| The Synergistic Toxicity of Pesticide Mixtures: Implications for Risk Assessment and the Conservation of Endangered Pacific Salmon Cathy A. Laetz,1 David H. Baldwin,1 Tracy K. Collier,1 Vincent Hebert,2 John D. Stark,3 and Nathaniel L. Scholz1 1NOAA (National Oceanic and Atmospheric Administration) Fisheries, Northwest Fisheries Science Center, Seattle, Washington, USA; 2Food and Environmental Quality Laboratory, Washington State University, Richland, Washington, USA; 3Department of Entomology, Ecotoxicology Program, Washington State University, Puyallup, Washington, USA Abstract
Background: Mixtures of organophosphate and carbamate pesticides are commonly detected in freshwater habitats that support threatened and endangered species of Pacific salmon (Oncorhynchus sp.) . These pesticides inhibit the activity of acetylcholinesterase (AChE) and thus have potential to interfere with behaviors that may be essential for salmon survival. Although the effects of individual anticholinesterase insecticides on aquatic species have been studied for decades, the neurotoxicity of mixtures is still poorly understood. Objectives: We assessed whether chemicals in a mixture act in isolation (resulting in additive AChE inhibition) or whether components interact to produce either antagonistic or synergistic toxicity. Methods: We measured brain AChE inhibition in juvenile coho salmon (Oncorhynchus kisutch) exposed to sublethal concentrations of the organophosphates diazinon, malathion, and chlorpyrifos, as well as the carbamates carbaryl and carbofuran. Concentrations of individual chemicals were normalized to their respective median effective concentrations (EC50) and collectively fit to a nonlinear regression. We used this curve to determine whether toxicologic responses to binary mixtures were additive, antagonistic, or synergistic. Results: We 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. Conclusion: Single-chemical risk assessments are likely to underestimate the impacts of these insecticides on salmon in river systems where mixtures occur. Moreover, mixtures of pesticides that have been commonly reported in salmon habitats may pose a more important challenge for species recovery than previously anticipated. Key words: acetylcholinesterase, carbamates, conservation, organophosphates, pesticides, risk assessment, salmon, synergy, toxicity. Environ Health Perspect 117:348–353 (2009) . doi:10.1289/ehp.0800096 available via http://dx.doi.org/ [Online 14 November 2008]
Address correspondence to N.L. Scholz, NOAA Fisheries, Northwest Fisheries Science Center, 2725 Montlake Blvd. E., Seattle, WA 98112 USA. Telephone: (206) 860-3454. Fax: (206) 860-3335. E-mail: nathaniel.scholz@noaa.gov We thank G. Jack, E. Culbert, J. LePage, B. French, J. Labenia, T. Linbo, and M. Boroja for their valuable assistance. We thank S. Hecht, T. Hawkes, and E. Gallagher for their critical review of this manuscript. The National Oceanic and Atmospheric Administration Office of Protected Resources and Coastal Storms Program provided financial support for this research. The authors declare they have no competing financial interests. Received 14 August 2008 ; accepted 13 November 2008.
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