How Did the Poison Get into the Trout? - poisoned trout in Lake Superior

National Wildlife, August, 1998

And other pollution puzzles of the Great Lakes region

One thousand feet below the surface of Lake Superior, a lead-weighted steel box settles into the lake bottom, cutting downward until it disappears in soft muck. On board a ship above, University of Minnesota chemist Deb Swackhamer signals a remote-controlled trapdoor to slide shut on the buried box. Then, with a squeal of cable, the box pulls free and returns to the ship.

This box of mud holds clues to a mystery that has perplexed scientists for more than a decade. In the 1980s, biologists found the banned pesticide toxaphene in the lake's trout, sediment and water. "We didn't expect this," says Swackhamer. "It just jumped out at us from the data." By 1992, the U.S. Environmental Protection Agency (EPA) was finding an average of 5 parts per million of toxaphene in the trout. That's far more than the level of the next worst contaminant in the region, about 3 parts per million of PCBs in Lake Michigan and Lake Ontario trout. And the toxaphene discovery was in Lake Superior-- previously believed to be among the most pristine of all lakes in the Western Hemisphere.

With these findings, researchers are confirming once again that the Great Lakes region is a hot spot for persistent toxic chemicals in the environment. The waters here are ringed with industry and development that have proven to be sources of pollution. Not only that, for reasons having to do with weather and geography, toxics tend to migrate to the Great Lakes region and then remain here, where they can end up in wildlife. That means scientists are on the alert for possible health hazards such as toxics in fish consumed by people.

Now toxaphene is presenting a new mystery: Although it was banned in 1982, recent research on old samples has found that toxaphene's levels in the Great Lakes have remained about the same for the last 20 years. And that means that new toxaphene is still somehow coming into the region. If it weren't, say scientists, levels would be decreasing significantly just as they did for toxics such as DDT and PCBs within a decade after they were banned.

With her boxes of mud, Swackhamer joins a group of scientists that for decades have been solving similar puzzles, many of which have been peculiar to the Great Lakes region. Take the case of DDE, a breakdown product of DDT. In the mid-1980s, 15 years after DDT was banned, biologist Dave DeVault became curious about why DDE was showing up in Great Lakes harbors. At the time, he was manager of the EPA's Great Lakes Fish Contaminant Monitoring Program. He and another researcher who found DDE in New York's Finger Lakes suspected smuggling or illegal manufacture. Then a chemist noticed that the structure of this particular DDE was unique. After nearly a year, DeVault's team traced it to imported Italian dicofol, a pesticide used primarily on rose bushes. The EPA quickly moved to restrict the contaminant.

In another case, in the late 1980s scientists used "fingerprinting" to trace sources of lead in Lakes Erie and Ontario. Because each geological source of lead is chemically unique, geochemist A. Russell Flegal of the University of California, Santa Cruz, was able to trace industrial lead in western Lake Ontario to Canada and lead in the southern half of Lake Erie to the United States.

Then there was the riddle of Great Lakes trout. For 30 years, in all the lakes but Superior, lake trout have not reproduced. Last year, toxicologist Richard Peterson of the University of Wisconsin, Madison, concluded from long-term research that the culprits are likely a group of dioxinlike chemicals. That explanation was not obvious: Once the dominant species in the Great Lakes, lake trout were hard hit by overfishing and predation by parasitic sea lampreys in the 1940s. By mid-century, only a few populations of Lake Superior trout remained. Once fishing was limited and sea lampreys were controlled, it seemed restocking would cure the problem. But introduced trout simply haven't reproduced.

The main reason, Peterson has determined, is the trout's susceptibility to the dioxinlike chemicals, which come from incinerators, pulp mills and a variety of industrial sources in the Great Lakes region and persist for decades. "We don't know why this species is so especially sensitive to these chemicals," he says. Dioxin levels are now falling in the Great Lakes due to stricter regulations put in place two decades ago, and scientists hope the trout will reproduce naturally again.

For the toxaphene detectives, the initial data from Lake Superior trout made no sense. Except for paper mills, the polluting industries that line the shores of the other Great Lakes don't do much business around Lake Superior, which has been less contaminated than its sister lakes by well-known villains like PCBs, dioxin and mercury. So why the toxaphene? "I didn't know what to make of it," says DeVault, now a U.S. Fish and Wildlife Service biologist. Perhaps other chemicals were interfering with the analysis. Perhaps the trout had switched to food sources laden with toxaphene from the past.