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'Pesticide Cocktails' Make a Deadly Synergy for Endangered Pacific Salmon by Staff Columbia Basin Bulletin, March 6, 2009

Mixtures of pesticides that have been commonly reported in freshwater salmon habitats may pose a more important challenge for species recovery than previously anticipated, according to a new study published in the March issue of "Environmental Health Perspectives."

"Salmon exposed to mixtures containing some of the most intensively used insecticides in the western United States showed either concentration-additive or synergistic neurotoxicity as well as unpredicted mortality," the research paper says.

"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." Organophosphate and carbamate pesticides are commonly used in the Columbia Basin and elsewhere.

The full article, "Synergistic Toxicity of Pesticide Mixtures: Implications for Risk Assessment and the Conservation of Endangered Pacific Salmon, can be found online at: www.ehponline.org/docs/2008/0800096/abstract.html

The researchers and collaborators are Cathy Laetz, David Baldwin, Tracy Collier of NOAA's Northwest Fishery Science Center, Vincent Hebert of Washington State University's Food and Environmental Quality Laboratory in Richland, Wash., John Stark of WSU's Department of Entomology, Ecotoxicology Program, and Nathaniel Scholz of the science center.

Studies have shown that more 90 percent of waterways affected by urban or agricultural land uses contain two or more pesticides, according to the research paper. It is also documented that the toxic effects of pesticide mixtures on an enzyme extracted from fish (in vitro) are additive. Yet little is known about the effects of pesticide mixtures when live fish are exposed to pesticide "cocktails," according to the new study.

In the research paper, NWFSC researchers and their collaborators evaluated the effects of five of the most extensively used pesticides in the Pacific Northwest and California --diazinon, malathion, chlorpyrifos, carbaryl, and carbofuran -- on the brains of juvenile coho salmon.

Nearly every pairing of the five pesticides inhibited activity of an important brain enzyme, acetylcholinesterase, after the salmon were exposed over a 96-hour period, the study says.

"This enzyme plays a critical role in several neurological pathways, and at reduced levels can diminish fish survival and even cause death," the study says.

The scientists said they expected that the harmful effects of pesticide combinations would simply add up as they accumulated.

"Surprisingly, for some combinations, they found that the toxic effects of reduced brain enzyme levels were much greater than predicted if each chemical was acting in isolation of the other," according to a study abstract posted on the center's web site, www.nwfsc.noaa.gov/.

"This deadly synergy made those particular pesticide combinations more harmful than the sum of their parts."

As the doses of individual pesticides increased, the synergistic effects were almost uniformly more pronounced. For some chemical combinations, the data suggest synergistic effects are possible at concentrations below the lowest levels used in the study.

"No salmon died when exposed to an individual pesticide, yet many died outright when exposed to combinations. The most deadly pairing was diazinon and malathion," according to the abstract.

The study shows that the in vivo toxicity of pesticide mixtures is additive and, in certain combinations, synergistic.

Testing individual pesticides to determine "safe" exposure levels may not capture the true risk to salmon, according to the research paper.

"Since synergistic effects occur with pesticide concentrations found in habitats supporting salmon stocks, which often include species designated as threatened or endangered, regulators may need to further consider multi-chemical effects when setting exposure standards," wrote the study authors.

"Although many salmon habitats are affected by agrochemicals and urban runoff, restoration priorities are usually developed without the specific inclusion of toxics in quantitative analyses of limiting factors," the research paper says.

"In the larger context of salmon conservation, a future priority will be to establish a quantitative connection between the mixture toxicity observed in this study and higher biological scales via effects on growth and survival."