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Pesticides in Combination Shown to Increase Endangered Salmon Threat

(Beyond Pesticides, March 5, 2009) A new study published in the March 2009 issue of Environmental Health Perspectives finds that pesticide combinations cause more harm to endangered salmon than ndividual pesticide exposure. This means that single-pesticide risk assessments required by the Environmental Protection Agency (EPA) inadequately assess hazards.

Mixtures of organophosphate and carbamate pesticides are commonly detected in freshwater habitats that support threatened and endangered species of Pacific salmon. According to the researchers from the National Oceanic and Atmospheric Administration (NOAA) Fisheries and Washington State University, these pesticides inhibit the activity of acetylcholinesterase (AChE) and thus have potential to interfere with behaviors that may be essential for salmon survival.

The researchers measured brain AChE inhibition in juvenile coho salmon exposed to sublethal concentrations of the organophosphates diazinon, malathion, and chlorpyrifos, as well as the carbamates carbaryl and carbofuran. The pesticides were tested individually and in combination. They plotted AChE levels on a curve to determine whether the toxicologic responses to binary mixtures were additive, antagonistic (lesser than additive) effect, or synergistic (greater than additive).

The authors observed addition and synergism, with a greater degree of synergism at higher exposure concentrations. Several combinations of organophosphates were lethal at concentrations that were sublethal in single-chemical trials. Combinations of diazinon and malathion, as well as chlorpyrifos and malathion had the greatest impact (lowest AChE activity). Previous studies have shown that many fish species die after high rates of acute brain AChE inhibition.

The authors believe that their results have important implications for ecological risk assessments, particularly those that focus on the toxicity of individual chemicals as the basis for estimating impacts to imperiled aquatic species.

“Salmon exposed to mixtures containing some of the most intensively used insecticides in the western U.S. showed either concentration-additive or synergistic neurotoxicity as well as unpredicted mortality. This implies that single-chemical assessments will systematically underestimate actual risks to ESA-listed species in salmon-supporting watersheds where mixtures of OP and CB pesticides occur,” the authors stated in their conclusion. “Moreover, mixtures of pesticides that have been commonly reported in salmon habitats may pose a more important challenge for species recovery than previously anticipated.”

On November 18, 2008, the National Marine Fisheries Service (NMFS) released a Biological Opinion that sets forth a plan for protecting Pacific salmon and steelhead from three toxic organophosphate pesticides. The decision comes after almost a decade of legal wrangling between salmon advocates and the federal government.

In the biological opinion, federal wildlife scientists comprehensively reviewed the science regarding the impacts of pesticides on salmon and ultimately concluded that current uses of the insecticides chlorpyrifos, diazinon, and malathion jeopardize the existence of these imperiled fish. The biological opinion prescribes measures necessary to keep these pesticides out of water and to protect salmon populations in Washington, Oregon, California, and Idaho.

Learn more about pesticide and water contamination at Bridge to an Organic Future, the 27th National Pesticide Forum, April 3-4, in Carrboro, NC.


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