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10-year Green Crab Study Links Warming Waters with Continuing Growth

Harpswell Anchor
August 2, 2023

By John Gormley, originally published in the Harpswell Anchor

Paul Joyce, at the helm of a 21-foot center-console motorboat, slowly maneuvered the vessel away from the dock at Bowdoin College’s Schiller Coastal Studies Center on Orr’s Island.

Standing next to the captain was David Carlon, a professor of biology at Bowdoin who for the last 10 years has been conducting a study of green crab populations in the waters of Harpswell. At the stern was Eva McKone, a rising Bowdoin College sophomore who is spending the summer on a fellowship at the Coastal Studies Center assisting Carlon with his research.

woman and man work to empty crab trap into bucket

Eva McKone and David Carlon shake crabs out of a wire-mesh trap into a bucket on June 29. McKone, a rising sophomore at Bowdoin, is helping Carlon with his research under a fellowship program at the Schiller Center. (John Gormley photo)

On this foggy morning in late June, the boat was scheduled to visit four Harpswell coves — Strawberry, Brewer, Widgeon and Ash — where the researchers would be hauling traps (two in each cove) and collecting green crabs.

Green crabs are an invasive species whose prey includes soft-shell clams. Soft-shell clams have long played an important role in Harpswell, both for the commercial harvesters who depend on them for their livelihoods and recreational diggers for whom they provide food and fun. In recent years, the numbers of soft-shell clams in Harpswell have plummeted. Predation by green crabs is thought to be one of the primary reasons why.

Carlon explained that the damage being done by the crabs to the soft-shell clam population is what prompted him to begin his study of the green crab population.

“It’s the central reason why I started this project,” he said. “I knew about its economic impact, so I knew it had a lot of local relevance.”

He has been conducting the study since 2014. Its purpose, he said, is “to understand the underlying drivers of green crab population dynamics at decadal scales,” in other words, over a period of 10 years.

The first stop of the day for the boat and its crew was Strawberry Cove, just southwest of the Town Office on Mountain Road. As the boat approached a small white buoy in the cove, McKone picked up a gaff and moved forward to the port bow. McKone leaned over the side, snared the buoy line with the hook of the gaff and began hauling a crab trap off the bottom.

Soon a black cylinder made of wire mesh broke the surface. McKone slid the trap over the gunwale and into the boat, then dragged it to the stern. After opening a hinged hatch on the cage, McKone, with Carlon’s help, began shaking the trap over a bucket. Most of the crabs dropped into the bucket. A few fell to the deck and scurried about, but they were quickly rounded up and placed with the others.

When the trap had been emptied, the bucket was perhaps a third full. “That’s not a lot of crabs,” Carlon said. “By July that bucket might be full.”

They moved to another trap in Strawberry Cove before continuing on to check the two traps in Brewer Cove, near the Coastal Studies Center at the northwestern end of Orr’s Island. Later they would cross Harpswell Sound to the eastern shore of Harpswell Neck to check the traps in Widgeon Cove before heading south for the long ride down to Ash Cove at the southern end of the Neck.

Long-term studies like Carlon’s are important because they allow researchers to differentiate between short-term variations and verifiable trends. To be valid, the data must be collected in a consistent way. In Carlon’s study, the traps are the same size and type from year to year, they are placed in the same locations, and they are checked at the same time intervals. Traps are set once a week and checked 24 hours later so that the time spent collecting crabs remains about the same.

With nine years of data in hand and more coming in, Carlon is working this summer with McKone to identify correlations between water temperature and abundance of crabs.

“We started this analysis this summer with the nine years of data we now have. We will add this summer for 10, and work the data up for publication,” Carlon said.

Their initial analysis indicates a distinct connection between temperatures and crab population. Rising water temperatures are “a prime suspect” in the growth of the green crab population, Carlon said.

The average temperature in Harpswell Sound has been rising at a rate of about 1 degree Celsius, or 1.8 degrees Fahrenheit, in 10 years, “and so has green crab abundance,” Carlon wrote in an email.

The variety of green crab found in Southern and Midcoast Maine arrived from Europe in the New York area in the mid-1800s. It expanded north but stopped when it got as far as Penobscot Bay, where colder water seemed to limit its range.

There is a second variety of green crab in Maine that arrived in Down East waters in the 1980s. It may have made its way here from the cold waters of Iceland and Scandinavia and it tolerates cold better than the green crabs to the south.

The two populations are known as the southern and northern ecotypes — groups within their species that have unique characteristics adapted to their environment. The ecotypes meet around Penobscot Bay. There is a zone in which the two seem to be interbreeding, Carlon said.

The fact that the southern ecotype tends to prefer warmer waters suggests that rising temperatures in the Gulf of Maine may be helping the southern green crab population to grow where it already exists and to expand its range farther north.

After trapping crabs, Carlon records the number caught and their size. He also records their sex. That last category has showed something surprising: many more males than females.

“We have found a male-biased sex ratio at all our sites, and throughout the season. It is more male biased in the early summer,” he said.

Over time this ratio should settle at 50-50. That could mean even more robust growth of the crab population in the future.

“The sex ratio is going to drive the population dynamic,” he said. “Maybe what we have now is not at equilibrium. We may see even higher numbers of crabs.”

Bottom line, the research affirms the link between population growth and water temperature. “The abundance has been rising,” he said. “If the water continues to warm, you are going to see more crabs.”

So what can be done? While there is some interest in green crabs as a menu item, Carlon is doubtful that commercial harvesting is the answer. “It will be difficult to control them by trying to fish them out,” he said.

young woman smiles while holding up fist-sized crab for camera

Eva McKone exhibits a green crab collected by the researchers. (John Gormley photo)

By riding currents, the larvae (the juvenile form of the crabs) can travel 50-100 miles from their place of birth. So if the crabs were eliminated from one area, new arrivals would quickly reestablish the population.

At present there does not seem to be an effective way to bring Maine’s green crab population under control. For now, the only option may be “you learn to live with them,” Carlon said.

While Carlon’s research may not address how to control the green crab population, it does serve as a clear warning that the problems posed by the crabs are likely to get worse as water temperatures continue to rise.

It also provides another valuable service: scientific education and experience for students like McKone.

This summer she is one of 15 Bowdoin undergrads with fellowships that allow them to get field and laboratory experience at the Coastal Studies Center. Her fellowship began in early June and will end in mid-August. McKone is a graduate of Andover High School in Massachusetts. Her family recently moved to Kittery, so she now calls coastal Maine home.

When applying to Bowdoin, she did not know about the Coastal Studies Center.

“I definitely knew I wanted to conduct research at Bowdoin and I knew I wanted to do it in biology. I was not aware of this specific program. I’m very happy that I stumbled upon it,” she said. “Working at Schiller has been an amazing experience.”

Holly Parker is the director of the Coastal Studies Center, a post she assumed a year ago. She describes the center as “an incredible living laboratory.”

Bowdoin undergraduates, by working closely with faculty members at the Coastal Studies Center, are getting almost graduate-level research experience, she said.

She sees the center as an important point of contact between Bowdoin and the people of Harpswell. She expects the work done by the researchers will address issues that impact Harpswell. And she believes in the importance of researchers and students engaging with the surrounding community by drawing on local people’s knowledge and experience.

She observed that the lobster industry, for example, has a long and successful history of protecting the lobster population from overfishing by “managing what they take out and what they put back in the water,” through rules such as size restrictions and notching the tails of egg-bearing females to make them off-limits to harvesting.

“The great thing about being in a place like Harpswell is the lived knowledge,” she said. “The research we do here is a piece of the puzzle. We need to get other sectors on board.”

John Gormley is a retired journalist who lives in Cundy’s Harbor. His interests include fishing, tennis and gardening.