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9/4/2013 2:00:00 PM
Scavenger Crabs Invade Local Waters
A green crab momentarily escapes from a trap during a statewide survey of the species Aug. 28. (J.W. Oliver photo)
A green crab momentarily escapes from a trap during a statewide survey of the species Aug. 28. (J.W. Oliver photo)
By J.W. Oliver

Numbers of green crabs, an invasive, scavenger species that feeds on soft-shell clams and could threaten the multimillion-dollar fishery, are rapidly growing in local waters.

"Last year, they were starting to become noticeable," South Bristol lobsterman Chester Rice said. "This year, they've taken over. There's been an explosion in the population."

"They're killing all the clams," Rice said. "All the clam diggers will tell you, there are no small clams left."

The Maine Department of Marine Resources, with the help of fishermen like Rice and other volunteers, conducted a green crab survey along the coast of Maine Aug. 27 and 28.

Rice set five traps for the survey early Aug. 27 and hauled the traps 24 hours later. The results - 284 crabs with a high of 79 in a single trap - did not surprise Rice.

"Quite often, they're fuller than this," he said. "I've been killing them by the thousands."

"We certainly have an invasion, if you look at the coast of Maine," Rice said.

Rice and others believe the crabs could pose a threat to other shellfish, such as lobsters, oysters and peekytoe or rock crabs, an important source of crabmeat in Maine.

One of Rice's traps brought in 63 green crabs and only one rock crab. "I just think these (green crabs) have driven (the rock crabs) all away," Rice said.

The biggest haul of green crabs came near an underwater oyster farm. "Looks like they like (oysters) from the looks of what's here," Rice said.

"They're bad for everything out here because they're eating everything," Rice said. "It's scary, what they might do to everything else that's here."

The Department of Marine Resources survey "is designed to provide a snapshot of the locations and relative abundance of green crab populations statewide," Shellfish Program Supervisor Denis Nault said in a letter to municipal officials.

The crabs can devastate important shellfish resources like clams and mussels, Nault said. The department will use the survey data "to raise awareness of the problem and assist municipalities with a plan for control of these aggressive shellfish predators."

The value of the Maine soft-shell clam fishery in 2013 was $15.64 million, trailing only lobster and elvers as the most lucrative fisheries in the state.

The average adult crab, about 3 inches in diameter, can eat about a dozen 1-inch quahogs a day, Nault said.

The hard-shell quahog requires more work for a crab than a soft-shell clam, however; thus, a single crab could potentially eat many more clams, particularly small seed clams, in the same time period.

Nault sees less of a threat to other shellfish. The green crab is a scavenger, and can feed on small lobsters, oysters and quahogs, but those foods are more difficult for the crab to eat, he said.

Scientists believe the European green crab, Carcinus maenas, came to the U.S. in the ballast water of ships. The species has been in Maine for about 100 years.

Warm temperatures and warm winters in the late 1950s "caused a massive population explosion of green crabs like what we're experiencing right now," Nault said, and a corresponding decline in commercial shellfish populations.

A cold Maine winter will kill green crabs and provide natural population control, but mild winters allow the crabs to survive and increase in numbers.

Biologists and clam diggers, after a series of mild winters and a serious surge in green crabs, are turning to man-made methods to protect soft-shell clams.

Freeport, borrowing a method from the 1950s and 1960s, has installed fences around some clam flats to keep the crabs out, Nault said, and aggressively traps crabs who skirt the fences.

The green crab has little value as a fishery, so a commercial harvest appears unlikely. "It's too small and it's a very poor quality meat; it's very soft and does not have a very long shelf life," Nault said.

There is a small market for green crabs as bait in the mid-Atlantic and the south, but the region already has plenty of green crabs and would have little reason to import them.

Scientists are examining the value of the crabs as an additive to food for fish farms, as well as the possibility of mechanically extracting the meat for use in products like seafood bisque.

The crabs offer some value as compost, but the costs of bait and fuel to harvest the crabs would likely outweigh the value of the catch.

The state plans to present the data from the survey at a summit about green crabs in the fall, but so far, the state has no easy answers for the clam diggers who fear what the crabs could mean for the health of their livelihood.

"How do we protect [the commercial shellfish] resource and, if we can, control the onslaught of the green crabs?" Nault said. "That's a hard question to answer."

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