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Mitten crabs from Asia and green crabs from Europe, clams from Asia and the East Coast, South American water hyacinth and Atlantic cordgrass are among the most damaging invaders in San Francisco Bay. The Asian clam Corbula amurensis has not only taken over huge parts of the bay bottom, it also seems to have altered the entire ecosystem. According to Cohen, since the Asian clam was first found in the North Bay in 1986, annual blooms of phytoplankton that were the basis of key food webs have ceased, and the zooplankton and tiny shrimp they supported have disappeared from most of the bay. The niche the plankton had occupied in the food web has since been dominated by a series of 15 Japanese zooplankton, which one after another have been devoured by the voracious filter-feeding Asian clams. "Over the last three years the pelagic fish that were supported by the native plankton have also gone into dramatic decline," said Cohen. Ecosystems are too complex for us to readily pin the blame for changes on a single species, he cautions, but these declines correspond almost exactly with the establishment of the Asian clam.

Cohen's 2004 and 2005 surveys, funded by the Coastal Conservancy, showed that invasives dramatically outnumbered natives in most areas. In sites near the mouth of the bay, 30 to 50 percent of the 80-110 species identified were aliens; near tributaries to the bay, only 20-30 species were found, and 70-80 percent were exotics; in parts of the South Bay, 90-100 percent of species identified were nonnatives.

It's difficult to determine what impact California's ballast-water regulations have had on this problem. Maurya Falkner, manager of the Marine Invasive Species Program at the State Lands Commission, believes that they have helped to slow the rate of new invasions. "Only about 30 percent of commercial ships discharge ballast," she said, "and about 80 percent of those have been compliant. There have been a few problem vessels, about five percent, mostly those coming up the coast from Mexico and South America. Some of them discharge ballast, but only 50 to 100 nautical miles offshore. Container ships retain their ballast water." Cohen is less sanguine in his view of the program's effectiveness, pointing out that statistics are based on reports from the ships rather than on inspections.

Theoretically, commercial vessels also have the option of eliminating invaders by treating ballast water onboard or ashore. However, Falkner said that no system for such treatment is yet available. The Lands Commission helped to install experimental systems in two Matson vessels, she said, "but we don't know much about those yet. There aren't even any performance standards in place."

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