FREEPORT, Maine — Small, green crabs are wreaking havoc along the Harraseeket River, and could soon devour the soft-shell clam population into extinction.

As water temperatures continue to rise and the winters get warmer, experts and clammers say the crabs, which eat spat — clams in larval stage — combined with coastal acidification, could drive 1,800 licensed clammers out of work and drastically alter the ecosystem.

Chad Coffin, president of the Maine Clammers Association, said the crabs have essentially taken over two-thirds of the productive clam flats in the last two decades, eating the mussels and scallops along the way. The devastation of clams has accelerated in the last decade and the problem is only getting worse, he said.

“It has taken 20 years for it to happen and essentially we’re the last thing left,” he said. “And it’s not just in Casco Bay; it’s happening all over the state. All significant shellfish populations will collapse in the next three to five years and it’s all related to the climate.”

Brian Beal, a University of Maine at Machias biologist, has been studying the decline of clam populations in the state and said warm weather, coupled with an extremely successful predator, historically spells disaster for clams.

Green crabs, originally from Japan, were first recorded on Long Island, N.Y., in the mid-1860s and weren’t seen in Casco Bay until the early 1900s, Beal said.

The green crab populations have been kept in check by severe cold snaps, Beal said, experienced frequently throughout the last century, allowing clams and other shellfish to recover.

But now, a warming climate has changed all that. Scientists fear the area might not have those same extended periods of cold experienced in previous decades, leading to larger and larger populations of green crabs, and as a result, the disappearance of clams.

“Most people don’t know it, but water temperature in the 1950s was as warm as it is now,” Beal said. “That temperature, combined with green crabs, wreaked havoc on the [clam] population.

Then in the 1960s, we had a period of three years with heavy snow and it knocked green crabs on their ass, and the clam population started to return. We just haven’t had a repeat performance of what we had in the 1960s.”

The explosion of the green crab population is a “weather-related phenomenon,” he said.

“Here in Washington County and Hancock, where they had the highest clam producing area in the state, it basically went away in the 1950s,” he said. “If we get an intense cold snap, that helps, but as water temperatures continue to get warmer — and when you look at weather patterns, every year is a little warmer than the one before — as far as green crabs are concerned, that’s the best thing that could happen. What I hope we don’t see is repeat performance of 1950s Washington County. That would be a major problem.”

On top of green crabs, clams have another less visible enemy: coastal acidification.

Joe Payne, of Friends of Casco Bay, said research shows when pH levels drop, caused by an increase in carbon dioxide, clam populations decline.

“Low pH makes it hard for clams to build shells, and when it’s really low it will actually dissolve spats,” he said. “And then when we find high pH, we find clams. If the green crabs don’t get them, then coastal acidification is going to. There’s two major whammies on soft-shell clams right now.”

Low pH has also been shown to cause stress in clams, Payne said, which can lead to a common soft-shell clam disease known as neoplasia.

And now, it’s not only clammers taking notice, it’s the lobstermen, too.

Eliot Thomas, a Yarmouth lobsterman who sits on the Lobster Advisory Board for the Maine Department of Marine Resources, said he has started to notice the crabs in his traps in areas where they were absent in recent years.

“The way I really notice it getting worse is, in years past, they were really only in shallow water. Now they’re in 40 to 50 feet of water,” Thomas said, noting that he’s heard stories of crabs eating lobster, but hasn’t seen it himself. Mainly, he said, the crabs now cause problems for lobstermen because they’re eating the bait.

Clammers, like Coffin, are trying to spread the word to other towns and clammers, but it’s proving difficult.

“The shellfish industry has never taken a regional approach to management. It has always been done through municipalities,” he said, noting that most believe the decline in clams is due to overfishing and poor management techniques. “It’s fragmented the industry. Nobody really wants to pay attention if they’re making money.”

Although it’s unclear what can be done to remove the crab species effectively, Freeport is one of the few towns taking steps to answer that question.

In May, the Town Council agreed to allocate $100,000 to work on the issue. So far, $10,000 has been used to buy traps, GPS units and to hire workers to help gather data, Town Planner Donna Larson said.

In late fall, the town dispatched 14 green crab traps in Casco Bay. About three weeks ago, Larson took a boat out and removed the traps, filling a 55-gallon drum to the rim with crabs — about 1,200 in total — ranging from 1-3 inches in diameter.

Larson said this was mostly a trial run to see if this mechanism of trapping works. Future efforts will focus more on gathering data.

“It’s part collecting data, and part to try and bring the population down,” she said. “Along the way we need to figure out if there is a minimal amount of data we can collect that doesn’t get in the way. If we’re going to measure them, we can’t do every single one. It’s not sustainable. We have to figure out a strategy of where the traps should go and when, and who’s going to do that.”

Trapping is only one part of the process, Larson said; the real challenge is finding a use for the crabs once they’re caught.

“The goal would be to find a use for these that could turn into a fishery,” she said, noting that the trapped crabs are now being used for compost. “Right now, they have no value, they’re a

detriment to the fishery. They have no monetary value. What we should be asking is, ‘how can we make this sustainable and lower the population?’”

On Dec. 4, the town council approved a $20,000 match for a grant that will help the town study the issue and look at water quality and temperature and pH levels, Larson said.

She said the town plans to advertise for the grant work soon and likely will begin work in early spring.

Although there doesn’t appear to be a simple solution for lowering green crab populations, Payne said pH can be offset by clammers and towns by buffering the clam flats with calcium carbonate — the stuff of clam shells.

This way, he said, they could at least keep pH at appropriate levels in the mud flats.

Coffin said he hopes other towns take notice of what Freeport is doing and take a regional approach to the problem.

“The reality is the town of Freeport is still the leader in the state. They’re actually investing the resources of the town,” he said. “We know we can’t eradicate green crabs, but we can try to knock down the population. We’re trying to prove the feasibility of defensive trapping and expand it to the state. That’s really the only hope for the shellfish industry.”

Barring an unpredicted return to the ice age, Beal said he doesn’t know of any fail-safe solution.

“One solution may be disease; that’s not unlikely. There’s natural diseases in any population. That might play a role,” he said. “I would hope that we would get some weather or some other phenomenon I’m not aware of.”

Although the disappearance of clams in Casco Bay would mean the end of an area industry, it could also be a devastating blow to the ecosystem, Beal said.

“Clams are a filter feeder, cleaning the water,” he said. “If you remove them, that function is missing or reduced, you get cloudier water, dirtier water; it may have an impact on other fauna as well. There’s many, many, many more species that are also susceptible to predation (by the green crab) that are also getting eaten, just maybe not at the level of soft-shell clams.”

In natural ecologies, when green crabs have been removed on a small scale, the density and the diversity of sea life increases, Beal said.

Coffin said he hopes more clammers start to take the problem seriously, noting that it’s hard to get people to pay attention, especially before it’s happening in their area.

“We didn’t care or really pay attention until we closed,” he said, referring to times when other areas of the state were seeing significant clam population declines. “A lot of the people we successfully recruited were like, ‘where have you guys been for the last 15 years?’ We’ve been digging clams.”