A discovery last week shows that another invasive aquatic species is continuing to take hold along the Maine coast, a development that could spell trouble for some native species on which fishermen depend.
According to officials at Acadia National Park, a molted shell of an Asian shore crab was found along the shore near Schoodic Point on Sept. 19. A middle school student found the discarded shell while participating in a Schoodic Education Adventure program at the Schoodic Institute, a research and education facility located at a former Navy base on the eastern side of Frenchman Bay.
“This is believed to be among the first confirmed reports of the opportunistic omnivore within Acadia National Park,” park officials said. “A molted shell means the animal that made it was likely still alive, scuttling around the area somewhere.”
The recent appearance of the crab, which grows to about the size of a silver dollar and has a diet that ranges from saltmarsh grass to small bivalves and barnacles, is not the first time the species’ presence has been documented in eastern Maine. Students at the University of Maine at Machias found live Asian shore crabs on Great Wass Island in the town of Beals when conducting marine habitat research in 2013.
In the 2000s, there were multiple documented sightings of the crab species along the Maine coast ranging from the town of York to Owls Head, according to data compiled by the U.S. Geological Survey. Asian shore crabs, which are native to the Pacific coast of eastern Asia, first were identified in the United States in 1998 on the New Jersey coast and since have spread south as far as North Carolina.
Brian Beal, director of research at the Downeast Institute and a professor of marine ecology at UMaine Machias, said Thursday that the Asian shore crab is similar to the European green crab in that they both are invasive and prey upon native marine organisms such as mussels and clams. Green crabs, however, are “thousands” of times more prevalent along Maine’s coast than Asian shore crabs, which he said are “rare” in eastern Maine.
“We haven’t found any Asian shore crab since 2013, and we’re out there all the time looking,” Beal said, referring to his students’ 2013 discovery. “That’s good news for eastern Maine.”
In the southern part of the state, however, Asian shore crab are more common. He said there’s a spot at Thomas Point in Brunswick where he can usually find “hundreds” of crabs in a few minutes. When found they tend to be in the upper intertidal zone, Beal said, whereas green crabs can be found in upper and lower intertidal zones, as well as in waters up to 20 feet below the low tide line.
Beal said the concern is that Asian shore crabs could become equally as prevalent as green crabs are now, given that water temperatures in the Gulf of Maine are on the rise. Warming conditions in the gulf, he said, seem to correspond with the spread of both green and Asian shore crabs.
If Asian shore crab numbers continue to increase, he said, they likely would hurt populations of marine worms, mussels, clams and other species. Green crabs and other changing environmental conditions already have had a significant impact on Maine’s softshell clams, the statewide commercial harvest volume of which dropped by more than 900,000 pounds from 2011 to 2018, from 2.36 million pounds to 1.45 million pounds.
There are economic implications if the Asian shore crabs and other invasive species continue to take hold along Maine’s coast and deplete the populations of native species. Marine harvesting in the intertidal zone along the coast last year netted a total of $51 million in revenue for Maine fishermen, and that figure could drop significantly if invasive crabs continue to damage native species, Beal said.
“They are another threat sitting out there,” Beal said of Asian shore crabs. “They are a ticking time bomb, probably.”