November 5, 2003

For More Information:

Philip Cook, Research Chemist, U.S. Environmental Protection Agency, Duluth, Minn., (218) 529-5202

Richard Peterson, Toxicologist, University of Wisconsin-Madison, (608) 263-5453

Donald Tillitt, Environmental Toxicologist, Columbia Environmental Research Center, U.S. Geological Survey, Columbia, Mo., ( 573) 876-1886

Scott Brown, Environmental Toxicologist, National Water Research Institute, Burlington, Ontario, (905) 336-6250

Stephen Wittman, Program Information Specialist, University of Wisconsin Sea Grant Institute, (608) 263-5371

Editors Note: High resolution photos for this story are available for downloading at

Toxic Chemicals Killed All Young Lake Trout in Lake Ontario for 40 Years

MADISON, Wis. (Nov. 5, 2003) – A team of researchers has determined that dioxin and similar toxic chemicals were high enough in Lake Ontario to kill virtually every lake trout that hatched there from the late 1940s to the late 1980s. Their findings differ from traditional explanations for the collapse of the lake trout population in Lake Ontario that focus on overfishing and attacks by the parasitic sea lamprey.

The findings also suggest chemical contaminants may have complicated efforts by the United States and Canada to restore healthy populations of lake trout across the Great Lakes basin, according to Philip Cook, a research chemist and environmental toxicologist at the U.S. Environmental Protection Agency in Duluth , Minn., and lead author of the study.

The research results also show the importance and the feasibility of investigating possible harmful effects of other contaminants that haven’t been studied well, Cook said.

The research was published in the September issue of the journal Environmental Science and Technology. The report details results from a 15-year collaboration among a team of toxicologists, chemists, chemical and environmental engineers, and sediment dating experts.

In one part of the work, a group of researchers led by toxicologist Richard Peterson at the University of Wisconsin-Madison found that, in their early life stages, lake trout are among the most sensitive fish to dioxin (specifically, 2,3,7,8-TCDD), PCBs and similar chemicals. At concentrations as low as 30 parts per trillion dioxin in egg tissues, mortality of newly hatched fish exceeds normal levels.

"Thirty parts per trillion is an extremely small concentration, approximately equal to one drop in 500,000 gallons of water," said Peterson, who directed the UW-Madison component of the study with support from the UW Sea Grant Institute.

Dioxin, PCBs and similar chemicals pass from water and sediments into small plants and animals near the bottom of aquatic food webs. Because they are retained in tissues, they accumulate as they are passed to higher levels of food webs. Animals near the tops of food webs, like lake trout, generally have the highest concentrations of such chemicals in their body tissues, Peterson said.

In their component of the study, Cook and his colleagues measured dioxin and other chemicals in samples of sediments, herring gulls, adult lake trout, other fish species and lake trout eggs from Lake Ontario. They used mathematical models to estimate from these measurements the concentrations in lake trout egg tissues between 1920 and 1990.

The researchers conclude that dioxin levels in lake trout eggs reached the 30 ppt mortality threshold in the early 1940s. By the late 1940s, concentrations reached 100 ppt. At that concentration, 100 percent of juvenile trout can be expected to die, the authors reported.

Concentrations remained at or above these levels until about 1976, by which time environmental regulations had sufficiently reduced toxic contamination levels to again allow some egg survival, according to the study. By 1982, egg concentrations had dropped to the point that no measurable direct mortality from dioxin was expected.

"That’s the good news of the study," Cook said. "It shows that pollution regulations can really be effective."

Cook points out, however, that researchers know much less about so-called "sub-lethal" effects of contaminants on lake trout, doses that do not kill the fish in laboratory tests but do impair critical functions like vision or swim bladder inflation.

"In natural environments, these low levels of contaminants could impair the recovery of lake trout populations," Cook said. "Young fish may not be able to flee from predators or find food, and that could be happening out there today in the Great Lakes. We don’t know for sure about that – we’re in a grey area with these low levels."

The work is an "elegant piece of science" that drew upon multiple sources of evidence to support its conclusions, according to Donald Tillitt, an environmental toxicologist in the Biological Resources Division of the U.S. Geological Survey in Colombia , Mo.

"It allows us to quantitatively look at the effects of these chemicals [on lake trout]," Tillitt said. "It’s a very significant piece of research."

"It’s one of the nicest case studies that have been done," agreed Scott Brown, an environmental toxicologist at Environment Canada’s National Water Research Institute in Burlington, Ontario.

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Conceived in 1966, Sea Grant is a national network of 30 university-based programs of research, outreach and education dedicated to the protection and sustainable use of the United States' coastal, ocean and Great Lakes resources. The National Sea Grant Network is a partnership of participating coastal states, private industry and the National Sea Grant College Program , National Oceanic & Atmospheric Administration , U.S. Department of Commerce. The University of Wisconsin Sea Grant College Program is administered by the Sea Grant Institute on the UW-Madison campus in Madison, Wisconsin.

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last updated 5 November 2003


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