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As unappetizing as that may sound, Mr. Johnson says it poses no risk to people, but it has substantially reduced the health of the whitefish, because the mussels lack a fatty acid they need for membrane development. (The acid is present in Diaphoreia amphipod, a tiny shrimp once favoured by the whitefish whose disappearance some suspect may also be linked to the mussel invasion.)

So now the fish are thinner, grow half as quickly, and spawn every seven years instead of every four. They are also being driven to greater depths to escape the clear water because the extra light causes cataracts on their eyes.

These developments concern Mr. Johnson, but Mr. Lancaster believes that the lake can fight back. "Our experience . . . is there are whitefish wherever we go, which suggests to me the assessment process has made a huge mistake."

He also says that this year there are more native clams among the mounds of mussel shells that carpet the beach, and the "windrows" of shells in the shallows are becoming smaller, suggesting that now-dominant quaggas reproduce less rapidly than zebras.

That said, mussels have plugged the entrance to Point Traverse harbour three times in the past five years; the dredged mounds of gravel and shell Mr. Lancaster points out are easily three metres high. And the beach, like many in the province, is carpeted by a thick layer of shards that makes it excruciating to walk barefoot.

Mr. Lancaster is also concerned by the number of dead birds he has seen in recent years, a phenomenon he thinks may be linked to botulism in the mussels. Mr. Johnson agrees, saying biologists are studying whether the round goby fish, another invasive species, may be acting as a "vector," by eating tainted mussels, struggling to the surface to die and be eaten by birds.

Companies such as Ontario Power Generation have used chlorine to control mussels on intake pipes, but eliminating them altogether would require treating all the water in the Great Lakes simultaneously. That's clearly impossible, so most scientists focus on keeping the next invasive species from coming in.

Since June, oceangoing ships that use water for ballast have been required to flush their tanks with saltwater to expel any unwanted hitchhikers before entering the St. Lawrence. However, most use cargo for stability, and so have the option not to flush -- a loophole that Natural Resources Minister David Ramsay is trying to close. Many scientists would like to have all shipboard ocean water treated -- Prof. Hebert suggests boiling it -- before it is released. The question becomes whether shippers, and ultimately consumers, will bear the cost.

Prof. Barton -- the most sanguine about the mussels' impact -- offers the most radical solution. A sad consequence of globalization may be the inevitability of invasive species, he says. In that context, perhaps we should consider introducing new species into the Great Lakes to compensate for what's being lost.

For example, he says, the little shrimp that's disappearing is "literally the only major, year-round food source for deep-water fish in the Great Lakes." Replacing it with another deep-water shrimp would hedge our bets and have the added benefit of increasing "global diversity," he says.

The idea makes many of his colleagues nervous. "Most biologists would be immensely cautious about plans to introduce species beyond their natural bounds because they're quite unpredictable things," says Prof. Hebert, clearly restraining himself.

"You really don't know what's going to happen." He cites the case of a tiny European fly that the Americans introduced in the eighties to control the gypsy moth population. Unfortunately, the tachinid proved to have a broader-than-anticipated palate, and now some of the more interesting species of large-winged moths -- including the luna -- are "basically gone from the eastern part of North America because of this goddamn thing," he adds, restraint slipping.

It is that kind of loss to biodiversity that may be the greatest consequence of invasive species. A new invader arrives in the Great Lakes every eight months -- 173 have been identified so far -- and each creates a chain reaction often too complicated to measure, much less control.

"We have a whole new ecosystem now that we have zebra mussels," Mr. Johnson says flatly. "And while 50 to 100 years of research on the Great Lakes hasn't exactly gone out the window, we're pretty close to starting from scratch."

Danielle Bochove is a freelance journalist in Toronto.

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