Critical perspectives on mercury toxicity reference values for protection of fish

Abstract

Environmental management decisions at mercury-contaminated sediment sites are predicated on the understanding of risks to various receptors, including fish. Toxicity reference values (TRVs) for interpreting risks to fish have been developed to assess mercury concentrations in fish or fish prey. These TRVs were systematically evaluated based on several lines of evidence. First, their conceptual basis and specific derivation were evaluated, including a close review of underlying toxicity studies. Second, case studies were reviewed to investigate whether TRVs are predictive of effects on fish populations in the field. Third, TRVs were compared with available information regarding preindustrial and present-day background concentrations of mercury in fish. The findings show that existing TRVs are highly uncertain, because they were developed using limited data from studies not designed for TRV derivation. Although field studies also entail uncertainty, several case studies indicate no evidence of adverse effects despite mercury exposures that exceed the available TRVs. Some TRVs also fall within the range of background mercury concentrations in predatory or prey fish. Lack of information on the selenium status of mercury-exposed fish is a critical confounding factor, and the form of methylmercury used in toxicity testing may also contribute to differences between TRV-based predictions and field observations of mercury effects on fish. On balance, the available information indicates that several of the TRVs reviewed are lower than necessary to protect fish populations. The 20% effect concentration from a previously published dose-response analysis appears closer to an effect threshold, based on available laboratory data. Additional research is needed to provide a stronger basis to establish dose-response relationships for mercury effects on fish.

Toxicity reference values for methylmercury effects on avian reproduction: Critical review and analysis

“…Sulfur deposition is relevant because it contributes to increases in Hg methylation and subsequent bioaccumulation [115]. These lines of evidence indicated that average naturally occurring Hg concentrations in forage fish are roughly 0.03 mg/kg to 0.1 mg/kg, with greater concentrations (on the order of 0.1-0.3 mg/kg) expected in predatory fish [155]. By this estimate, all of the previously published diet-based avian TRVs for Hg are similar to or lower than naturally occurring Hg concentrations in fish.…”

“…Because Hg is naturally occurring (e.g., in volcanic eruptions, natural seeps, geological deposits) and bioaccumulative, there must be a lower limit to the Hg concentrations in prey that could plausibly harm bird populations; that is, TRVs should not be lower than naturally occurring Hg concentrations in the normal prey of the bird species being assessed. However, as reviewed by Fuchsman et al , it is challenging to define naturally occurring Hg concentrations in fish and other avian prey, because Hg released by humans to the environment over centuries has become globally distributed. Fuchsman et al evaluated 3 lines of evidence relevant to estimating natural background Hg concentrations in prey fish: a modeling exercise by Hope and Louch , Hg concentrations in control fish from toxicity experiments , and Hg concentrations in fish collected from areas minimally impacted by anthropogenic increases in aerial deposition of Hg and sulfur .…”

“…However, as reviewed by Fuchsman et al , it is challenging to define naturally occurring Hg concentrations in fish and other avian prey, because Hg released by humans to the environment over centuries has become globally distributed. Fuchsman et al evaluated 3 lines of evidence relevant to estimating natural background Hg concentrations in prey fish: a modeling exercise by Hope and Louch , Hg concentrations in control fish from toxicity experiments , and Hg concentrations in fish collected from areas minimally impacted by anthropogenic increases in aerial deposition of Hg and sulfur . Sulfur deposition is relevant because it contributes to increases in Hg methylation and subsequent bioaccumulation .…”

Learned Discourses: Timely Scientific Opinions

“…Beckvar et al (2005) discuss various approaches to linking whole-body fish tissue residues to potential biological effects (relative to Hg and DDT) and weigh the technical advantages of each. Fuchsman et al (2016) reviewed the various proposed toxicity reference values for Hg (based on adverse effects to fish) and found that many of these represent ambient or "background" concentrations. Thus, selecting LOAEL values as toxicity benchmarks for environmental receptors involves uncertainty.…”

Use real‐world data to inform the National Coastal and Great Lakes condition

“…Beckvar et al () discuss various approaches to linking whole‐body fish tissue residues to potential biological effects (relative to Hg and DDT) and weigh the technical advantages of each. Fuchsman et al () reviewed the various proposed toxicity reference values for Hg (based on adverse effects to fish) and found that many of these represent ambient or “background” concentrations. Thus, selecting LOAEL values as toxicity benchmarks for environmental receptors involves uncertainty.…”