Selenium:mercury molar ratios in fish from the Savannah River: Implications for risk management

The risk to humans from contaminants in fish is dealt with by issuing consumption advisories, which are devised from consumption patterns, contaminant levels, and toxicity of those contaminants. The primary contaminant of concern in fish is mercury, and recently it has been suggested that the protective effects of selenium on mercury toxicity should be considered in risk management. The relationships between mercury, fish species and fish size, and selenium:mercury (Se:Hg) molar ratios were examined for freshwater fish from the Savannah River. The data indicated: (1) significant variation in Se:Hg molar ratios among and within species, (2) mean selenium molar ratios for the 11 species were negatively correlated with mean mercury levels and mean fish length, (3) greater variation in individual variances within species than the variation in mean ratios among species, (4) some species with high mean ratios had individuals with ratios below 1, and (5) even the species with the highest mean ratios had some individuals close to a molar ratio of one. These data suggest that the great variability of the Se:Hg molar ratios within a species results in the ratios being unpredictable. That is, knowing the mean Se:Hg molar ratio for a given species does not predict the ratios of individuals, or thus the protectiveness of selenium on mercury toxicity.