However, the crab most likely was introduced to San Francisco South Bay much earlier, in the late 1980s. A hearty and fecund invader, the crab has multiplied to such numbers that it now overruns parts of the bay. In 1998, state workers hauled away about 50 tons worth of mitten crabs that had clogged fish screens at state water pumps near Tracy. Besides clogging fish salvage tanks, mitten crab burrows undermine banks and levees, accelerating erosion and unwanted slumping.
To date, there are no strategies in place for controlling the crab's numbers. Fishers are suggesting that the crab population be culled by opening the mitten crab to commercial fishing, but this is problematic, too.
Dugan said, "The idea is to figure out a way to get rid of the crabs. If you open a fishery, suddenly, they have a value."
Another concern is that parasites could spread like fire in a hayloft through the bay at some later date, because of the sheer number of crabs. Not just people but also raccoons and other animals that feed on mitten crabs could be infected.
Because crayfish are a potential intermediate host for the flukes, the scientists also dissected more than 400 crayfish from the estuary. Like the crabs, all the crayfish appear free of lung flukes.
Despite their findings, the scientists are cautious about concluding that eating the crabs is safe now or in the future.
"Until we finish our survey of the region's snails," Walter said, "we can't assess the risk of future introductions of lung flukes."
The scientists are in the process of analyzing hundreds of freshwater snails from the area to look for evidence of lung flukes. Even if the snails turn up clean, Culver said, there are other major health concerns - such as b
Contact: Carrie Culver
National Sea Grant College Program