Volunteers turned out on a drizzly afternoon last week to load 200 mesh onion sacks filled with quahogs onto boats, ride out to the clam beds of the North River, and broadcast the quahogs into the shallows.

Led by the Marshfield and Scituate harbormasters, the volunteers transplanted contaminated clams from the Taunton River in what is known as a shellfish relay. The relay has become an annual ritual for the two communities, supplying clams for the winter-through-spring harvest and for spawning. Before the beds open for recreational clamming in December, the quahogs are expected to cleanse themselves and be ready for people to dig.

Debee Tlumacki for the Boston Globe

Delphis Guillette of Scituate, wearing a bug net, volunteered to spread seed clams.

“They always taste better when you earn them,” said volunteer Russell Clark, who owns a mooring service in the Humarock section of Scituate and piloted one of his work boats laden with clams for the relay. Come winter, he’ll know just where to dig.

“There’s nothing better on a cold day,” he said. He goes out onto the flats with a couple of buddies, digs, and brings home the clams to eat fresh in front of a football game.

Together, the towns transplanted more than 100,000 clams of various sizes. Most were 2 inches or more but not fully grown.

The clams come cheap for the relay at $19 per roughly 80-pound bag, but they harbor bacteria from closed shellfish beds. This year, the fishermen who hauled them up to Marshfield’s Damons Point Road in a pair of trailers said the quahogs came from the Taunton River in the vicinity of the closed Somerset Station power plant, also called Montaup.

Before fishermen deliver the clams to Marshfield and Scituate, they are tested for PCBs and heavy metals to make sure they fall well within federal standards, according to Reginald Zimmerman, a spokesman for the state’s Executive Office of Energy and Environmental Affairs.

Michael DiMeo, the Marshfield harbormaster, said they usually purge themselves of bacteria in 30 days. They will be retested before the beds open to make sure they are safe to eat.

Last Wednesday, DiMeo, two assistant harbormasters, and another volunteer piled into Clark’s boat around 3 p.m. with half of Marshfield’s 100 bags and headed northeast, around the marshes toward the mouth of the river. Although the clams arrived about an hour late on the trailers, the crew was still able to time the trip with the incoming tide. They seeded the deeper areas first, moving in as the tide rose.

Debee Tlumacki for the Boston Globe

The clams are from Taunton and will be ready for harvesting when the Marshfield shellfish beds open.

They balanced each sack on the edge of the flat-bottomed boat, cut the mesh, and let the clams slide into the water a few at a time. DiMeo coached as he worked: Spread them out, not too many at once, and keep the boat in the shallows.

They wove back and forth like the shuttle on a loom, covering several acres, most of which is not easily accessible by land.

“We have a lot of acreage, but unfortunately you need a vessel to get to it,” DiMeo said.

Marshfield has conditional approval for shellfishing, which means that instead of being open year round, the beds are opened seasonally based on environmental testing, he said.

On the North River, which flows along the Marshfield-Scituate line, the shellfish beds expected to open in December run from Route 3A eastward to a line near the mouth of the river. A map is available on the web page of the Marshfield Shellfish Division.

Resident shellfish permits in Marshfield cost $10, nonresident permits $25. Marshfield residents 65 and older can get their permits for free. In Scituate, the rates are $20 for residents and $50 for nonresidents.

David Dauphinee, a former commercial fisherman who runs an auto repair shop, owns a rustic cottage — it has no running water — on the Scituate side of the river, and when his family goes there, he enjoys digging quahogs for dinner. He was among the volunteers spreading clams last week.

“It’s giving back to a place that’s given way more to me than I’ll ever be able to repay,” he said.

Dauphinee said he looks for places that are relatively easy for people to access, yet protected from seagulls. Placing the clams toward the bottom of the tide protects them when the tide rises.

“What’s exciting is when you’re out there digging and you see juvenile quahogs that are a product of the ones we’ve brought in,” he said. That means the quahogs are reproducing, replenishing the population naturally.

Among the others who turned out for the afternoon were Sean McCarthy of Carver, who works for the Scituate Engineering Division; Dan Smith of Pembroke, who started digging clams last year and wanted to help; and Simone Zimmerman of Scituate, who heard about the relay through the North and South Rivers Watershed Association.

Shellfishing is important to Scituate for recreational, economic, and historical reasons, said the town’s harbormaster, Mark Patterson. Each quahog acts as a filter, cleaning the water of impurities; permits generate revenue for the town; and families can enjoy an activity that is part of the community’s heritage.

“Shellfishing is something that families from Scituate have done for generations,” he said.