Mitten crab discovery spurs alert along coast

The discovery of four Chinese mitten crabs in the Delaware Bay prompted scientists to broadcast an alert to the entire East Coast yesterday amid the increasing likelihood that the native Asian crustacean is reproducing in the region.

"To me, this suggests that they are more likely to be reproducing here and in the system here than continuing to be delivered fresh," said Greg Ruiz, the senior scientist at the Smithsonian Environmental Research Center in Edgewater, Md.


"It's not likely that we would see a continuing supply of mitten crab larvae from overseas, so that we would continue to catch these mitten crabs in the Delaware and Chesapeake bays."

Watermen have previously found three confirmed examples of the hairy-clawed, long-legged, brownish-olive crab in two different areas of the Chesapeake Bay over the past three years. Two more reports were unconfirmed.


Researchers in Maryland have been watching the spread of the species closely, concerned that they could compete for food with native blue crabs or cause erosion by burrowing into stream banks. Isolated appearances of mitten crabs in fishing nets have also raised alarms recently in Lake Erie, the St. Lawrence Seaway and along the Louisiana coast of the Gulf of Mexico.

In China, they are regarded not as a pest but as a valuable seafood, with their eggs rumored to be an aphrodisiac.

Ruiz said that the four mitten crabs, caught in traps over the last week by watermen going after blue crabs, were about 15 to 20 miles south of the Chesapeake & Delaware Canal, which connects the two estuaries and serves international ship traffic.

He said it's still possible, although less likely, that the crabs are not laying eggs here but just keep swimming out of the ballast tanks of ships from Asia or Europe.

Lynn Fegley, a fisheries biologist with the Maryland Department of Natural Resources, said a key reason it's still hard to say whether the crabs are reproducing in the region is that all seven of the confirmed examples have been males.

No females, eggs or larvae have been found in the Chesapeake or Delaware bays, she said. If researchers find burrows in the banks of freshwater streams, where the mittens spend most of their lives, then that would be a telling sign that they've taken up residence here, Fegley said.

Ruiz said that his organization is working with the U.S. Fish and Wildlife Service, the National Oceanic and Atmospheric Administration, and state wildlife agencies to broadcast alerts to fishermen up and down the East Coast, asking them to report the distinctive-looking crabs.

In Maryland, people are asked to call the "mitten crab hot line" at 443-482-2222 or send e-mail to


Mitten crabs are about the size of a dinner plate, with a notch near the front of their shells and black fuzz around their white claws. They lack the swimming legs that Chesapeake Blue crabs have.

The mittens have been found by watermen in the Chesapeake Bay in 2005 and 2006 near the mouth of the Patapsco River, south of Baltimore, and, last month, off Chesapeake Beach in Southern Maryland.

In the Delaware Bay, a fisherman found that waterway's first confirmed Asian crab on May 25 off Liston Point, between Woodland and Augustine beaches, said Craig Shirey, a program manager at the Delaware Department of Natural Resources and Environmental Control.

A photo of the crab was published in local newspapers. Then on Tuesday and Wednesday, a second waterman checking his pots in the same section of the Delaware Bay reported catching three more. He turned them over to scientists, who verified yesterday that they were mitten crabs.

This fisherman, who asked officials not to release his identity, told researchers he'd caught more, but didn't keep them, Shirey said.

"He said that he has seen a few of them on and off for the past at least six years, but never gave it much thought or reported it," Shirey said. "He said, 'We normally see a few of these things this time of year, and have for years.'"


This report has not been confirmed by scientists, Shirey said.

In the San Francisco area, the mitten crabs boomed in population in the 1990s, clogging intake pipes for municipal water systems, Fegley said. But then their numbers died off quickly, and were no longer a nuisance, she said.

It's hard to say what impact the Asian crabs would have on the Chesapeake Bay, she added. Some nonnative species, such as brown trout and largemouth bass, have become established in Maryland, without causing any substantial harm to the ecosystem.

"It's a value judgment," as to what's an invader and what's just an introduced species, she said. "Some people don't want them here, but others appreciate them as sport fish."