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Monster Vs. Mitten Crab

For the past several years federal and state fish facilities have been beseiged by an armored army: hundreds of thousands of Chinese mitten crabs, clogging screens and pumps, injuring or killing sensitive fish, and even bringing operations at the federal facility to a halt at one point. But the ubiquitous crustacean may have finally met its match.

Crabzilla, a monstrous 8-foot-wide by 18-foot-high traveling fish screen, now straddles the conveyance channel at the Tracy facility, scooping up crabs on a giant revolving wheel while allowing fish to slip through tiny mesh openings. Although the wheel usually spins at speeds of about 2 feet per minute, it can be sped up to around 20 feet per minute if lots of crabs are entering the channel. While the salvaged fish are trucked back to the Delta far from the pumps, the crabs are brushed and pressure-hosed off the screen onto a conveyor belt that dumps them into a container. From there they are hauled to Modesto and ground into fertilizer.

Crabzilla is actually a modification of a fish screen created by Visalia's Farm Pump and Irrigation Company, according to BurRec's Brent Mefford, one of several engineers who worked on design. How effective is it at fending off the furry-clawed invaders? The first of its kind, the giant screen underwent extensive testing during the fall of '99 and has been used at the Tracy facility ever since. So far, it has been removing approximately 90 percent of the crabs that enter the channel, says BurRec's Charles Liston although Mefford believes that number is closer to 99 percent. It may be a few years before Crabzilla is put to its truest test though: crab numbers were down this year, particularly in Suisun Marsh and the Delta (they were still high in the South Bay), according to Cal Fish & Game's Kathy Hieb. That doesn't mean we won't see large numbers of crabs again. "This could be part of the typical 'boom and bust cycle' of some introduced species," says Hieb. "More likely there will be some cycle of high and low numbers over time. In Europe, the mitten crab population has a 15-20-year cycle, but of course, we have no idea of what it is here." Contacts: Charles Liston; Kathy Hieb; Brent Mefford: (303) 445-2149 LOV

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