Invading armies of green crabs have taken over clam flats and sea grass meadows in southern Maine, threatening the soft shell clam industry and gobbling up the eelgrass in the meadows that provide nursery habitat for young ocean fish, clams and mussels, according to the Maine Department of Marine Resources (DMR). Just how widespread the problem is remains an open question.

While some efforts are focused on limiting the invasion, since there is no way to actually stop it, others are exploring how to develop a market for green crabs, which have tough shells and little meat.

Green crab fish-food may be an option. The protein and Omega-3 rich green crab may be able to be used in aquaculture operations in Maine and possibly be exported, according to the DMR.

University of Maine researchers are also working to develop a food additive paste made from green crabs, others are trying to market them as bait, and some crabs have found their way into compost.

"We have no interest in managing an invasive species that is so destructive to our fisheries," said DMR Commissioner Pat Keliher. The DMR will continue to work with the fishing industry, municipalities and researchers to focus on reducing green crab populations rather than managing it as a commercial fishery.

The invaders, which have been around for a century, probably came to the East Coast by way of European bilge water dumped from ships. Historically, they were kept in check by cold Maine winters.

The recent green crab population explosion coincides with warming ocean temperatures, according to the DMR. A similar warming trend in the early 1950s devastated the soft-shell clam fishery in Maine, but clams rebounded during the colder winters in the 1960s that pushed green crab numbers down. Many are hopeful a couple of cold winters with ice on the clam flats will do the same this time around, but the current warming trend has held on for several years, with crabs taking over the sandy flats and rocky shores and now moving into the deeper non-tidal sea floor.
Current control options include fencing, protective nets placed over seeded clam flats, and trapping.

The Army Corps of Engineers and the DMR work together with towns to put up green crab fencing, which has proven successful in keeping crabs off some clam flats. The DMR has also issued a dozen permits to towns to allow them to trap and get rid of as many crabs as possible without having to buy a license or submit landings reports.

Putting up nets to cover flats that have been seeded with clams is another proven deterrent that some towns are using. The nets protect the clams until they have burrowed into the flats and are less likely to get caught by a green crab.

But green crabs are effective diggers and, now that they are being found in deeper water, may get around fences by going below the intertidal zone.

The town of Penobscot is experimenting with new crab trap designs to be deployed in the intertidal zone and in deeper water, and University of Maine Marine Biologist Brian Beal is developing a scientific study method to better determine the extent of the invasion.

Other scientific studies of marine invaders are under way this week along the New England coast. Under the direction of the Massachusetts Office of Coastal Zone Management, the National Oceanographic and Atmospheric Administration, and Sea Grant, teams of scientists from around the world are doing rapid assessments on the coastline from midcoast Maine to Rhode Island. Conducted every three years since 2000, marine scientists visit commercial ports and marinas and rocky intertidal sites in each of the four states to identify and document the spread of known invasive species, the introduction of new species, and changes in location of native marine species. The rapid assessment gives a snapshot of changing conditions over time.

On the management end of the spectrum, the DMR is preparing guidelines for municipalities on crab control. They expect to release the guide this fall.