The Green Crab Invasion

Will Carcinus maenas destroy Maine’s $15 million soft-shell clam industry?

By Edgar Allen Beem

Chad Coffin and a volunteer crew of half a dozen Freeport clammers plod out onto the muddy flats of Recompense Cove in hip boots, not to dig clams but to erect a low, two-thousand-foot-long fence designed to deter green crabs from invading the cove, eating all the soft-shell clams, and destroying their livelihood.

As he looks out over the cove at low tide, Coffin, president of the Maine Clammers Association, describes how just a few years ago there were thick mats of mussels and dense stands of eelgrass in the intertidal zone. Now there is just gray mud and green crabs.

“Everything is being systematically eaten,” Coffin says. “All the shellfish resources disappeared based on proximity to the deepwater channel.”

Large male green crabs live in the channel, females and juveniles in the mud and along the shore. Upper Casco Bay is literally crawling with crabs. Turn over a handful of rockweed and dozens of green crabs, ranging from the size of a quarter to the size of a human hand, scurry for cover. Coffin stomps on them like bugs.

Sometimes called “cockroaches of the sea,” European green crabs are voracious eaters, consuming as many as forty clams a day each, and prodigious breeders, with females laying as many as 185,000 eggs a year. The crabs appear to favor estuaries where fresh water empties into salt, and Freeport has a lot of estuaries. Local clammers have removed fourteen thousand pounds of crabs from the Harraseeket River on the other side of Wolfe’s Neck, Little River Cove is littered with pens and netting designed to deter green crab predation, and Recompense Cove has the low fence of two-by-fours, plastic mesh netting, and aluminum flashing. The green crabs have already colonized the cove, but the clammers hope they can trap the existing crabs and keep new ones from moving in while the clams make a comeback.

All of these efforts are part of a $70,000 local offensive aimed at combating the green crab invasion and saving the Freeport shellfishery. Communities up and down the Maine coast are watching Freeport carefully.

The fact that green crabs are posing a serious threat to Maine’s $15 million soft-shell clam industry and the livelihoods of the state’s 1,700 licensed soft-shell clam harvesters is big news, but it’s nothing new.

“Since 1949,” noted a 1961 U.S. Department of the Interior report on The Soft-Shell Clam Industry of Maine, “the green crab has been the most serious clam predator in Maine. In 1951, an experimental clam farm in Scarborough, operated by the Department of Sea and Shore Fisheries and the Fish and Wildlife Service, was completely destroyed in three weeks by green crabs. Entire populations of clams outside experimental fenced areas in Islesboro, Wells, Bremen, Jonesport, and Searsport were destroyed by these crabs.”

The European green crab (Carcinus maenas) is an invasive species that first came to American shores in the early nineteenth century, probably in the ballast of ships. They were first reported in Casco Bay around 1900 and had reached the Washington County coast by the 1950s. The rise of the green crabs coincided with a decline in Maine clam harvests, which fell from thirty-three million pounds in 1950 to just seven million pounds in 1959. A series of cold winters in the early 1960s is credited with stalling the green crab onslaught and allowing the clam population to rebound. The Maine clam catch peaked at thirty-eight million pounds in 1977 and then went into a long, slow decline to around eleven million pounds a year today.

Warmer water temperatures are the most frequently cited cause of the accelerated growth of the green crab population in recent years. At a news conference at Winslow Park in Freeport held to announce a one-day statewide green crab census project this past summer, Department of Marine Resources (DMR) Commissioner Patrick Keliher told reporters that virtually everything happening to Maine fisheries, both good and bad, “is attributable to warming ocean temperatures.”

Whatever the causes, the impacts of the green crab invasion are clear — loss of shellfish, loss of sea grass, and soil erosion where crab burrows undermine the shore.

Clammer Chad Coffin says he began noticing the changes almost five years ago, but it was not until last year that Freeport clammers began actively doing battle with the crabs. In May 2012, the Freeport Town Council agreed to appropriate $100,000 to research and respond to the threat. The project was held up by delays getting permits from the U.S. Army Corps of Engineers and discussions over the direction of the project such that Freeport is now committed to spending $70,000 over the next year to determine what, if anything, works.

“Freeport stands alone as a town in Maine that has put down hard money and the only reason is that Chad is relentless,” says Darcie Couture, a former DMR scientist whose Brunswick-based Resource Access

International was hired to research the causes of Freeport’s soft-shell clam collapse.

The lead scientist on the Freeport project is Dr. Brian Beal of the University of Maine at Machias. Beal, who dug clams in high school and college and whose father was a shellfish dealer, is an expert on the soft-shell clam and has been studying green crab predation since the 1980s. “Green crabs have always been there in the background,” says Beal. “What has happened is that clammers, through Chad’s persistence, have come to recognize a problem that has always been here.”

Beal’s base of operations is the clam flat at the mouth of the Little River. There he has erected a dozen thirty-by-thirty-foot exclosures and placed ten fourteen-by-twenty-two-foot nets on the mud. The experiments are designed to determine the most effective way to keep the crabs off the clams. Currently, there are no soft-shell clams left on what used to be one of Freeport’s most productive clam flats. “We did five core samples from each of the twenty-two areas,” says Beal. “We found no live clams. Zero for 110.”

The hope is that enough green crabs can be trapped, fenced out, or otherwise deterred so that the clams can rebound. But is it too late to save a fishery once all the clams are gone?

“I don’t know,” says Beal. “But to do nothing is an invitation to watch the soft-shell clams disappear. The game is up.”

Just around Flying Point in Maquoit Bay, Dr. Hilary Neckles, a U.S. Geological Survey scientist, has been working with the town of Brunswick to research what happened to the bay’s lush expanses of eelgrass, an important breeding ground for fish and shellfish. “Maquoit Bay used to have over five hundred hectares of eelgrass,” says Neckles. “Now there is none left in the intertidal zone.”

Last summer, Neckles observed rafts of eelgrass the size of houses floating on the bay and she thinks she knows why. Neckles gathered samples of the severed eelgrass and “all had the signature of green crabs. They had been uprooted, and they had snipped and shredded bases.” Neckles has installed exclosures in Maquoit Bay to keep crabs out while she determines the best way to transplant eelgrass.

Neckles calls the green crab an “ecological catastrophe” for Maine, describing what clammer Chad Coffin witnessed over the past five years as “an absolute cascade” — scallops disappear, mussels disappear, clams disappear, sea grass disappears, and with them go the seabirds and fish that once depended on them for food and habitat. Clammers just have to hope they won’t be the next to disappear.

Given that green crabs are an old problem and their increasing numbers were reported a decade ago, it seems that Maine has been slow to respond to the crab crisis. “Getting the DMR to recognize the crab issue was a big ship for us to turn around,” says Chad Coffin, who now feels that the department understands the severity of the threat and is helping to solve the problem.

Some folks up the coast aren’t so sure.  “We’re pretty much a year ahead of the DMR,” says Bailey Bowden, chair of the Penobscot Shellfish Conservation Committee. “We’re waiting for them to catch up with us. We were aware of the problem last year, but we got wiped out in one summer. We lost eight hundred acres. The catch went from one hundred thousand pounds a year to ten thousand pounds.”

When town officials and clammers realized that the estuarine landscape of Penobscot’s Northern Bay was under attack by green crabs, they sent a delegation to meet with DMR staff in Boothbay Harbor. “They really didn’t believe green crabs were the problem,” reports Bowden. “They thought it might be neoplasia [a cancer in clams] or ocean acidification.

St. Joseph’s College professor Dr. Mark Green, who has received several National Science Foundation grants to study ocean acidification in the Gulf of Maine, is taking part in the Freeport study. Clam larvae float in the water column and eventually settle out of the water into the mud. Green has found that the larvae evaluate the mud for acidity. If there is too much, they will lift off in search of a better bottom. If they are forced to settle in acidic sediment, clam larvae simply dissolve. Buffering clam flats by adding ground shell to the mud may reduce the acidity, Green says, but the main problem remains the green crabs. “Green crabs are a calamity,” he says, explaining that green crabs eat the bigger clams, and young clams are not repopulating the flats.

So far, the war against the green crab has been fought entirely on the local level. Maine actually had a green crab fencing program statute on the books, authorizing the DMR commissioner to spend up to $25,000 annually on crab fencing, but the program had no money in it and was repealed in 2012, just as it was really needed. Racing to catch up with the problem, the state also started issuing municipal permit exemptions to a 2001 law requiring a license to harvest green crabs.

“It was missed. It was completely missed,” says Darcie Couture of the green crab problem. “This was one of those things that went under the radar. In Maine, lobster is king.”

Some now worry whether lobsters may be the green crabs’ next victims. Green crabs do eat lobster larvae and small lobsters. One laboratory experiment found that green crabs beat lobsters to a food source every time, and that lobsters were only able to displace the crabs in two of sixty-five attempts. The experiments also found that green crabs devoured juvenile lobsters in six out of eleven trials.

Recompense Cove “used to be a lobster-shedding ground in June and July, but there’s not a single trap out there now,” says Chad Coffin. He believes that “lobstermen have been hugely impacted by green crabs. The crabs have herded lobsters together and made it easy to stay on the lobsters.”

So Freeport clammers may be protecting lobsters as they fight to protect clams, but DMR Commissioner Keliher said at the Winslow Park news conference, “You know the efforts from just the clamming industry alone will not be enough. It’s going to need some sort of business entity that’s going to want to come in and try to remove [the crabs] at a very high rate along the coast.”

Ultimately, it may be Maine’s $350 million lobster industry that comes to the rescue of the $15 million clamming industry. At least that’s what seems to be happening in Nova Scotia.

Chris McCarthy, a Parks Canada ecologist at Kejimkujik National Park in Nova Scotia, says that six years ago that maritime ecosystem suffered exactly what Freeport and other Maine coastal communities are experiencing now — clams and eelgrass disappearing from estuaries suddenly overrun with green crabs. The green crabs plaguing Canada, however, were not from the first introduction that worked its way up the eastern seaboard, but a hardier strain (or clade) adapted to colder waters and brought over to Halifax in ships from Iceland and the Faroe Islands in the 1980s.

Those green crabs stripped away all but 1 percent of the Keji Seaside’s eelgrass by 2010. A combination of trapping crabs and restoring eelgrass was undertaken the same year. “We’ve had a 70 percent survival rate where there are fifteen crabs per trap per night or less,” reports McCarthy.

To reduce the crab population to such a manageable level required a market incentive, which Nova Scotia found in the demand for live lobster bait. Forty-five licensed green crab fishermen now earn as much as $100 for a one-hundred-pound tote full of green crabs, which are spiked live, two to a lobster trap. “This is now a commercial fishery,” says McCarthy, “but we need something else beside bait fishery. Bait people only want the big ones.”

Ironically, Nova Scotia fishermen are already worried that the new green crab fishery may not be sustainable.

The DMR prefers to focus on reducing the green crab population rather than managing it as a commercial fishery, according to Keliher, who has said, “We have no interest in managing an invasive species that is so destructive to our fisheries.” But managing a green crab fishery may be what it takes to save the soft-shell clam industry — that, or a few good old-fashioned Maine winters.