The Chinese mitten crab is likely here to stay, according to scientists who plan to broadcast alerts to fishermen up and down the East Coast to be on the lookout for the Asian crustacean.

Four mitten crabs turned up in late May in the Delaware Bay—a sign that they may be reproducing in the region. (In June, one showed up in the Hudson River, above New York City.)

“To me, this suggests that they are more likely to be reproducing 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.”

Ruiz said his organization is working with the U.S. Fish and Wildlife Service, the National Oceanic and Atmospheric Administration, and state wildlife agencies to alert fishermen that they should report the long-legged, brownish-olive crabs.

Researchers are concerned that mitten crabs could compete for food with native blue crabs or burrow into stream banks, causing erosion.

Watermen had previously found three confirmed mitten crabs in two different areas of the Chesapeake Bay over the last three years.

The crabs recently caught in Delaware Bay were trapped by watermen going after blue crabs, Ruiz said. They were about 15–20 miles south of the Chesapeake & Delaware Canal, which connects the two estuaries and serves international ship traffic.

Ruiz said it’s still possible, albeit less likely, that the crabs are not laying eggs in the mid-Atlantic but are merely being brought to the region in the ballast tanks of ships from Asia or Europe.

Lynn Fegley, a fisheries biologist with the Maryland Department of Natural Resources, said it’s still difficult to know whether the crabs are reproducing because all seven of the confirmed examples have been males. No eggs or larvae have been found, either.

The mittens spend most of their lives in the banks of freshwater streams, and if researchers find burrows there, it would be a telling sign that the crabs have taken up residence, Fegley said.

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

Fegley added that it’s difficult to predict what impact the crabs would have on the Chesapeake Bay. Some nonnative species, such as brown trout and largemouth bas, have become established in Maryland without doing any major harm.