Lethal and Sublethal Effects of Binary Mixtures of Cyanide and Hexavalent Chromium, Zinc, or Ammonia to the Fathead Minnow(Pimephales Promelas) And Rainbow Trout (Salmo Gairdneri)

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Abstract
Various models have been proposed to predict the combined interactive effect on fish of mixtures of poisons from separate toxicities of individual substances. The success of these models was tested, using data describing the lethal and sublethal effects of individual substances or binary mixtures of HCN and Cr(VI), Zn(II), or ammonia to the fathead minnow (Pimephales promelas) and rainbow trout (Salmo gairdneri). Using the strictly additive toxic unit and additive index approach, it was determined from log-dosage mortality curves that the Zn–HCN and ammonia–HCN mixtures were more acutely toxic and Cr–HCN less toxic than predicted. The concentration and response addition models, which have been proposed for toxicants whose joint action is similar or independent, respectively, could not be used to predict dosage–mortality curves for the HCN mixtures. Linear regression lines representing the growth response of fish to log concentration for toxicants alone and in binary combinations were not significantly different. Thus, for the toxic substances tested, the sublethal joint action of individual toxicants was not predictable from existing models and, in most cases, no interaction was indicated. The interactive nature of toxicants may be a function of the concentrations tested causing different biological processes to be affected (e.g. mortality vs. growth), and therefore different responses to be measured. A need still exists for development of a valid multiple toxicity approach for evaluating and predicting the toxicity of chemical combinations. Key words: multiple toxicity, binary mixtures, joint action, fish, bioassay, toxic substances