The rapid growth of oyster aquaculture along Maine's coast, one of the bright spots of how the changing climate is affecting the state's marine economy, is producing another side benefit along much of the coast.

Oysters are starting to reappear in the wild along the shoreline, too, not just in plastic cultivation mesh bags in licensed aquaculture lease sites.

The oysters are not really "wild" in the traditional sense, but seem to be a direct result of the presence of nearby oyster farms. Some might be escapees — scattered by currents after a mesh bag accidentally ruptures — but oyster farmers and researchers suspect most found in the wild are the product of farmed oysters that spawn, releasing their seed or spat into surrounding waters, where they grow and then settle along the shoreline, holding fast to rocks as if fastened with superglue.

"They're definitely spawning before they are being harvested" at aquaculture sites, said Brian Beal, a marine scientist at the University of Maine at Machias and Downeast Institute in Beals. The tiny, newly hatched spat is small enough to be washed by currents out of mesh grow bags or cages at aquaculture sites and into the surrounding area, he said.

Eastern oysters, also known as American oysters, were native to Maine for millennia, as indicated by ancient shell middens found along the coast where members of Wabanaki indigenous tribes discarded oyster shells for generations. But their numbers slowly declined over hundreds or even thousands of years, likely because waters in the Gulf of Maine simultaneously rose and cooled, according to a 2017 Maine Sea Grant report on the oyster aquaculture industry.

In the mid-20th century, interest grew in trying to revive Maine's oyster population, and in the early 1970s University of Maine researchers began trying to cultivate them in the Damariscotta River. Oyster growers had to learn how to breed the bivalves so they could survive cold spells and other factors such as increasing carbon levels in the ocean brought on by climate change.

Dana Morse, who works for Maine Sea Grant in South Bristol at UMaine's Darling Marine Center, which spearheaded the oyster aquaculture research, said that as the Gulf of Maine has been getting warmer again, farmed oysters are spawning more than they did decades ago.

"It would happen every once in a while, but generally it didn't happen," Morse said of the late 1990s, when he started working at the research site.

Now, it happens annually, and when it does "those fertilized eggs and larvae will drift" into the current, he said, and get distributed into surrounding waters.

As the gulf continues to warm, conditions will become even more suitable for oysters and other warmer-water species such as quahogs, researchers said. Before cultivation began in the 1970s, small pockets of wild oysters hung on in some warmer and less salty shallow inland bays and tidal rivers, generally from the midcoast area to areas farther southwest.

Over the past couple of decades, oyster habitat has expanded east as far as Taunton Bay near Mount Desert Island, according to Beal. Oyster lease sites approved by the state Department of Marine Resources extend from the Piscataqua River on the New Hampshire border to Steuben, on the western edge of Washington County.

"You won't find them Down East," Beal said, referring to waters near his research base in Beals, which tend to be colder than waters further west. "Water temperature is definitely a factor."

Chris Petersen, who teaches marine ecology at College of the Atlantic in Bar Harbor, said there were no oysters to be found in Frenchman or Blue Hill bays when he started working at the college in 1990. Now, since Acadia Sea Farms started growing oysters in 2012 off Haynes Point, just west of the causeway onto Mount Desert Island, he and his students can easily find oysters not too far away.

"There are a lot of oysters in Clark Cove," he said, adding that some loose ones he has found near the northwest corner of MDI might be escapees who started growing before drifting away from the aquaculture site. "They've probably all been in the past five to seven years."

As oyster habitat has expanded, they've been starting to show up in places where other bivalve species have been on the decline.

Beal said that blue mussels, for example, used to have widespread distribution along the entire coastline, growing in densely packed colonies along the shore both above and below the low-tide line. Invasive green crabs, however, have made such mussel beds more scarce, he said. Softshell clams, which like mussels have long been harvested commercially along the coast, also have seen their numbers decline significantly due to green crabs, but oysters are not expected to share the same fate.

"Oysters are a little more hardy than mussels are," Beal said, and have thicker shells that are significantly harder for green crabs to crack. Also, oysters are filter feeders, like mussels and clams, he added, which can help maintain water quality along the shore if mussel and clam populations continue to decline.

According to state marine resource regulators, a commercial shellfish harvesting license is required for anyone who may want to take oysters found along the shore, but the vast majority of those who hold such licenses and who aren't also licensed for aquaculture instead dig for clams or marine worms.

But Beal, whose research often focuses on how to boost the viability of commercially harvested species along Maine's coast, believes there could be a future for clam or worm diggers who also want to harvest oysters found in the intertidal zone. As temperatures in the Gulf of Maine continue to increase, conditions along the shore are expected to become "excellent" for oysters, he said.

"We're just going to continue to get warmer and warmer," Beal said. "They do very well in warmer temperatures. I think, in time, there will be more opportunities."

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