Green Crabs

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Bailey Bowden, chairman of the Penobscot Shellfish Conservation Committee, a small town located at the mouth of the Penobscot River, said an invasion there became apparent in 2012, when the committee noticed the disappearance of previously problematic moon snails, combined with numerous divots on the mudflats. The committee performed an experimental set of lobster traps, and came up with hundreds of green crabs (Carcinus maenas) in a short period of time.

“The number of crabs we have is unbelievable, and they have eaten everything in our bay,” said Bowden.

The crabs also undermine the bay’s banks, causing massive erosion of the shoreline, Bowden said.

“We really need to find a way to get rid of these things, not only to save the clam industry, but to save everything else,” he said, adding that green crabs have destroyed the area’s economically valuable population of clams and it’s clam-harvesting industry. Green crabs have forced the area’s lobstermen to move their gear, he said, because the creatures get into the traps and eat the bait before the lobsters ever get in.

“We’ve seen green crabs eating horseshoe crab eggs,” Bowden said. “We think green crabs will eat anything that can’t get away from them. They’re able to grab onto most anything. Probably one of the saddest things I’ve seen is the shoreline destabilization. Because that will really change our ecosystem.”

Kohl Kanwit, director of the Department of Marine Resources’ (DMR) Public Health Bureau, said the DMR and the clam-harvesting industry began holding formal talks about the problem in the spring of 2013. The consensus at the time was that green crabs were a serious problem on the Maine coast, but it was unclear how extensive the problem was. The DMR collaborated with University of Maine at Machias biologist Brian Beal to design a one-day, coastwide sampling protocol. The idea received responses from more than 30 municipalities, and 28 ended up participating. Volunteers set up to 10 traps each on August 27. Most used lobster traps with closed vents, due to the traps’ widespread availability; some used specialized crab traps. Municipalities selected locations with current or historic high abundance of clams. Traps were set on soft bottom in water less than 20 feet deep at low tide.

The counts

In all, 208 were traps set. Of those, 193 collected green crabs. The highest count seen was in Stockton Springs, with 191 crabs in a trap. Freeport had 181, Scarborough 151, Waldoboro 146. Trenton, Sorrento, Biddeford and Brunswick had more than 100.

The project verified reports from harvesters statewide that green crabs are present in most areas in numbers detrimental to bivalve shellfish resources, Kanwit said.

“Across the board, anyone who hadn’t realized they had a problem was really shocked at the amount of crabs,” she said.

Dr. Daniel Belknap, a professor of geology at the University of Maine/Orono, discussed the green crabs’ impact on the health of the coastal environment. As a resident who lives on a cove on the Damariscotta River, Belknap said he observed first-hand the cove’s “complete devastation,” over the past two years. This was manifested by severe erosion; the appearance of rills, or parallel channels, that are visible signs that the underlying marsh is being undermined; clipped marsh grass; and overhanging marsh “flaps” at the edge of banks that had washed away. There were hundreds of broken native crab carapaces and clam shells – evidence that these were eaten by green crabs. Belknap aired a video that showed hundreds of green crabs swarming out of the bank like cockroaches.

“This, to me, is very frightening,” Belknap said.

Dr. Hilary Neckles, from the U.S. Geological Survey’s Patuxent Wildlife Research Center in Augusta, discussed the loss of eelgrass in Maquoit Bay, about 2,500 acres, in the northwestern corner of Casco Bay. Here, she said, favorable environmental conditions historically produced a “continuous, extensive, gorgeous eelgrass meadow” right up through 2010. In the summer of 2013, there was no eelgrass.

Neckles said she sought to pinpoint the cause, given that there are many potential causes of eelgrass loss. These include water quality degradation, organic enrichment and sulfide toxicity of the sediment, direct human disturbance such as dredging and dragging, storms/wind/wave scouring, ice scouring, wasting disease, toxic pollutants, and animal disturbance. Evidence of damage by green crabs could be seen in shoots she collected from the shore and floating in the bay last July, which showed the clipping and shredding that is characteristic of green crab damage. Because green crabs at work could not be directly observed, Neckles and her team aimed to gather indirect evidence by performing “exclosure” experiments that could test whether environmental conditions in the bay were suitable for eelgrass growth in the absence of green crabs. The experiments employed fencing around eelgrass, to protect it from green crabs. Eelgrass within the “exclosures” thrived; outside the fences, the grass was destroyed.

“It showed green crab disturbance rises to the fore as a major player,” Neckles said. “We can’t rule out the effects of other interacting stressor. It does suggest restoration is possible, but it will depend on limiting green crab disturbance. It also suggests other eelgrass beds are definitely at risk.”

Factors affecting the resiliency of eelgrass in relation to green crabs will be an important area of research,” Neckles said.

According to Dr. April Blakeslee, an assistant professor of biology at Long Island State University in Brookville, N.Y., and part of a team looking at the spread of green crab from a genetic point of view, green crabs have been listed as one of the top 100 worse invasive species globally. They have been shown to have major impacts on native species and ecosystems.

“It’s found almost on every continent and is only native to Europe,” said Blakeslee. Many areas have potential risk of further invasion.

Blakeslee said factors that keep green crabs under control in their native European habitat include disease, parasites and competition. One of the parasites castrates the crabs, making it impossible for them to reproduce. In North America, the crab has escaped that parasite, she noted.

According to the DMR, scientists suggest that green crabs reached U.S. shores in the mid-1800s after riding cross the Atlantic in the ballast water on ships. Once in North America, they traveled to Maine, where they have been present for more than a century. They were first spotted in Casco Bay in 1900, and had reached Jonesport by 1951. The DMR conducted regular green crab surveys in the 1960s and 1970s, but did not conduct the surveys over the last 15 years.

“The green crab population in some areas of Maine has increased dramatically in recent years, and they have been feeding on shellfish resources such as blue mussels and soft-shell clams, threatening the state’s third-largest wild fishery,” the DMR said. “Recent anecdotal information indicates green crabs are expanding into subtidal habitats. The increase in the green crab population has coincided with an increase in ocean temperatures. A similar cycle occurred in the early 1950s when the ocean temperatures rose and the green crab population increased, devastating the soft-shell clam resource in Maine. This trend reversed during colder winters in the 1960s, effectively reducing the green crab population.”

There is currently no viable commercial market for green crabs, said the DMR; however, efforts are underway in the private sector to pursue a value-added process that converts green crab protein in to a sustainable aquaculture feed for use in Maine and possibly for export. There are also attempts to augment commercial compost with green crabs. Research has been conducted at the University of Maine to produce a food additive paste made from the meat. And there have been efforts to develop a bait and pet food market.

“However,” the DMR said, “because the green crab is a non-native species and is so destructive to native resources, the department will focus its regulatory efforts on reducing green crab populations rather than on their management as a sustainable commercial fishery.”

Beal, the University of Maine/Machias professor, said eradication efforts should focus on females. A 2-inch female produces 165,000 eggs, and they shoot their eggs widely into water column. Larvae have relatively wide salinity and temperature tolerance. Adults can withstand seemingly improbable conditions out of water for more than 10 days at summer temperatures. They inhabit a wide array of ecosystems, including mud, sand, rock, and eelgrass, and they feed on organisms in all these habitats. They’re omnivorous. And they are gregarious, so they don’t have trouble finding mates.

“The green crab is the consummate invader,” Beal said, adding, “This is the second coming today of green crabs.” Maine was also infested in the 1950s. Fishery officials at the time conducted experimental clam farming, and lost most or all seed clams, depending on the area, to green crabs. “They had a green crab invasion on the tidal flats,” he said. “The entire Casco Bay area was nearly depleted of soft-shell clams.”

Beal said it was useful to look at environmental conditions in the 1950s that might explain the influx of green crabs. Data from 1951-1954 show the average seawater temperature was warming. “It seemed green crabs responded to that,” Beal said.

Beal said he has been collecting clam flat data with his marine ecology class since 1991. “I never saw a green crab from 1999 to 2006,” he said. A small number of green crabs appeared in 2007. The numbers ramped up considerably by 2013. Their presence coincided with warm water temperatures, he said. Water temperature might be useful as a predictive tool, he said.

In correlation, clam studies in various regions show the profitable bivalve is disappearing, he said.

“There’s a huge problem, and what the clammers are reporting is right on the money,” Beal said. However, he said, another study, in April 2013, showed that planting clams in protected structures successfully produced hundreds of offspring. “This system has tremendous bounce-back potential,” Beal said of the experiment. “We just need to be vigilant and attentive in our response. Clams are there.”

Chris McCarthy, an ecologist with Parks Canada, said that Kejimkujik National Park Seaside, in Nova Scotia, has also seen the green crab invasion.

McCarthy’s team studied the national park’s Little Port Joli estuary, a special preservation area about two hours southwest of Halifax on Nova Scotia’s south shore. The estuary is normally full of life, he said.

In 2007, park staff began to see serious problems, McCarthy said.

“Our eelgrass beds were declining quickly,” he said. “Smaller soft-shell clams were disappearing. Lots of sedimentation. Some desertification happening as well. In general, things weren’t well. Things were either in poor or fair condition under the surface, despite the beauty above.”

Clams are the most prolific bivalve at Kejimkujik Seaside. The small clams, a favored food of green crabs, had virtually disappeared. There isn’t a blade left of once-abundant eelgrass beds.

Referring to the green crab as an “ecosystem engineer,” McCarthy noted that other species, such as geese, no longer appear on the estuary. “It’s very aggressive and very invasive,” he said of the green crab. “Can we control green crab effects without hurting the estuary? We needed a way to remove large numbers of green crabs, and a use for green crabs, at minimal operational costs.”

A trap, dubbed The Terminator, was developed with the help of the local fishing industry. In one instance, the trap captured 1,187 green crabs in one night. Their meat is thought to be useful as bait for the lobster fishery, and for compost and fertilizer. Some Nova Scotia lobstermen are now targeting green crab as a marketable product, he said.

In conjunction with the development of the new fishery, he said, park staff also developed an ecosystem monitoring program, and saw a 30 percent decline of green crabs in the estuary after the first year of fishing, followed by another 30 percent decline the second year, 2009 and 2010.

“One of the biggest things we learned was to get local fishermen involved,” McCarthy said. “We had a lot more crabs than we thought. We thought there might be 300,000. We didn’t even know we had this problem. We thought we could bury them – but not a million crabs.”
Efforts in the area show there is good income potential and prospects for restoring the ecosystem, he said.

“But this is the short-term and, in terms of where we’re going, there’s a lot to learn,” McCarthy said.

In 2013, Nova Scotia saw a small number of a new invaders, the Chesapeake blue crab, he said.
Shellfish harvester Rachel Huntley, who is connected with Atlantic Shellfish of Harrington, said she and a friend harvested green crabs in 2013 with the goal of developing a market.

“I called all over the globe,” Huntley said. “The only market we could find was a bait market.”

After expenses, they earned only 20 cents per pound.

“You’re not going to have people bust their rear ends for nothing,” Huntley said.

“This has the been the missing piece – where’s the market?, der Kinderen said. There’s just so much green crab you can compost. So somehow we’ve got to find the market. We’ve got to find a way where this problem becomes a resource.”