out that the Kickemuit River, a focal point for environmentalists concerned about water pollution, is a good place to grow oysters.

“We attribute it to the fact that there’s more freshwater for the oysters to thrive in up here than there is in the ocean,” said Patterson, pointing to a map.

The Kickemuit is rife with nitrogen. In fact, tests show that it has a little too much of it. But oysters thrive on nitrogen, Patterson said.

He likens oysters to powerful little water filters that clean more than 50 gallons of water a day, straining out the particles they feed on and depositing the rest at the bottom of rivers, salt ponds and estuaries. This reduces the turbidity of water, which allows more sunlight to support aquatic life, which bolsters fisheries, including habitat for lobster and winter flounder.

The demise of coastal oyster populations wasn’t a result of water pollution, the experts say.

It was from the spread of dermo — a parasite that kills oysters.

For that reason, the oyster-gardening program has focused on restoring the Bay’s oyster beds with a species that resists and survives the parasite.

In theory, oysters could be deployed to highly polluted waters for the express purpose of filtration, Patterson said.

However, Rhode Island aquaculturists haven’t been aggressive about that so far because they worry about people eating oysters harvested from dirty water, he said.

Nonetheless, over the long term, the program hopes to make a visible impact on water quality in Narragansett Bay.

“If we can get a gazillion of these things out there filtering, within our lifetime — and I like to think we’re young — we should begin to see a difference,” Patterson said.

The origin of the effort at Roger Williams University dates back about five years, according to Dale Leavitt, an associate professor of aquaculture. That was when the school’s aquaculture program launched its first big batch of oysters in its hatchery, he said.

Later, in 2005, about 50,000 oysters had grown to the point that they could be released into the Bay.

A year later, the effort grew, recruiting a force of volunteers. The Rhode Island Oyster Gardening for Restoration and Enhancement was established and an even greater number of oyster larvae was developed in the hatchery at Roger Williams.

The larvae, visible only by microscope initially, are put in tanks where they attach to shells. In a short time, they are processing more water and food than the hatchery can provide.

At this point, the juvenile oysters, or spat, are moved into cages and hung in the waters of Bristol Harbor, the Sakonnet and Potowomut rivers and Point Judith and Quonochontaug ponds.

The cages keep the youngsters out of the sediment and away from predators: crabs and lobsters.

Oysters don’t grow in the wintertime, so that’s when the program brings them to various sites in the Bay. Volunteers hang oyster cages off their moorings and docks.

Thanks to the help of about 18 volunteer oyster growers, the program released about 100,000 oysters into the Bay last year, Leavitt said.

This year, the corps of volunteers reached 58. The program will release 276,000, Leavitt said, as he piloted an 18-foot boat into Bristol Harbor yesterday.

This time, many of the oysters are older and bigger than the ones released into the Bay last year.

Meanwhile, some of the oysters released in 2005 have begun to produce their own spat, Leavitt added. The oysters have reached a size of three inches, he said.

A minute later, he and Chelsea Apito, a 21-year-old marine biology student, pulled up to a buoy in the harbor. They dumped one more load.

“I think they’re interesting organisms,” said Apito. “They’ve been around for such a long time… I think it’s great we’re trying to bring

mreynold@projo.com