Biologist studies crab invading valley

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Biologist studies crab invading valley

California crops and water systems are facing a serious threat from the prodigious Chinese mitten crab, according to Dr. Brian Tsukimura, assistant professor of biology at California State University, Fresno.

Working on a committee to investigate the newly-immigrated crab from China, Tsukimura said its exponential growth is astounding.

“The numbers went from 20 crabs pulled in 1996 from the Tracy Pumping Station, which feeds the Delta Mendota Canal, to about 20,000 pulled the next year,” said Tsukimura. “What really alarms me is that last year they pulled 20,000 per day.”

Native to the coastal rivers and estuaries of China’s Yellow Sea, the Chinese mitten crab is thought to have been accidentally brought by fishing vessels to the San Francisco Bay from which it worked its way to the Central Valley via the waterways.

“No one really knows too much about this species of crab,” said Tsukimura. “We’re not sure of the affects it will have on agriculture in the Valley.”

Tsukimura said that more than 50 tons of crabs were hauled away last year from the Tracy pumps. “These crabs have invaded our waterways from as far north as Colusa to as far south as Merced.”

The crabs are a menace for any number of reasons; from clogging Delta Mendota Canal pumps to damaging levees. The crabs, which burrow into banks

and levees sometimes two-feet deep, get sucked into canals that crisscross California — thus spreading wherever the waterway leads.

“The crab mates in saltwater, its offspring (larvae) settles in brackish water before moving to fresh water to grow up and mature,” said Tsukimura. “They can survive out of water for more than 24 hours.”

Tsukimura said that mitten crabs are adept walkers and can travel upstream sometimes 5 mph and as fast as 10-12 mph downstream. “In Germany, swarms of mitten crabs have been found in the streets at night and have even entered houses,” he said.

Another concern for Tsukimura is the potential damage the crab could have on crops in the Valley. “As far as we know, it is an encounter feeder, which means it eats whatever it meets.”

Although there is little documentation on the mitten crab consuming crops, it has reportedly damaged sprouting row crops in China and Korea. Tsukimura says little is known about the crab because the majority of literature is written in Chinese. “The mitten crab is commercially consumed in China and research is almost nonexistent,” he said.

Because the crab has had exponential growth in the Valley, Tsukimura says it is important to study its life cycles and find out how often the crab breeds. “This is just one aspect that could give us insight on how to control its population,” he said.

Tsukimura will specifically be studying the hormonal and environmental stimuli for reproduction. “These aspects will be targets for eventual management programs,” he said.

With the onslaught of summer, Tsukimura said the crab is at its high season and should hit the Valley waterways in droves. “By August, this crab will be on the minds of anyone in agriculture that gets irrigation water from the Tracy pumping station, which includes west Fresno County.”