title: The synergistic toxicity of pesticide mixtures: implications for risk assessment and the conservation of endangered Pacific salmon.
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 anticholin-esterase insecticides on aquatic species have been studied for decades, the neurotoxicity of mixtures is still poorly understood.
  
  
  OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: NOAA (National Oceanic and Atmospheric Administration) Fisheries, Northwest Fisheries Science Center, Seattle, Washington 98112, USA.
authors.forename: Cathy A
authors.initials: CA
authors.lastname: Laetz
authors.forename: David H
authors.initials: DH
authors.lastname: Baldwin
authors.forename: Tracy K
authors.initials: TK
authors.lastname: Collier
authors.forename: Vincent
authors.initials: V
authors.lastname: Hebert
authors.forename: John D
authors.initials: JD
authors.lastname: Stark
authors.forename: Nathaniel L
authors.initials: NL
authors.lastname: Scholz