DEI Experiments to Conserve the
Future of Clamming

by Sarah Craighead Dedmon

Above, young clam. Green crab below is small enough to get through netting mesh used to cover clam flats. Green crabs molt several times in a season. It is large enough to eat several newly recruited clam spat. Kyle Pepperman photo

Softshell clams are not especially bothered by the warming waters endangering so many fisheries in the Gulf of Maine. But neither are their predators. In fact, they’re thriving. “Down at Cobscook we’ve literally brought the drag up and dumped it out, all green crabs,” said David Cale, a fisherman from Machiasport.

According to Dr. Brian Beal, the scuttling green crabs Cale pulled up in his drag aren’t even the most damaging kind — it’s the billions of green crabs that we can’t even see. “They’re smaller than a tick,” said Beal, research director of the Down East Institute (DEI), a marine research laboratory located on Beals Island. “When they first settle, they’re smaller than the aperture of window screening, and most people don’t realize that.” Crabs that measure just 3 millimeters across are devouring softshell clam spat so young, they’re still invisible to the naked eye.

This year DEI began a three-year study called the “predator exclusion experiment.” Working with the shellfish committees of seven Downeast towns, DEI’s Kyle Pepperman led groups of commercial harvesters to the flats last spring where they planted hundreds of thousands of softshell clam spat, all grown in DEI’s hatchery.

Each group laid out a checkerboard of squares, seeding their area 5,000 clams at a time. Some of the squares were left unprotected, and the rest were covered with nets. The goal? To measure the survival rates of clams protected from green crabs, as compared with the survival rates of clams that are not.

In November, Pepperman led groups to take core samples from the experiment sites. Final data from the experiment’s first year will be compiled in January.

Beal performed a similar experiment in Freeport this summer, with overwhelming preliminary results. Samples taken from beds protected by nets produced roughly 2,500 juvenile clams per cubic foot, but an uncovered sample produced only eight.

Predators

Kyle Pepperman said that Maine’s softshell clam industry began to decline in 1984, largely due to predation. That year the state’s landings came in at 25 million whole pounds of clams. In 2016, Maine landed 7.3 million whole pounds, a number that has decreased by more than one million pounds per year since 2010.

Green crabs pose one formidable threat to the clam fishery, but Pepperman said there is another predator that worries him more. “I am way more scared of milky ribbon worms than I am of green crabs, because we can’t protect against [the worms],” he said. “Crabs, we can put a net over the top and it offers pretty good protection. The worms, they’re decimating flats in this state.”

Milky ribbon worms can grow to be eight feet long. They prey on the clams below the mud, injecting them with a digestive enzyme then sucking out what Pepperman calls a “clam milkshake.”

“They’re basically unstoppable,” said Beal. “A gull will eat one, they break them into pieces, and these pieces can become new worms. They don’t die.”

Beal is beginning to wonder if the growth of the green crab population and milky worm population might be connected. “Are the green crabs preying on something that would eat the juveniles of the milkies?” he said. Or, are the green crabs preying on the worms and doing an incomplete job, leaving more pieces to regenerate into more worms?

“I think that the clamming industry for much of the state of Maine is going to look very different in the future if we continue to see warming water temps,” said Beal. “The only thing that’s going to happen is we’re going to have more predators.”

Arctic Surf Clams

Netting going over clam flat. Beds protected by nets produced roughly 2,500 juvenile clams per cubic foot, but an uncovered sample produced only eight. “The only thing that’s going to happen is we’re going to have more predators.” Brian Beal. Sarah Craighead Dedmon photo

Since 2010, DEI has been working with a different clam species not native to the intertidal zone, the arctic surf clam. “It’s a cold water species, and it’s a deep water species, but it can grow here if it’s protected from predators,” said Beal. “The best growth and survival that we have seen was actually in the Harraseeket River in Freeport.”

DEI has seen good success raising the clams through the first two stages of bivalve aquaculture: hatchery, and nursery. “It’s now come down to my part in the field,” he said. Earlier DEI research confirmed that arctic surf and softshell clams can coexist in the same flats, with no adverse effects.

Arctic surf clams normally live in waters 100 - 200 feet deep in a range stretching from Rhode Island to Labrador. Because densities are low off of the U.S. coast, Canada is currently home to the world’s only arctic surf clam fishery, valued at $50 million annually.

The market for the clam is almost entirely in Japan, where it is prized for its stunning color. “Ninety five percent of their value is in the foot,” said Beal. “which is a purplish color in a live animal that turns to a beautiful orange tinge when it’s processed.” Wild clams harvested in Canada range in age between 25 and 40 years in shells four-five inches across, but only the two-inch foot is sold for food. DEI is working to raise an animal that could be harvested whole after 18 months in the field, a much smaller harvestable product at approximately one and a half inches.

Beal said that no one has ever sold a live arctic surf clam before. “So that’s exciting for us. We’re trying to diversify the domestic seafood market with this species.” However, they both share common enemies. “It’s been pretty tough, because it turns out that green crabs really love this species,” said Beal, “even more than softshell clams.”

DEI conducted an arctic surf clam market assessment in 2015, and found that there is a viable niche with restaurants, specialty markets, and wholesalers. DEI served some of the clams during a meeting in Portland. “The people who ate them were just over the moon over its flavor and taste,” said Beal.

“The first thing is to keep green crabs at bay,” he said. “If we can’t, it won’t go anywhere.”

For more information about the work of the Downeast Institute, visit http://www.downeastinstitute.org/