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A 15-year federal study suggests that traces of dioxins probably played a large role in killing off trout in the world's largest freshwater system the Great Lakes.
"This is as close to a smoking gun as we've found," said Stephen Whiteman, spokesman for the University of Wisconsin Sea Grant Institute in Madison.
The results of the study fly in the face of long-held theories that overfishing and the invasion of sea lampreys wiped out the lake trout in the middle of the last century.
The study examined the drop in the number of fish in Lake Ontario. Led by the U.S. Environmental Protection Agency and the Sea Grant Institute, a team looked at trout population dating back to 1865.
Philip Cook, an EPA research chemist, said the team found that the climbing levels of dioxins, an industrial pollutant that first showed up in measurable levels in the 1930s, directly correspond with the quick demise of the trout in Lake Ontario.
"The toxicity alone explains what happened" to the lake trout, Cook said.
Cook said that during the 1960s, the dioxin level was so high it was impossible for lake trout larvae to survive. "The mortality rate was 100 percent," he said.
He said there was heavy commercial fishing for nearly a century before the lake trout population crash. And lampreys, an eel-like, parasitic creature, first found their way into the Great Lakes when the Welland Canal opened in 1829.
The study focused on Lake Ontario. But researchers believe it will help determine what happened in all the Great Lakes, including Lake Michigan, where the lake trout was wiped out in 1956only 12 years after the commercial catch of lake trout was 6 million pounds.
Some are skeptical about the study's findings. Dan Thomas, president of the Great Lakes Sport Fishing Council, maintains that commercial overfishing is to blame for the lake trout's disappearance.
One question the study didn't answer is why even though the dioxin levels have been dropping since the 1970s, only in Lake Superior is there a self-sustaining population of lake trout.