December 27, 2013






Green crab invasion hurts area fisheries
by JD Rule


     An aggressive invader has come to Maine and hardly anybody noticed. Not, at least, until the occupation began and the enemy's infiltration was well along. In the beginning, the European green crab was thought to be a problem faced only by the shellfish community, but participants at the Maine Green Crab Summit held in Orono on December 16 now understand that the impact extends much wider. The summit, which attracted over 200 participants from all over New England and the Maritimes, was sponsored by Maine Sea Grant.
     According to scientists and marine researchers who spoke at the summit, the green crab has been responsible for major losses in soft‑shelled clam populations, the elimination of huge swaths of eelgrass, shoreline erosion that threatens to go beyond valuable marshlands, and it is now the threatening of Maine's second most valuable fishery: scallops. It seems, according to the scientists, that green crabs will eat nearly everything, including moon snails, which have been the subject of eradication efforts by the Lubec shellfish community and others.
     Dr. Brian Beal, professor of marine biology at the University of Maine at Machias (UMM), described a study he and his students have conducted for many years in Cutler, involving measuring the growth rate of clams that were seeded under various controlled conditions. "Before 2006 there were no crabs," Beal said, "but after the crabs showed up only 32% of the clams survived." Beal presented information that showed a high correlation between green crab population increases and seawater temperature. Increased water temperatures, according to recent publications, are also implicated in scallop "gray meats," the lobster shell disease that has severely impacted the Long Island Sound lobster fishery, and some of the other resource declines experienced by fishermen.
     Park employee Chris McCarthy outlined an eradication effort in Nova Scotia's Kejimkujik National Park that was successful in restoring eelgrass and reducing the green crab population by aggressive trapping of the invasive species. McCarthy displayed aerial photographs that showed eelgrass coverage reductions that had been subsequently largely restored. Eelgrass, termed "the canary in the coal mine" by several speakers, is often viewed as an important indicator of water quality. In response to a question, McCarthy confirmed that despite the reliance on volunteer labor, the cost to restore the estuary in Kejimkujik Park was "unsustainable" over the long haul and was dependent on "uncertain" government subsidies.
     During one of the panel discussions it was noted that if trapping can reduce the population of large, mature crabs, then shorebirds and other native predators may be successful at reducing the population of the more‑vulnerable juvenile crabs, which in their earlier stage are the size of a dime. A typical mature male measures approximately four inches across the carapace, according to figures produced by several speakers, with females slightly smaller.
     The green crabs came here from northern Europe, according to Dr. April M.H. Blakeslee of Long Island University, arriving "probably from rocks used in ships as ballast," a common practice during the early 1800s when the crabs were first found. Blakeslee followed the genetic trail of the first invaders from waters south of Cape Cod, following currents and opportunities up the Maine coast and over to Nova Scotia. A second genetic strain, said Blakeslee, dates to roughly 1950 and was found in Newfoundland, "probably from ballast water, since they no longer used rocks by that time." The two groups appear to be merging, said Blakeslee, in the Bay of Fundy. Reportedly, European green crabs have also been found on the West Coast, moving northward from San Francisco. Reports by organizations such as the Smithsonian Environmental Research Center also show incursions by the Chinese mitten crab, which is now found in western and southern waters.
     Participants were shown graphic images of crabs burrowing under grassy berms adjacent to mud flats, to the point where moderate storm action led to collapsing banks, exposing fresh soil to both the invaders and accelerated erosion. A brief video clip was shown of a small section of grassed‑over berm being pulled back to reveal a large number -- possibly thousands -- of crabs.
     In an interview, Beal was asked whether crab damage might have been responsible for the partial collapse of South Lubec Road, repaired recently by the Maine Department of Transportation. Beal immediately proposed a few "alternate hypotheses" but did not rule out undermining of the shoulder of the road by crabs. He promised to visit the area "sometime after the new year" to inspect. Beal agreed that this burrowing behavior by crabs might pose a threat to Lubec's 93 miles of shoreland.
     To help gauge the extent of crab infestations, the Maine Department of Marine Resources (DMR) conducted a survey in August, with "about 30 towns" participating by setting approximately 400 traps during the same 24‑hour period. At a meeting of the Lubec Shellfish Committee, Selectman Michael Scrivani, who is active with the committee and who also attended the summit, stated that "nearly 3,000" crabs were collected by Lubec clammers, with even more small crabs falling out of the modified lobster traps that had been utilized. "We absolutely plan on doing this again," said Denis‑Marc Nault, the DMR scientist responsible for shellfish management.
     Subsequent to the summit, Robin Hadlock Seeley made available an underwater video taken of a lobster trap placed in Freeport's Harraseeket River, in 40 feet of water for 90 minutes, on November 9. The video, available on Facebook through the Lubec Community Bulletin Board, shows a large number of green crabs swarming over the bait bag. According to the description, when the trap was hauled there were "two gallon buckets' worth of large green crabs," but no lobsters.
     As for what to do with the population of green crabs, several speakers described using them for fertilizer or processing them for use as aquaculture feed. Dr. Denise Skonberg of the University of Maine Orono School of Food and Agriculture described a test she had performed processing green crabs with a meat‑extraction machine similar to one used in the poultry industry. Skonberg's team cooked the "purée" into an "empanada," which she described as a "popular South American stuffed fried pastry." The results of a taste‑test by a panel of 87 individuals yielded results that Skonberg termed "promising," while cautioning that further work is needed. "You probably want to keep [the pastry] small," she quipped, "so people don't look inside."
     A number of speakers talked about the commercialization of green crabs, but all of them came back to the same point: The market price of the product -- whatever it is -- must sustain the cost of processing the crabs and provide a profit to the fishermen who catch them.
     DMR Commissioner Patrick Keliher was the final speaker. "This meeting needs to be the first step," Keliher said. "We're not going to throw money at this. ... We need to find a way to eradicate these critters." He continued, "It will not be the DMR alone, and it will not be the clammers alone." Keliher got a round of laughter by saying, "People ask me if we should start managing these. If we look at fisheries management, perhaps we should."
      While no single speaker had a conclusive answer as to next steps, the consensus appeared to be that complete eradication of a pest that has damaged fisheries and real estate from New Jersey to Labrador may not be feasible, but their impact may be mitigated by developing a way to make their capture economically profitable.

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