Scientists point to pollution as cause of lake
By Christopher Steiner
Knight Ridder Newspaper
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
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 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
"The mortality rate was 100 percent," he said.
"It wouldn't even matter if there were sea lampreys
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
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
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
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
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
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
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