MACKWORTH ISLAND, Maine — It’s a chilly August morning off the southern coast of Maine and the mudflats extend as far as the eye can see. Clad in heavy rubber boots, Carissa Maurin and Sheba Brown splash through small pools of water as they crunch over shells of all kinds — razor clams, oysters, periwinkles — while green crabs scamper sideways across their path.
As they walk farther out, they start seeing the familiar deep blue shells that they’re looking for: wild blue mussels, first in small clumps, and later in piles several feet across. The good news is that they have found them at all. The more perplexing news is where they found them — this mussel bed was once on the other side of the island.
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Across the Gulf of Maine, wild blue mussels are on the move, and on the decline. A 2017 study found that as the gulf’s waters have drastically warmed in the past 40 years, the population of wild blue mussels has dropped 60 percent along the coast. Since then, those who live on and know these waters — fishermen, coastal residents, aquaculture farmers — say the mussels seem to be disappearing.
“Where there used to be big beds of blue mussels, there just aren’t anymore,” said Mike Doan, staff scientist at the conservation group Friends of Casco Bay.
Now, across the gulf, scientists are seeking to understand the extent of the loss and the mechanisms causing it, while mussel farmers work to adopt new techniques, including starting mussels in a lab to harden them to a more resilient state before they are returned to the gulf.
“Climate change is happening, and we’re trying to figure out how that’s playing a role in these changes and documented losses in mussel beds,” said Maurin, the aquaculture program manager at the Gulf of Maine Research Institute.
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Mussels play a relatively small role in Maine’s commercial fishery compared to other species, representing just 1 percent of the state’s more than half-billion dollar catch in 2022, according to preliminary data from the state. But they play a crucial role in the ecosystem, providing habitat for other small sea creatures that fish feed on and filtering the water, and researchers see the dramatic decline of this historically prolific species as a potentially dire warning about the gulf’s overall health.
When ecologist Cascade Sorte started looking at the state of wild blue mussels in the Gulf of Maine more than a decade ago, she made a stunning discovery: Where sites once had between 15 and 62 percent of their surfaces covered by mussels based on historic surveys, she found as little as 2 percent covered in some places. “The bottom line is that mussels are declining, and they’re declining really significantly,” said Sorte, now at the University of California Irvine. Sorte said similar declines have also been observed in California and elsewhere.
It wasn’t long ago, Maurin said as she stood amid piles of seaweed and surrounded by tidal pools, that mussel beds were easy to spot along Maine’s shore — ubiquitous piles of tell-tale blue shells, mounded atop each other.
These days, they’re harder to spot as the beds appear to be retreating farther out to sea, as numbers shrink, making it harder for scientists to track the status of the species. One hypothesis is that the mussels are moving to deeper waters, which are cooler, but once there, they are more vulnerable to predators.
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Maurin and Brown are enlisting community members to log sightings of mussels on days with extreme low tides, noting what they’re seeing — or not seeing — and where, and whether they’re seeing such predators as invasive green crabs or dog whelk, a predatory sea snail. Later, scientists from the institute plan to take boats to places where mussels were spotted and perform acoustic mapping of beds.
Since the early 1980s, the Gulf of Maine has been warming at more than three times the average rate of the world’s oceans: 0.86 degrees per decade, compared to 0.27degrees. Last year, the Gulf of Maine’s sea surface temperature was the second warmest since 1982, according to a report by the Gulf of Maine Research Institute, and all of the top ten warmest years have occurred since 2012.
That warmer water is contributing to a boom in the green crab population, which is bad news for the mussels. For years, the crabs have been moving steadily north and booming in population as the water warms. In the past, Maine’s severe winters kept the green crab population in check, said Aaron Whitman, a senior research association at the institute. “Now, with a little bit warmer water in the winter, there are more surviving green crabs from year to year.”
The warmer water also appears to be affecting the mussels’ ability to spawn, a problem that mussel farmer Evan Young has seen first hand. Like many other mussel farmers, Young’s Blue Hill Bay Mussels business was catching wild mussel spat — tiny young mussels — on ropes dangled from the sea surface and then growing them to full size. When he started 25 years ago, it was like clockwork. “We would put empty ropes in for collection around the first of July and mother nature would just give us our product,” he said.
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About 15 years ago that started to change, and what was once consistent became sporadic. Some years, there were few or no spat on the ropes at all. “For a couple years there I barely got paid, or I worked on the side doing other things to get through,” he said.
Faced with what appeared to be an existential crisis, Young consulted Kyle Pepperman, a marine biologist who runs a research hatchery at the Downeast Institute in Beals, Maine. The institute had been growing clams since 1987, so when Young told them about the problem with young mussels, they thought, “OK, this is kind of our jam,” Pepperman said. “So we embarked on a project to figure out how to grow mussels in a hatchery. And not just how to do it — how to make it economical.”
It took years, but Pepperman and Young said it finally paid off. Starting the mussels in the protected hatchery allows them to grow a more resilient size, with shells less vulnerable to inhospitable temperatures and the ocean’s acidification, a byproduct of warming. This year, Young says he is on track to harvest 300,000 pounds, more than double his harvests before the decline.
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Matt Moretti, co-owner and CEO of Bangs Island Mussels, started working with the Downeast Institute more recently, after putting his ropes out a few years ago and failing to catch any spat. Instead, he said, the ropes were filled with sea squirts — a tube-like sea creature that has been appearing in larger numbers in Maine’s warming waters. “It was devastating,” he said.
Now, in addition to buying spat from Downeast Institute, Moretti said they’re trying to monitor mussel larvae, sampling water to find out when the mussel larvae are there. “It used to be every spring, very reliably,” he said. “But for the last few years, it’s happening over the winter instead.”
Moretti said they will use that information to better anticipate when to expect spat.
He also said they may have to consider moving away from Casco Bay, where the waters are warmer than they are farther north.
As mussel farmers search for ways to safeguard their catches against predators and warming waters, the Gulf of Maine Research Institute scientists are still seeking to better understand how wild mussels are faring.
On a recent extreme low tide day, seven volunteers searched tidal flats near six communities from Rye, N.H., to just north of Bar Harbor, and logged what they found.
A volunteer in Southport, Maine, saw just a few mussels. At Moody Point in Wells a volunteer reported a lot of snails and algae and barnacles, but no mussels.
In Rye, another respondent visited a beach that used to have abundant mussels in the 1980s and 90s. On this day, at least, the volunteer wrote that there were none.
Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her @shankman.