Crab Threatens Delta Waterways
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?
Maybe not, say the experts, but a couple of crabs headed south did take a dip in a Stockton lawyer’s pool recently. They have even been found wandering around parking lots in cities east of San Francisco.
The crabs have shown up in Merced--60 miles south of the nearest delta slough. Making it as far as Los Angeles, Heib said, “would be nothing to them.” She said mitten crabs in China have been found 800 miles from the sea water where they hatch.
“They’re definitely going to migrate to you guys in Southern California,” and in growing numbers, said biologist Jeff Bridges at the federal pump station.
An exploding population of mitten crabs is inevitable, said Heib: “Each female carries a quarter of a million to a million fertilized eggs that hatch in the spring.”
For now, the immediate threat is mostly just that--the specter of the crawling beast carpeting creek banks and river bottoms across the state. But scientists say they are concerned about a range of eventual potential dangers.
Heib has noted in scientific reports that the crab’s burrowing habits have destroyed levees in China and could do the same here, although delta levees so far remain stable.
The crab is a carrier of a parasite known as the Oriental lung fluke that, if consumed by humans in a crab not fully cooked, causes tuberculosis-like symptoms. No such cases have been reported, however.
Mitten crabs by the thousands clogged a cooling system in a delta power plant last year. There could be more such problems as the crab population explodes.
The crabs feed on plant and animal life and could reduce stocks of clams, worms and other water life needed for ecological balance.
Their taste for tender rice shoots has wiped out crops in China. In California, said Bob Herkert of the California Rice Industry Assn., what the mitten crab could do to the crop “is a $500,000 question.”
Hundreds of thousands of acres of rice-farming land in the inland northern counties lie in the path of the advancing crab, Herkert said. So far, there have been no crops damaged, he said.
Mitten crabs have fouled commercial fishing nets, and for sport anglers, said fisherman Richard Hutcheson of West Sacramento, who fishes delta sloughs for catfish and bass, “You can’t put bait in the water. As fast as you get it in, they hit it.”
Hutcheson said, “It’s amazing. I’ve fished those waters for 15 years and never saw anything like this year.”
How to dispose of the creatures is just as much a problem as how to prevent their proliferation, Heib said.
At the pump stations, biologists gather all the crabs they can catch, then truck them live back into the delta, far from the pumps.
However, said biologist Scott Siegfried at the federal pump station near here, “we really don’t know how many we’re catching. They crawl around and go wherever they want.”
What he does know, said Siegfried, is that “this year is going to be crazy.”