| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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habitats Chinese Crabs Discover San Francisco Bay
We are headed south on Highway 880 in Kathleen Halat's black Jeep. She is driving a good bit over the speed limit and seems to be thinking more about our conversation than the road. She is wearing gym shorts, Teva sandals, and a sweat shirt. She is smart. And, at age 24, she is probably the world authority on what may be the most significant invasion of the San Francisco Bay since the coming of the Spanish in the eighteenth century. Halat studies the Chinese mitten crab. I ask Halat if she's seen the recently leaked U.S. Fish and Wildlife Service report about invasions in the San Francisco Bay and Delta. She hasn't yet, but she knows one of its authors, Andrew Cohen, a researcher at the University of California at Berkeley, and she does not question its finding that San Francisco Bay "can now be recognized as the most invaded aquatic ecosystem in North America...[and that] in the period since 1850, the San Francisco Bay and Delta region has been invaded by an average of one new species every 36 weeks." The Bay ecosystem has been so profoundly altered, in fact, that it seems impossible to say which end is up. The indicator species for the Bay's health is the striped bass: an exotic introduced in 1879 and an archenemy of the Bay's native fishes. Maybe that's why there is so little outcry over new invasions or the careless policies permitting bilge dumping in the Bay. It is a paradoxical twist, and an addition to the complexity of the moral equation, that the Bay has probably achieved greater biological diversity in the past century and a half, though it has probably lost many of its unique species and characteristics. Because the Bay was so disturbed so early on, there is no baseline data and very little understanding of what it was like before the mass invasions began. We can't mourn (or study, or insist on restoring) what we do not know we have lost. I suspect this is the rationale behind those politicians trying to gut the National Biological Service. Halat and I turn off 880 near the southern tip of the Bay at the Sunnyvale exit and, after winding our way through the tiny, old-fashioned, Hispanic town of Alviso, turn onto a dirt road running atop a levee and then, finally, stop the Jeep where a Santa Clara County Water District pump blocks the way. From the levee we look over a junkyard filled with the shells of old cars on one side and a sea of bulrushes hiding Alviso Slough, one of Halat's study sites, on the other. In November 1994 fisherman Don Baker of Crockett caught an unfamiliar crab in his shrimp net south of the Dumbarton Bridge. He brought it to Karen Grimmer, educational director of the Marine Science Institute (MSI) in Redwood City. Grimmer brought the crab to the California Academy of Sciences, where invertebrate specialist Robert Van Syoc identified it as a Chinese mitten crab (Eriocheir sinensis). Realizing the potential impact of the crab, MSI recruited Halat. It is still unclear how or exactly when the mitten crab arrived in North America. It may have come in the bilge water of a tanker. Or it may have been introduced purposefully as a food source; it is cherished in China, where its gonads are considered a delicacy. "The mitten crab is a physiologist's dream," says Halat, a graduate student in Berkeley's Department of Environmental Science Policy and Management. Perhaps most remarkable from a scientific point of view, she says, is its high tolerance for variations in salinity. It can move from seawater saltiness to fresh water with no adjustment time or apparent metabolic stress. It is also one of only two species now established in North America that are born, have sex, lay their eggs, and die in sea water but spend the bulk of their adult lives in the fresh water of rivers. (The other is the Atlantic river eel.) Called catadromous, these creatures adopt a reverse commute schedule--salmon, trout, and other anadromous species go upstream only when they are ready to reproduce and die--and it has paid off in spades for mitten crabs, who begin their long walk up river at the energetic young age of only five or six months. They spend their prime earning years (two or three of them) fortifying themselves upstream and then, in the late fall and winter, the older crabs glide downstream to mate and die. The crab grows to be about six inches wide, including its legs, and its name refers to tufts of hair growing on its husky claws making them resemble--at least to some namemaker's eye--winter apparel. If it is a physiologist's dream, the mitten crab may turn out to be a land manager's nightmare. Halat's research is partially supported by the Santa Clara County Water District, which is concerned about damage the crab may do to its levees and dams. Doug Padley, the water district's wildlife biologist, says the mitten crab poses a "major problem" in the south Bay and its tributaries. "They may burrow in such high densities that they cause levees to fail," he says. "It's not only a potential maintenance problem, it is also a public safety issue. If a levee fails, there is life and property at risk as well as the levee itself." Inflating these fears are mitten crab horror stories from introductions past. The crab was first identified in Europe in 1912, and by the 1930s millions of crabs migrated up Germany's major rivers clogging dams and climbing onto shore, where they wandered city streets and entered homes. They devastated fisheries and destroyed river banks and levees causing floods and other damage. No one knows why populations exploded when they did, or why they have not exploded in Europe since. Nor does anyone know in what ways they altered the river ecosystems they overran, but they must have been biologically significant. Cohen says that species frequently balloon in numbers shortly after their arrival in a new ecosystem, later leveling off as they settle in to their new niche. Other ballooning populations never collapse, such as the zebra mussel in the Great Lakes, which has completely overrun entire ecosystems and seems content to be its own only neighbor. If the south Bay county of Santa Clara is concerned, folks to the north in the Bay's Delta region are horrified, or they should be. The levee systems of the south Bay would be a nuisance to replace and bolster, but undermining the structural integrity of the thousands of miles of north Bay Delta levees and dams would be catastrophic. Of course there may not be a crab population explosion such as the one in Europe, but even a stable population would cause tremendous damage. Indeed, Halat and I, now--despite the hot sun--dressed in long pants and long-sleeve shirts and tennis shoes, are pushing our way through the bulrushes toward the slough. Suddenly we reach the edge of the vegetation and the brown rushing water is visible. We jump into the slough, whose banks have already begun to erode due to the diligent burrowing work of mitten crabs over the last couple of years, and sink into the gray-green mud. Halat's project is to measure the mitten crab's reproductive success, migration patterns, and population densities in the Bay. Once inside the slough, evidence of the crab is everywhere. Small burrows, most with a circumference of a golf ball, penetrate the banks. Not only is their presence evident, but so is the damage they are doing. In only a few generations the burrows have caused chunks of the banks to break off and rush downstream. Halat's population study, now only a year and a half old, has found densities in the south Bay ranging from a couple of crabs per square meter to more than 50. The further upstream you go, in general, the lower the densities. We are two miles upstream from the Bay, but unconfirmed reports place the crab as far as 25 miles upstream. On the Yangtzee River in China, they are known to migrate eight hundred miles upstream. There was a spate of news stories in November 1994, when the crab was first discovered in the Bay and it was feared that it carried a human parasite, the Oriental lung fluke, which has given millions of people in China a sometimes fatal tuberculosis-like lung disease. Halat sent 50 Bay mitten crabs to a University of California at Santa Barbara lab to be tested for parasites, and none were found. Apparently a freshwater snail that has not (yet) invaded the Bay is the fluke's primary host. Until it arrives, eating mitten crabs should not be dangerous. End of news story? I don't think so. "A few years ago I'd find one here and there, but now they're all over the place," says fisherman Don Baker. "Sometimes I get as many as two hundred in a single drag [for shrimp]," he says, "and we have to throw the whole thing back. That many crabs start eating the shrimp." "They must really be horny little buggers, too," says Baker. "Throw two of them in a bucket and before you know it they'll be mating." (Maybe this explains the popularity of their gonads.) Halat is one of only two people licensed to pull mitten crabs out of their burrows. In 1986, eight years before they were first caught in the Bay, the California Department of Fish and Game found living specimens for sale in Asian food markets in San Francisco and Los Angeles. They must have been imported directly from Asia. Afraid that they might get loose and establish themselves in the Bay, the federal and state governments designated the crab an "injurious species," making its capture or possession (without a license) a serious crime. "There we go," Halat says, pulling out a little mitten crab, harmless looking, cute even. Halat--whose Ph.D. advisor at Berkeley told her to concentrate on getting published in a respectable scientific journal and not to worry about getting on the evening news--tries to remain level-headed about the damage mitten crabs may do. Never resorting to "catastrophe" or "disaster," she says instead that they may have a "significant impact" on the levees and the estuary ecosystem. Neither she nor other scientists I spoke to would hazard a guess on what trouble the crab will cause for other Bay species. It is omnivorous, eating "everything," says Halat. And it eats a lot of everything, too. The truth, says Cohen, is that remarkably little is known about the ecology of San Francisco Bay. "It's a very disturbed ecosystem and ecologists like to study more pristine areas," he says. "Ironically, it's often those impacted areas like the Bay that, for very practical reasons, we want to know more about." Baker, who has been fishing the Bay for more than a decade, has no problem speculating on record: "I don't know much about this, but if they're eating in the same food chain as other things, I don't see how they could not have a big effect." "The important thing to get in the story," pleads Halat, suddenly dropping her scientific detachment, "is to tell people who find these crabs to leave them where they are. Tell them not to move them. They're advancing fast enough on their own." Halat turns over the tiny crab and shows me how to sex it. This one, having a wide, flush, V-shaped abdominal flap is a female, Halat says. (Each female carries somewhere between 250,000 and a million eggs.) Halat grabs a clump of bulrushes near the roots, pulls herself expertly out of the slough, and then offers me a hand. Our pants and hands are covered with mud. Halat's face is striped with the stuff and she looks a little like a new species herself. Walking back to the Jeep she says, "I guess the main point is that you never know what's going happen when you introduce a new creature into an ecosystem."
Gordy Slack is Assistant Editor of California Wild. |
Summer 1996
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