Researchers Seek Better Understanding Of Invasive Crabs

by Alan Pollock
The green crab has few predators, aggressively hunts and eats its prey, destroys seagrass, and outcompetes local species for food and habitat. NOAA PHOTO The green crab has few predators, aggressively hunts and eats its prey, destroys seagrass, and outcompetes local species for food and habitat. NOAA PHOTO

CHATHAM – Shellfishermen are worried about little green aliens. And purple ones.

They’re invasive crabs, and while not new to local waters, they seem to be on the rise, threatening to take a greater and greater share of commercially important shellfish species. The species has few predators, aggressively hunts and eats its prey, destroys seagrass, and outcompetes local species for food and habitat.

Last month, the shellfish advisory committee heard a presentation by Owen Nichols, the director of marine fisheries research at the Provincetown-based Center for Coastal Studies. The center regularly conducts habitat studies in places like the Nauset estuary and Pleasant Bay, using dredges, underwater cameras, eel traps and seine nets to sample the different species of fish and invertebrates that call the waterways home. By comparing findings to similar surveys done in the 1970s, Nichols said researchers can see trends.

For one thing, there are fewer large fish observed than in the 1970s, particularly with winter flounder.

“It’s not just in Chatham, but throughout where winter flounder are,” he said. “The big fish just aren’t spawning inside the bays and lagoons anymore like they used to.” Instead, they seem to spawn just outside the protected waters, and the juveniles then swim inside to grow. The theory is that rising water temperatures have influenced the change, he said. “And we saw a number of tropical fish,” he added.

With warmer temperatures, there seem to be more blue crabs and black sea bass coming from the south, he noted.

The most recent surveys also turned up thousands of green crabs, a year-round invasive species that came to North American waters from Europe in the late 1800s. It’s not known what their predators might be, or whether they’re deterred by cold water.

“We have a lot of concern about what they might be eating out there,” since green crabs decimated the softshell clam fishery in parts of Maine, Nichols said. With help from Claire Williams of AmeriCorps Cape Cod, Nichols has been trap sampling for green crabs in Salt Pond in Eastham and has also discovered plenty of purple marsh crabs and shore crabs.

“These things love to eat salt marsh grass,” he said. “They can really be destructive to the marshes.”

Green crabs tend to keep these other species in check, Nichols said.

“We really want to understand how they fit in the ecosystem before we go just wiping them out,” he said. “Unintended consequences are often the norm when we try to manipulate the system like that.”

To further that understanding, a grad student from the University of New Hampshire is analyzing the diets of crabs captured at four sample sites in Nauset Harbor, some close to aquaculture grants with tempting shellfish, and others from farther away. The researcher analyzes the DNA of the crabs’ stomach contents to determine which species it has been consuming. The study is still in its early phases, but Nichols said he’s eager to see the results.

“Knowing what these animals are eating — and when and where — is going to be really, really important to understand their effect on the ecosystem,” he said.

Shellfish committee members say green crabs appear to be far more prevalent now than in years past. One member said he’s seen Asian crabs at a productive flat near the lighthouse.

Green crabs don’t seem to target quahogs except as a last resort. “It seems to be the last thing they’ll eat if they’re presented with other shellfish to eat,” he said. They are said to be particularly attracted to mussels, but that may not be the case.

Shellfish Constable Renee Gagne said the town is conducting an experiment with ribbed mussels in Muddy Creek, “because they want to apply them onto restoration projects for a living shoreline,” using shellfish to stabilize erosion and improve water quality. They chose ribbed mussels rather than “using things like oysters or whatever that people would be enticed to pick,” she said. The mussels are in subdital cages, along with a large number of blue mussels.

“We’re also seeing a lot of crabs in there,” including Asian crabs and green crabs, “and we’re seeing no mortality,” Gagne said. One would have expected the crabs to treat the mussels as an “open buffet,” but that hasn’t seemed to happen, she noted. “That means they’re living in harmony.”