Oysters survive winter in Charles

The Massachusetts Oyster Project, a nonprofit water cleanup effort started in Charlestown, confirmed yesterday that about half of the 150,000 pollution- filtering oysters the group deposited near the Charlestown Bridge in October have survived the winter.

"When you think of how polluted that water can be, you'd expect them all to die," said Anamarija Frankic, an assistant professor of coastal ecosystem management at the University of Massachusetts at Boston.

"For half of them to grow, it's reasonable to assume they'll survive and reproduce," she said.

The Massachusetts Oyster Project hopes this batch of Eastern oysters - once a native species that could grow as large as a man's shoe - will thrive again in the river, and help clean it as well.

As it eats, each oyster filters about 30 gallons of water per day, removing bacteria and organic pollution that runs downstream from sewer overflow during heavy rains.

Rendered inedible by their work, the oysters are a potentially powerful contributor to a cleaner Charles River and Boston Harbor.

Given the number of survivors found in the group's informal survey, 75,000 or so remaining mollusks will be able to clear approximately 2.25 million gallons daily.

The chief project organizer, Andrew Jay of Charlestown, hopes the colony's early success proves sufficient to win the Massachusetts Oyster Project its next grant.

Jay and members want to bring more oysters to the Charles, and are developing plans for colonies in the Neponset River as well.

"A large part of the population is surviving and growing," he said. "Clearly there is enough evidence to warrant the continuation of the project."

As volunteer divers Mat Brevard of Worcester and Rich Bradshaw of Watertown emerged from the murky riverbed - thumbs up in a sign of success - Frankic navigated a slope of rocks, slippery with a pea-green seaweed, to gather the bag of about 30 oysters Brevard had retrieved.

The freshly retrieved oysters were easily one-and-a-half to two times the size they were when they were dropped into the river.

Though three or four of the shells inside the bucket were empty - the husks of bivalves not so lucky - the majority of the specimens were clean, their lavender and charcoal shells clear of growths. "They look healthy," Frankic said.