Pre-anthropocene mercury residues in North American freshwater fish

Abstract

Mercury (Hg) has been entering the environment from both natural and anthropogenic sources for millennia, and humans have been influencing its environmental transport and fate from well before the Industrial Revolution. Exposure to Hg (as neurotoxic monomethylmercury [MeHg]) occurs primarily through consumption of finfish, shellfish, and marine mammals, and regulatory limits for MeHg concentrations in fish tissue have steadily decreased as information on its health impacts has become available. These facts prompted us to consider 2 questions: 1) What might the MeHg levels in fish tissue have been in the pre-Anthropocene, before significant human impacts on the environment? and 2) How would these pre-Anthropocene levels have compared with current regulatory criteria for MeHg residues in fish tissue? We addressed the first question by estimating pre-Anthropocene concentrations of MeHg in the tissues of prey and predatory fish with an integrated Hg speciation, transport, fate, and food web model (SERAFM), using estimated Hg concentrations in soil, sediment, and atmospheric deposition before the onset of significant human activity (i.e., ≤2000 BCE). Model results show MeHg residues in fish varying depending on the characteristics of the modeled water body, which suggests that Hg in fish tissue is best considered at the scale of individual watersheds or water bodies. We addressed the second question by comparing these model estimates with current regulatory criteria and found that MeHg residues in predatory (but not prey) fish could have approached or exceeded these criteria in some water bodies during the pre-Anthropocene. This suggests that the possibility of naturally occurring levels of Hg in fish below which it is not possible to descend, regardless of where those levels stand with respect to current regulatory limits. Risk management decisions made under these circumstances have the potential to be ineffectual, frustrating, and costly for decision makers and stakeholders alike, suggesting the need for regulatory flexibility when addressing the issue of Hg in fish.

Critical perspectives on mercury toxicity reference values for protection of fish

“…Taken together, the available information supports the suggestion by Hope and Louch [102] that the estimated pre-Anthropocene fish tissue mercury concentrations may serve as a reasonable, although approximate, floor level at or below which predictions of adverse effects attributable to mercury in fish are unrealistic. The prey tissue TRV of 0.04 mg/kg developed by Depew et al [16] is consistent with background concentrations of mercury in lower trophic level fish, which is expected, given that the TRV was set equal to a mercury concentration in prey from a control treatment.…”

“…The Beckvar et al [8] TRV of 0.2 mg/kg falls at the high end of present-day background concentrations of mercury in larger fish ( Table 5), but fish tissue mercury concentrations [129] comparable to this TRV apparently can occur naturally under conditions that favor mercury methylation, such as in dystrophic lakes or in watersheds with abundant forest and wetland cover [32,35,102]. Taken together with the uncertainties in the data underlying this TRV (see section Review of fish tissue TRVs), it would appear that the Beckvar et al [8] TRV is lower than necessary for protection of adult and early life stages of fish.…”

“…Inclusion of data from more recent museum specimens would risk overestimating natural background concentrations. Therefore, 3 other lines of evidence are considered in an effort to understand whether available TRVs are lower than background conditions: modeled natural background concentrations ; measured mercury concentrations in fish used as controls in toxicity tests ; and measured mercury concentrations in fish collected from remote areas , considered together with regional estimates of mercury deposition increases. Given the broad array of fish monitoring studies available on present‐day mercury concentrations, the studies supporting the second and third lines of evidence are intended to be representative, rather than exhaustive.…”

“…Hope and Louch modeled fish tissue concentrations of methylmercury under a variety of scenarios reflective of pre‐Anthropocene conditions, defined as natural background or the period prior to significant anthropogenic effects on the environment. This exercise used the USEPA's Spreadsheet‐Based Ecological Risk Assessment for the Fate of Mercury (SERAFM) model.…”

“…This exercise used the USEPA's Spreadsheet‐Based Ecological Risk Assessment for the Fate of Mercury (SERAFM) model. Hope and Louch reported that mean natural background mercury concentrations in whole prey fish were estimated to range from 0.03 mg/kg to 0.1 mg/kg, with the lowest concentrations estimated for rivers and the highest estimated for stratified dystrophic lakes. Median concentrations in whole prey fish were somewhat lower than the means, ranging from 0.005 mg/kg to 0.03 mg/kg.…”