Crustacean invasion: Beware the green crab

Scientists seek volunteers to keep an eye out for invasive critters

Posted: Monday, May 04, 2009

Beware of the small, insidious green crab.

Linda Shaw / Noaa Fisheries
Linda Shaw / Noaa Fisheries

Scientists say it's not a matter of if but when it's seen in Southeast, and they are looking for more volunteers to keep an eye out for the four-inch, formerly European crustacean.

The crabs came to Cape Cod nearly 200 years ago, probably in the hulls of European ships or on rock ballast. They were blamed for crashes of clam landings there in the 1940s and 1950s, said scientist Linda Shaw of the National Oceanic and Atmospheric Administration. They have dug into eelgass beds and loosened the sediments. They dent shellfish populations that humans and other creatures rely on, either by eating or competing with them.

An infestation appeared in the San Francisco Bay Area in 1989, they're thriving on Vancouver Island, and they may already be on the Queen Charlotte Islands.

The green crab is a capable and vivacious critter: good at surviving in lots of different conditions, a voracious eater, and an avid digger of sediment. All those things make it a big potential danger here, Shaw said.

"They can go out of control," she said. "Because they haven't evolved in the area, the area hasn't evolved to keep them in check."

Green crab has been dubbed the "cockroach of the sea" because of its adaptability and the astounding 200,000 eggs it can produce in a single reproductive cycle.

There's also the fact that while most Alaska crabs have either big crusher front claws or smaller manipulative ones, the green crab has one of each. Though our local red rock crab may be able to beat up the green crab in a fair fight, the green crab may have the more versatile toolbox.

They may arrive as larvae in plankton being carried northward on a conveyor belt of a current on the Pacific coast. Or they may arrive, as Prince William Sound worries, with the help of mankind, in ballast water. If they come on the currents, Southeast is probably the first spot they'll land. They tend to make jumps in El Niño events with warmer water.

Unlike the Lower 48, population density is too low here for humans to guard every inch of shoreline against these crabs. Nonetheless, NOAA has a burgeoning trained volunteer force in Gustavus, Sitka and Ketchikan, plus Prince William Sound and Kachemak Bay.

The most extreme green-crabwatcher is certainly Gary Freitag, who once again this year will be dropping crab pots from a float plane.

Ketchikan resident Freitag, an associate professor at the University of Alaska Fairbanks, works for Alaska Sea Grant.

He's heading to rugged areas on the outer coast, where he and scientists at the NOAA in Juneau have ascertained the green crabs are most likely to show up. Into an indexed map of Southeast, they input a query: certain ecological conditions green crab are known to prefer, such as eelgrass or soft sediment, estuarine areas. They receive hot spots of likely habitat. The spots also have to be close to those plankton-bearing currents. That's where they'll start looking.

On the outer coast of Prince of Wales and Doll islands, Freitag's volunteer pilot will drop down to settle the plane on its floats. He'll let the wind drag the plane, while Freitag surveys the depth using a depth sounder he strapped to the undercarriage of the plane. When Freitag finds the right spots, he - wearing e-PIRB and lifejacket, but never leaving the plane - will drop crab pots from the open door. The next day he'll return to measure, photograph and release everything.

"It took some time to figure out the technique," Freitag said.

• Contact reporter Kate Golden at 523-2276 or

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