Mitten crabs fan out in area: Species could be a threat to levees
Nancy Vogel (The Sacramento Bee)
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Aug. 27, 1998 

Boys playing in a Rio Linda canal have discovered Chinese mitten crabs, proof that the fast-multiplying creatures -- which weaken levees and eat rice crops in their native lands -- have established themselves in the Sacramento region. 

"They're pretty much anywhere water got this winter, up to the major dams," said California Department of Fish and Game biologist Kathy Hieb, who has taken dozens of reports of the crabs from Roseville to Knights Landing to Stockton since they were discovered in San Francisco Bay in 1992. 

While the crabs are just one of 200 species that don't naturally belong in Central Valley rivers and the San Francisco Bay, they raise more fears than most. 

Young Chinese mitten crabs burrow holes in levees and stream banks where tides regularly fill the burrows and keep them wet. In their native China and Korea, the crabs have damaged rice crops and carry a lung fluke that has given millions of Asians tuberculosis-like symptoms. 

So far, Hieb said, there are no reports of crabs damaging levees or crops in California, although many anglers have complained of crabs stealing their bait. Nor do the California crabs appear to carry the parasitic lung fluke. 

Hieb expects many more sightings in coming weeks as the long-legged crabs walk down rivers and streams to San Francisco Bay, where they will spawn and die. 

Liz Maxwell's 13-year-old son has been pulling the palm-size crabs out of holes in a Natomas East Main Drainage Canal bank for a month, she said. But it's only in the last few days that they've been identified as the Chinese mitten crabs. That happened after her boss, general manager of the Rio Linda/Elverta Water Community District, passed around a crab in a Styrofoam cup at a meeting of the Sacramento Area Flood Control Agency. 

A South Bay shrimp fisherman first caught a Chinese mitten crab in 1992. In 1996, just 45 crabs were found in the Delta. By 1997, 20,000 had been captured, according to state biologists. 

"Whoever introduced this crab -- if it was deliberate -- was very, very short-sighted," said Hieb. She suspects someone established the crabs in California in order to eat them. 

In China, mitten crabs -- named for their hairy claws -- are the most expensive crabs for sale, she said. Especially desirable are the bright orange, developing ovaries of female crabs, which fill as much as half of a crab's body cavity. In the 1980s, Hieb said, live mitten crabs were found in Los Angeles and San Francisco markets selling for $10 to $20 each. The hardy crabs can survive weeks out of water if kept moist, and were probably carried by passengers on flights to California from Asia. 

Any crab found in freshwater north of the Delta, Hieb said, is most likely a Chinese mitten crab, because there is no native freshwater crab in the Central Valley. She's gotten reports of the crabs from Cirby Creek in Roseville, Putah Creek in Yolo County and the Port of Sacramento. The crabs have been spotted as far north as Meridian. A few hundred a week are now showing up at the fish screens of federal water project pumps near Tracy. 

Given that currents carry some crab larvae out of San Francisco Bay, Hieb said she wouldn't be surprised if the crabs spread to coastal streams. They also could reach Southern California via state and federal water projects. 

Fish and Game biologists say they don't yet have a means for controlling the crabs and won't tackle that issue unless the crabs begin to inflict damage. 

Ted Smith, manager of the American River Flood Control District, said his inspection crews have noticed no crab damage along the Natomas canal levees in Rio Linda and Elverta. 

But Maxwell, whose son found the crabs, worries. "If these things are multiplying to the levels they're estimating," she said, "what's going to happen in two or three years?" 

Copyright © 1998 The Sacramento Bee 

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