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California and the West | CALIFORNIA ALBUM

Crab Threatens Delta Waterways

In just three years, the Chinese mitten species has risen to the top of the region's pest list, clogging pipes and burrowing in levees. L.A. area might be creature's next stop.


TRACY, Calif. — Here on the southern edge of the sprawling, murky waters of the Sacramento-San Joaquin Delta, another of nature's intruders has appeared.

It's called the Chinese mitten crab, and in sheer numbers alone, it packs the potential for trouble.

At just one delta location, officials reported last week that they were hauling 40,000 crabs a day out of a fish-collection facility.

The delta, a fragile place held together by 1,100 miles of earthen levees crisscrossing 1,154 square miles of former marshland, already is one of the most invaded ecosystems in North America, biologists say. More than 200 life forms here are immigrants--everything from huge sturgeon to tiny clams inhabiting the meandering sloughs and channels that define a huge chunk of the variegated California landscape.

And now comes the ornery little critter with the beady, protruding eyes and small white pincers sticking out of puffy brown claw coverings (hence the name mitten crab). It is a delicacy in its native China but a problem here, as this notorious burrower and potential carrier of disease settles into one of California's most important water basins, one fed by the state's two biggest rivers, the Sacramento and the San Joaquin.

It is California's main source of water, supplying about two-thirds of the state's drinking supply and even more for Central Valley farmers.

Between the sloughs and canals, pear orchards lying below the water line thrive in rich soil, forming part of the special blend of delta hues--pastel greens, grays and browns. Ducks, geese and a rare breed of elk attract hunters, and the area is used by recreational boaters as well. But tourists are a rarity.

A drive on the levee-top roads winding through the system brings a traveler across stately old tree-shrouded mansions standing alone in fields and occasional small towns such as Locke, still bearing the look of its Chinese founders a century ago.

What worries biologists is that the palm-sized mitten crab finds this atmosphere just as appealing as the delta's few human residents.

The crab hides from its enemies by burrowing, and the mitten variety has already become expert at snuggling into the soft delta mud lining levees that could one day collapse if the burrowers stay at it.

The mitten crab first appeared in delta waters about three years ago. Researchers figure it made its way from the southern end of San Francisco Bay, where it first established a California beachhead. Paddling north, it discovered the less salty waters emanating from the delta through the Carquinez Strait and thus found its way to a perfect home.

Mitten crabs spawn in brackish waters, then head upstream seeking pure, fresh water to mature before returning to salty water to hatch their eggs and die in a two- to three-year life cycle.

State biologist Kathyrn A. Heib, who studies the creature, suspects the mitten crab got to California in the first place by "people carrying them from China," attracted by the fact that single crabs in San Francisco's Chinatown were bringing $20 apiece in the early 1980s.

Other theories suggest the crabs came as stowaways in the ballast of ships arriving from the Far East.

In any case, cashing in on the crab is now illegal. In an attempt to prevent its spread, the federal government banned any transport of live mitten crabs 12 years ago. Dead, they have no commercial value.

Researchers have no clear fix on a central gathering place for the crab in the vast tracts of the delta. They say it seems to spread itself throughout the watery basin, tending to cluster around pipe outlets in sloughs used to irrigate crops.

Thousands are massing at water-pumping stations near Tracy. One is a federal operation that ships water to Central Valley farmers. The state runs the other pump plant, which sends water to millions of households in the Southland.

At the state pumps, officials said, an onslaught of crabs is expected later in the year. It is at the federal facility where the crabs are clustering in the tens of thousands per day, said Jeff McCracken of the Bureau of Reclamation. They have accumulated in such numbers there that the sheer mass of crabs is clogging the traps used to divert fish before they reach the pumps, he said.

There were only half a dozen found there two years ago, scientists said. Last year they gathered 20,000 mitten crabs, and this year the numbers could go to the millions at the one pumping station.

But many are getting through the fish traps uncounted, some being sucked into the pumps, others clambering out of the channels--they can scale steep concrete walls--only to plop back into the canals downstream.

New Areas to Conquer

In its quest for fresh running water, said Heib, the beast has already wandered north, east and, yes, south toward Los Angeles.

Can it beat the killer bees into Hollywood swimming pools?

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