Attack of the green crabs

Meet the invaders putting Maine shellfish at risk
By DEIRDRE FULTON  |  February 27, 2014

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TINY MENACE Green crabs may be small, but they're making a big impact on the clamming industry.

They’re green, they’re mean, and they’re endangering not only Maine’s soft-shell clam population, but also oysters, mussels, lobsters, and eelgrass. This menace is known as the green crab (a/k/a carcinus maenas), an invasive, omnivorous species that has been in Maine for 114 years but only recently began affecting the productivity of clam flats in places like Freeport, Brunswick, and on down the coast.

The Maine Clammers Association calls the issue a “crisis;” shellfish harvesters say the voracious predators, which originally came over on ships from Europe, are decimating the third-largest commercial fishery in the state. A 2013 study in Freeport showed that soft-shell clams are all but missing from mid- and low-intertidal areas, having become breakfast, lunch, and dinner for aggressive green crabs, who also enjoy dining on juvenile lobsters and blue mussels. 

“The Maine shellfish industry is in deep trouble,” Freeport clammer Chad Coffin told MPBN last year. “We think that we’re only maybe two years away from really no commercial viability in the state on soft-shell clams, which has been, historically and traditionally, one of the most important and economically valuable resources on the coast of Maine.”

While researchers don’t know they exact cause of the green crab invasion, everyone points to one likely culprit: increasing seawater temperatures. Whereas Maine’s icy winters used to keep green crab populations in check, milder temperatures have allowed the buggers to flourish since the 1990s. (This has led, incidentally, to an alliance between clammers and climate-change activists.) Add to that the green crabs’ incredible ability to reproduce (a two-inch female can produce 165,000 eggs at one time) and a relatively wide tolerance for temperature, and you’ve got what one expert calls “the consummate invader of new ecosystems.”

Brian Beal, the professor of marine ecology at the University of Maine at Machias who led last year’s study in Freeport, points out that the crabs have a destructive effect not just on commercial species, but on the environment as a whole.

“Green crabs burrow into marsh embankments, causing the bank plants to die,” he says in an email to the Phoenix. “When this happens, the bank breaks away from the upper shore, and this erosion has a number of negative effects not only on the marine environment, but the adjoining terrestrial environment as well.”

“They’ve mowed down hundreds of acres of eelgrass,” adds Darcie Couture, formerly of the state Department of Marine Resources. “There’s a lot of serious habitat degradation.” Couture has designed and is implementing a green crab study in Brunswick, to better understand not only the habits and physiology of the species, but also to find the best ways to control it.

Beal’s study in Freeport, completed with the help of local shellfish harvesters, suggested that use of selectively placed (and consistently maintained) fencing and netting can enhance baby-clam survival. The study also showed that fall is the best time of year to catch breeding female crabs. Thanks to a $200,000 grant through the University of Maine system, Beal will revisit in 2014 some of the work he initiated in Freeport last year. “The funds will allow me to work directly with clammers who are the ones that have the most at stake in this situation,” he says.

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