Eat the Invading Alien Crabs, Urge U.K. Scientists

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The crab is now established throughout much of Europe and the U.S., giving rise to similar concerns over its environmental impact.

In San Francisco Bay, California, they proved so successful that in 1998 they threatened to shut down the state's water supplies. The crabs, which migrate downstream to estuaries to breed, collected in such vast numbers that they blocked screens covering the water intake facility in the Sacramento-San Joaquin delta. To prevent southern California running out of freshwater some 20,000 crabs had to be removed from the screens each day.

Other problems include damage to riverbanks, bait-stealing from sport anglers and clogging of water cooling systems at power plants, according to Kathy Hieb, biologist with the California Department of Fish and Game. She says the mitten crab's wider ecological impact has yet to be established.

Although the population fluctuates enormously from year to year, Hieb suspects the mitten crab has become a permanent resident in San Francisco Bay.

"They are here to stay," she said. "Although numbers are now low, we predict they will increase again and be cyclic. We do not believe the crab can be eradicated as it has pelagic [free floating] larvae and is widespread in the bay and its watershed. A single female can have up to a million eggs, so many young crabs can result from a few adults if physical conditions are beneficial to their survival."

Prohibited Species

The mitten crab is a prohibited species in California, in order to prevent it spreading to other rivers. This makes it illegal to transport the crustacean while alive.

Despite this, the species does have commercial possibilities, according to Richard Tullis, biology professor at California State University, Hayward, who has studied the crab.

"I think the idea of eating the crabs into retreat is a marvelous idea," he said. "There is a thriving black market in the Chinatowns of Oakland and San Francisco. If open to commercial fishing the population would dwindle in the wild, like it has in China."

Fears about the crab's ability to colonize new areas are underlined by the staggering distances it can travel inland. In China, the crab has been reported 1,300 kilometers (800 miles) from the sea, while in Europe it has already reached Prague, capital of the Czech Republic.

In Britain, Rainbow believes there could be home demand, as well as an export market, for commercially-fished mitten crabs, particularly within London's Chinese community.

"We did give some we'd caught to a Chinese restaurant who cooked them and said they were delicious," he said.

Tullis agrees, adding, "Fixed Asian style, stir-fried with garlic, soy and ginger, it will also turn on non-Asians."

The potential for British mitten crabs is enhanced by the fact they are free of lung flukes which infest Chinese populations. This parasitic flatworm requires three different hosts to complete its life cycle: freshwater snails, mitten crabs, and mammals. In the Far East, humans who eat fluke-infected mitten crabs are vulnerable to a tuberculosis-like disease which can be fatal.

Rainbow says the snails that harbor lung flukes need warm waters and are absent from northern Europe. "We don't think the British crabs are infected by the parasites," he added.

This leads Rainbow and his research team to conclude: "The culinary route may represent our best culling strategy if we are to limit its potentially damaging environmental effects."

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