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Great Lakes Article:

Scientists point to pollution as cause of lake trout's disappearance
By Christopher Steiner
Knight Ridder Newspaper
11/11/03


CHICAGO - American Indians knew them as the namaycush, or "tyrant of the lakes," and before their mystifying disappearance in the middle of the last century, lake trout sat atop the Great Lakes food chain as a prodigious predator.

When the fish disappeared, it devastated the Great Lakes commercial fishing industry, opened the door for invasive species to run wild and left scientists with a riddle: What killed off the lake trout?

A new federal study strongly suggests that there was an invisible perpetrator that eradicated the lake trout, one that finally explains how the king of the largest freshwater system in the world vanished in a few decades.

The results of the 15-year study suggest that minute traces of a type of industrial pollution - dioxins - likely played a large role in killing off the fish.

"This is as close to a smoking gun as we've found," said Stephen Wittman, spokesman for the University of Wisconsin Sea Grant Institute in Madison.

The findings challenge old theories that overfishing and the invasion of sea lampreys drove down the numbers of the fish.

Lake trout need clean, deep, cold water to survive. This makes their populations one of the most fragile in the freshwater world. The fish's deep-orange meat resembles a fresh salmon filet minus the shiny silver skin. The fish mature slowly and have long life spans, sometimes more than 20 years.

American Indians sustained villages on the protein-rich meat of the lake trout, and European settlers harvested the fish commercially beginning in the early 1800s. Millions of fish were hauled out of the depths, and hundreds of fishing camps sprang up across the Northwoods to take advantage of a seemingly endless bounty.

Despite these human pressures, populations of the deepwater predator held relatively steady.

From the late 1930s through about 1960, however, Great Lakes lake trout numbers plunged until the fish was virtually extinct outside of some isolated pockets in Lake Superior. In Lake Michigan, the population fell off a cliff.

In 1944, the commercial catch of lake trout weighed more than 6 million pounds, according to the Wisconsin Department of Natural Resources. In 1954, only a few fish were caught. By 1956, Lake Michigan's lake trout had been wiped out.

Efforts to restock the Great Lakes with lake trout have failed to create a self-sustaining population - millions of fingerlings are released annually into Lake Michigan, but most fail to mature and, outside Lake Superior, no evidence of any successful natural reproduction exists.

The lake trout puzzle has haunted Great Lakes scientists for half a century. Most have accepted that a combination of commercial overfishing and the invasion of sea lampreys clobbered the species.

The U.S. Environmental Protection Agency and the Sea Grant Institute led a team that examined Lake Ontario's lake trout population dating to 1865. The researchers examined only Lake Ontario, but they believe their work will help determine what happened to lake trout in all of the Great Lakes.

The study found that rising levels of dioxins directly correspond with the rapid demise of the lake trout in Lake Ontario, said Philip Cook, a research chemist with the EPA.

"The toxicity alone explains what happened" to the lake trout, Cook said. During the 1960s, he explained, the dioxin level was such that no lake trout larvae could survive.

"The mortality rate was 100 percent," he said. "It wouldn't even matter if there were sea lampreys or overfishing."

Dioxins aren't manufactured intentionally - they are byproducts of industrial processes. Dioxins most commonly form during the burning of trash with chlorine or during the production of herbicides such as Agent Orange.

Dioxin levels have been on the decline since the mid-1970s.

The Great Lakes have been absorbing pollutants since the first sawmill went up on the shores of Lake Ontario around 1800. But no other pollutants have proved to be so lethal for lake trout in such low doses as dioxins, which explains why lake trout numbers held steady into the 1930s, when dioxins first showed up in measurable levels, according to the study.

Cook's team employed several scientific methods to determine the historic levels of dioxins in Lake Ontario trout eggs. The EPA had direct dioxin measurements from lake trout eggs dating back through 1978. From 1971 to 1978, the researchers were able to extrapolate what dioxin levels in trout eggs would have been from toxicity levels measured in Lake Ontario herring gull eggs by the Canadian Environmental Protection Service.

For data before 1971, the study examined core samples taken from the lake's floor. Scientists found a link between the amount of toxins in core sediment and what would have been in water and, in turn, trout eggs.

Scientists in the lab then had to determine at what levels dioxin impedes trout reproduction.

Dioxins prove lethal for some lake trout larvae at levels as low as 30 parts per trillion, or one drop in 500,000 gallons, the study said. At 100 parts per trillion, dioxins kill all lake trout larvae. Even for the exacting science of toxicology, those amounts are especially minute.

The study found that Lake Ontario dioxin levels exceeded the 100 percent mortality level for more than 20 years. Based solely on dioxin levels, lake trout larvae had no chance of surviving from about 1950 through 1975.

Moreover, dioxin levels in Lake Ontario were high enough to prove lethal for at least some lake trout larvae from about 1940 through the mid-1980s, the study shows.

The study applies only to Lake Ontario, but its findings may help solve how lake trout disappeared in all of the Great Lakes.

The findings have been met with skepticism from those who have lived and fished around the lakes for years.

"It definitely could be one piece of the puzzle," said Mark Gadsen, spokesman for the Great Lakes Fishery Commission, which was created by the U.S. and Canadian governments in 1955 to curb the introduction and impact of invasive species and pollution on native Great Lakes fisheries.

Dan Thomas, president of the Great Lakes Sport Fishing Council, rejected the notion that dioxins are mostly to blame for the trout's disappearance.

"That's bull," he said. "This is typical of the EPA approach, because they only deal with one approach - the worst-case scenario," he said.

Thomas holds to the theory that commercial overfishing "made all of this possible."

Some see holes in the old theory that lampreys and overfishing killed the lake trout: Sea lampreys and commercial fishing came to the Great Lakes long before the trout's steep decline.

Lampreys are eel-like, parasitic creatures that attach themselves to fish, gnaw through the skin and rob the host of nutrients and bodily fluids. Lampreys first found their way into most of the Great Lakes after the Welland Canal opened in 1829, circumventing the natural barrier of Niagara Falls. That, combined with nearly a century of heavy commercial fishing on the lakes before the trout population crash, leads the EPA's Cook to believe that dioxins are mostly to blame.

Even though dioxins in Lake Ontario are now below lethal levels, scientists can't pinpoint a reason that lake trout fail to reproduce naturally there.

Lake Michigan dioxin levels weren't as high as in Lake Ontario, Cook said, but its lake trout faced added stresses from other causes.

"There are other problems in Lake Michigan," namely PCBs, he said. "But common sense tells you that if you are being stressed out by one thing and you get hit with another toxic chemical, you are that much more vulnerable."

Lake trout have returned to the Great Lakes, but only Lake Superior boasts a population that is self-sustaining without annual restocking. In fact, natural reproduction rates in Lake Superior are creeping back toward historical levels.

People should realize this isn't an intractable problem and "the lakes can recover if you remediate these sites," said Richard Peterson, a professor who teaches in the environmental toxicology program at the University of Wisconsin-Madison and who worked on the study. "Lake Superior is a great example of what the rest of the lakes can be."

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