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Deadly mix.
In lab studies, some combinations of pesticides were much more lethal than exposure to single compounds.

NOAA,/Doug Walker

Pesticide Brew Spells Trouble for Salmon

Salmon in the U.S. Pacific Northwest, and elsewhere, have been in a world of hurt for decades. One of their main enemies is agricultural chemicals, such as chlorpyrifos. The pesticide interferes with salmon brains and harms their ability to feed, according to studies by zoologist Nathaniel Scholz of the National Oceanic and Atmospheric Administration in Seattle, Washington. Now Scholz's research is showing that mixtures of pesticides are even worse for salmon and can be surprisingly lethal.

Chlorpyrifos and other so-called organophosphate pesticides kill cells by inhibiting acetylcholinesterase, an enzyme that helps neurons communicate. These pesticides are sprayed on crops and are widespread in streams in the Northwest; half of the waters sampled by the U.S. Geological Survey contain six or more pesticides. In their previous work with salmon, Scholz and his colleagues had only looked at the effects of one pesticide. To get a more realistic idea of exposure, they designed lab experiments to test effects of mixtures of chlorpyrifos and four other pesticides, exposing juvenile salmon to two compounds at a time.

At the highest concentrations, which exceeded natural conditions, all the various combinations of pesticides inhibited the activity of acetylcholinesterase by at least 50%--a level which impacts behavior. The two lower concentrations were more realistic, and at that level, a quarter of the combinations put a crimp on acetylcholinesterase. What's particularly important, Scholz says, is that the total impact was greater than the sum of the two pesticides, demonstrating a synergistic effect.

The biggest surprise was the strength of the synergistic punch from the pesticides diazinon and malathion, which killed all the salmon exposed to them. Even at the lowest concentration, fish were extremely sick, Scholz says. "It was eye-opening," Scholz says. "We're seeing relatively dramatic departures" from what happens with each pesticide by itself.

Scholz says the findings, which are in review for publication, mean that the U.S. Environmental Protection Agency may be underestimating the hazard pesticides pose to salmon. Given the hundreds of millions of dollars being spent to help salmon populations recover, it's crucial to have a good handle on the biggest threats, he adds.

"It's quite an advance that they were able to examine this in such detail," says toxicologist Derek Muir of Environment Canada's Aquatic Ecosystem Protection Research Branch in Burlington, Ontario. "It's quite significant work," he says. Because there is a good deal of information about where pesticides are sprayed, Muir continues, it may be possible to estimate the impact on wild populations. But factoring in all the other chemicals in streams will be difficult, he cautions.

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