Tracking Maine's wild mussel beds: declining or retreating into the deep?

Yahoo

31-05-2025

May 31—The wild blue mussel beds that once blanketed Maine's dynamic intertidal zone are disappearing, driven out by warming water that not only hurts the mussels themselves but benefits one of its chief predators, the highly invasive and always hungry green crab.
Scientists at the Gulf of Maine Research Institute want to know if the intertidal disappearing act is a sign the blue mussel population is in decline or in retreat, with its local beds gradually moving out of the easy-to-spot intertidal into the colder waters of the more far-flung subtidal zone.
"For the last 10 to 15 years, everybody has been saying mussels are disappearing," said research associate Aaron Whitman. "We think the beds are just moving out past the low tide line. But just because you don't see them twice a day doesn't mean they're not out there."
A 2017 study estimated Maine's wild blue mussel population has dropped 60% since the 1970s. Like baby lobsters, however, the mussel beds may have simply traded warmer for cooler, fleeing the warm intertidal for the somewhat cooler subtidal, inch by inch, one generation at a time.
But that makes the beds much harder to find. That is why GMRI is enlisting the help of citizen volunteers to help it find, measure and track these subtidal beds, the edges of which are only visible at extremely low tide, so it can document the health of the local population, especially in a changing climate.
On Friday morning, Whitman and Carissa Maurin, GMRI's aquaculture program manager, led a group of two dozen employees of M&T Bank out to Mackworth Island in Falmouth to document its subtidal mussel bed during peak minus tide, or one that was about a foot lower than normal.
"I love the environment," said Roxanne Gray, a mortgage originator in the Brunswick branch. "I've been a Mainer my entire life, so for me, being able to be a part of what keeps Maine a beautiful, healthy state, is really important."
They documented conditions (muddy), recorded the presence of predators (green crabs) and measured individual mussels (from three-quarters of an inch to almost five inches) at a mussel bed that Whitman said appears to have shrunk in size since GMRI surveyed it last year.
The bed has slowly moved, too, not just into colder subtidal waters but around the island itself, according to old state survey maps. The maps date to the early 2000s, which is the last time Maine surveyed mussel beds. Updating the state list will be harder now that many beds are submerged most of the time.
That is why GMRI encourages groups like M&T as well as everyday Mainers all along Maine's 3,500-mile coastline to monitor their coastal areas on minus tide days, record what they see (even if there aren't any blue mussels), and share it by entering the data into GMRI's Ecosystem Investigation Network.
"We can't just take our boat and map all of Casco Bay, much less the rest of the state," Maurin told M&T employees gathered around her in ankle-deep mud. "We don't have that much time, or enough staff. So knowing where to start looking is super important. That's where you come in."
M&T offers all of its 22,000 employees 40 hours of paid time to do volunteer work like this, according to Regional President Philip Cohen, who was trudging across the flats alongside employees Friday despite a braced knee from a recent surgery.
GMRI scientists will use that information to decide where to employ acoustic equipment to investigate a mussel bed, Whitman said. The equipment bounces sound waves off the sea bed and measures the echo, much like a dolphin does, to detect different bottom types and find active mussel beds.
Mussels are important because people like to eat them, making them a part of Maine's fishing economy. And the wild mussel beds are critical suppliers of the spat, or fertilized eggs, needed to seed Maine's next mussel crop, Maurin said. Oyster farmers can get spat from hatcheries; Maine's mussel farmers need wild spat.
Mussel beds provide critical habitat for juvenile fish, with the kelp and seaweed that grow there able to hide them from predators. Mussels also function as living water filters, removing excess nitrogen and phosphorus, while sequestering carbon into their shells.
While beds are retreating into colder, deeper waters, the Gulf of Maine is also rising. The gulf is warming three times faster than the planetary average and rising about 2.5 times faster in recent years than it did over the last century, according to the Maine Climate Council.
Some data suggests deeper waters in the Gulf of Maine may be cooling because of a recent shift in currents, while surface temperatures that affect mussel beds have been rising fast.
About 90% of global warming is occurring in the ocean, causing the water's internal heat to increase, according to the U.S. National Aeronautics and Space Administration. Heat stored in the ocean causes the water to expand, which is responsible for one-third to one-half of global sea level rise.
The sea surface temperatures in the Gulf of Maine in 2021 and 2022 were the warmest on record. The gulf spent 97% of 2022 in a marine heat wave. With an annual average of 52 degrees, the surface temperature of the gulf is about 2 degrees hotter now than it was 30 years ago.
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