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Study: Combining pesticides makes them more deadly
By JEFF BARNARD AP Environmental Writer
Posted: 03/02/2009 07:27:36 PM PST
GRANTS PASS, Ore.—Common agricultural pesticides that attack the
nervous systems of salmon can turn more deadly when they combine with
other pesticides, researchers have found.
Scientists from the NOAA Fisheries
Service and Washington State University were expecting that the
harmful effects would add up as they accumulated in the water. They
were surprised to find a deadly synergy occurred with some
combinations, which made the mix more harmful and at lower levels of
exposure than the sum of the parts.
The study looked at five common
pesticides: diazinon, malathion, chlorpyrifos, carbaryl and carbofuran,
all of which suppress an enzyme necessary for nerves to function
properly.
The findings suggest that the current
practice of testing pesticides—one at a time to see how much is needed
to kill a fish—fails to show the true risks, especially for fish
protected by the Endangered Species Act, the authors concluded in the
study published Monday in the journal Environmental Health
Perspectives.
"We need to design new research that
takes into effect the real-world situation where pesticides almost
always coincide with other pesticides," co-author Nathaniel Scholz, a
research zoologist at the NOAA Fisheries Service Northwest Fisheries
Science Center, said from Seattle.
Inge Werner, director of the aquatic
toxicology laboratory at the University of California at Davis, was
not involved in the study. She said while the idea was not new, the
findings were definitive, even at levels that don't kill fish
outright.
"We may not see the big fish kills out
there anymore like we used to," she said from Davis, Calif. "But the
subtle, sublethal effects that basically render them unfit for
survival in the wild are much more important. In certain areas,
pesticides really are a very important factor" in salmon survival.
Jeffrey Jenkins, professor of
environmental and molecular toxicology at Oregon State University, was
not part of the study. He said the study was well done, but it would
take more research to push the U.S. Environmental Protection Agency to
change its pesticide testing standards as they relate to fish, which
are defined by law.
Last year, NOAA Fisheries issued findings
under the Endangered Species Act that diazinon, malathion and
chlorpyrifos jeopardize the survival of all 28 species of Pacific
salmon listed as threatened or endangered in the West.
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The three chemicals, found by the
U.S. Geological survey to contaminate rivers throughout the
West, interfere with salmon's sense of smell, making it harder
to avoid predators, locate food and even find their native
spawning streams and reproduce. At higher concentrations, they
kill fish outright.
NOAA Fisheries and EPA must
evaluate 34 more pesticides by 2012 under terms of a settlement
reached in a lawsuit brought by Northwest Coalition for
Alternatives to Pesticides and others.
In the study, scientists combined
the pesticides two at a time at various concentrations, then
exposed juvenile coho salmon in tanks for four days. Many of the
fish died outright.
Fish that survived were killed, and
their brains analyzed for the levels of the enzyme
acetylcholinesterase, which allows impulses to move between
neurons in the brain. In every fish, the levels of the enzyme
were below the level considered healthy.
Earlier research found that lower
levels of the enzyme affected the ability of fish to feed and
swim, which would affect their ability to survive, Scholz said.
The researchers suggested that the
reason harmful affects of combinations of chemicals were greater
was that they also suppressed another enzyme, which helps the
body rid itself of toxins.
The amounts of the individual
pesticides were calculated to have a standard effect on the fish
nervous systems, and in some cases were higher than would be
expected to be seen in the environment, Scholz said. Some
combinations produced effects that added up to the sum of the
parts. But as the doses of the individual pesticides increased,
the effects became more synergistic—in effect multiplying rather
than just adding.
The results indicated that similar
effects would occur at much lower levels, and future research
will consider just how little exposure is needed to harm fish,
he added.
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