FREEPORT, Maine — Despite cautious optimism earlier this year that midcoast mudflats would be spared, invasive European green crabs were apparently only waiting for warmer water to scurry back into the area and begin wreaking havoc on the shellfish industry.

“Since the beginning of last week, they’ve increased considerably,” said Sara Randall of the Downeast Institute for Applied Marine Research and Education, which is conducting six studies on the crustaceans in Freeport. “Compared to exactly this time last year there aren’t as many, but we think that within the next couple of weeks we’ll see a lot more.”

“They’re here,” said Brunswick’s Marine Resource Officer, Dan Devereaux. “Not to the extent that they were here last year — I still think the catches are about 80 percent down from last year — but they’ve doubled in the last couple of weeks.”

For the last couple of years, the crabs have decimated lucrative clam flats in Casco Bay, prompting researchers to study why the crabs are here and how best to eradicate them.

But perhaps due to the unusually cold spring, the crabs didn’t arrive during June. In late June, Devereaux pulled a clam rake through the mud of Harpswell’s Buttermilk Cove and found no crabs.

Whether or not the crabs return in the numbers seen last year, Devereaux said the damage has been done: Most of the clams being harvested now are between two-and-a-half and three inches long. Smaller clams — the size little green crabs can best handle — just aren’t around.

“Over the next few years, clammers are going to be in trouble,” Devereaux said.

And with anecdotal stories about lobster traps coming up full of green crabs, Couture said lobstermen may want to pay close attention too.

This year, the majority of the crabs seem to be in deeper water like Quahog Bay in Harpswell, where some 600 pounds of crabs were pulled up in traps last week, according to Darcie Couture of Resource Access International, who is working with Harpswell and Brunswick harvesters.

But Patrice McCarron, executive director of the Maine Lobstermen’s Association, said Friday that she hasn’t fielded any calls from lobstermen about the invasive creatures, nor did she hear about them from anyone gathered last week at a meeting of the Lobster Advisory Council.

Still, Couture said if the crabs are crawling along the ocean floor, “they’re going to be eating what’s out there.”