Eagles-Smith et al. 2016 — Fish Hg spatial and temporal patterns across Western US + Canada (USGS broad synthesis)
This USGS-led broad synthesis compiles total mercury (THg) concentrations in 96,310 individual fish from 4,262 unique locations across 15 U.S. states, 3 Canadian provinces, and 2 Canadian territories — covering 206 species sampled 1969-2014. It is the largest freshwater-fish-Hg dataset in the Western North American literature. Across all fish, total THg in muscle ranges from 0.001 to 28.4 µg/g wet weight with a geometric mean of 0.17 µg/g. Crucially:
- 30% of individual fish samples exceed the EPA Fish Tissue Residue Criterion for methylmercury (0.30 µg/g ww), the threshold designed to protect noncommercial-fish-consuming humans
- 17% of site-level mean concentrations exceed the same criterion
- 3.9% of fish samples exceed the FDA action level of 1.0 µg/g ww for commercial seafood
- 34% of whole-body samples exceed 0.20 µg/g ww, an impairment threshold associated with potential effects on fish themselves
The paper develops a relativized THg-estimation framework that accounts for fish-species, fish-size, habitat, and ecoregion variability so that comparisons across watersheds and ecoregions are robust. Riverine habitats are consistently higher than lacustrine; semi-arid and arid regions (Great Basin, Desert Southwest) higher than temperate. Fish THg concentrations are not correlated with sediment THg concentrations at the watershed scale, indicating that factors influencing MeHg production (microbial methylation activity, sulfate, dissolved organic carbon, redox) are more important than inorganic-Hg loading for predicting fish-MeHg exposure.
Key numbers
Dataset scope:
- 96,310 individual fish records
- 4,262 unique sites
- 206 species (sampled 1969-2014)
- 15 U.S. states (Alaska, Yukon Territories, Northwest Territories, British Columbia, Washington, Oregon, Idaho, California, Nevada, Arizona, New Mexico, Colorado, Utah, Wyoming, Montana) + Saskatchewan, North Dakota, South Dakota (Great Plains expansion)
- 3 Canadian provinces + 2 territories
- Habitats: lakes, ponds, reservoirs; rivers, streams; wetlands; canals
- Foraging guilds: piscivore, generalist invertivore, omnivore, benthivore, planktivore
THg distribution:
- Range: 0.001 to 28.4 µg/g ww (5 orders of magnitude)
- Geometric mean (all fish, all sites): 0.170 ± 0.001 µg/g ww (very narrow SE on n=96K)
- Site-specific geometric mean range: 0.006 to 2.98 µg/g ww (average across sites 0.124 ± 0.002)
Exceedance proportions:
| Threshold | Source | % of individual fish exceeding | % of site means exceeding |
|---|---|---|---|
| EPA Fish Tissue Residue Criterion (0.30 µg/g ww MeHg, noncommercial human consumption) | EPA Borum et al. 2001 | 30% | 17% |
| FDA action level (1.0 µg/g ww commercial seafood) | FDA | 3.9% | — |
| Whole-body fish impairment threshold (0.20 µg/g ww) | Beckvar et al. 2005 | 34% | 20% |
Variability drivers (per the mixed-effects linear models):
- Habitat type (lake vs river vs stream): significant, riverine > lacustrine
- Fish species: highly significant (piscivores have higher THg via biomagnification)
- Fish size: positive correlation within species (THg accumulates with age/size)
- Site within ecoregion: significant (site-level Hg cycling chemistry varies)
- Year: significant temporal trends (variable direction across ecoregions)
- Ecoregion (Level 1, 10 ecoregions): largest single source of variation
- Sediment Hg concentration: not significantly correlated with fish Hg at the watershed scale
Methods
Database compilation: Original THg fish-tissue data assembled from USGS, state, federal, and Canadian monitoring databases. Tissue types standardized to skinless boneless fillet (76.8% of original data) using literature-derived conversion factors. Wet-weight basis (dry→wet conversion using moisture-content data where available, default 76% moisture).
Length standardization: THg concentrations within species standardized to the species’ median length using linear mixed-effects models with fork length as a fixed effect and species × site as random effects (Eagles-Smith and Ackerman 2014 method). For species with sparse data, similar-species groupings used.
Statistical models: Tiered linear mixed-effects models (4 levels). Initial descriptive (site-specific geometric means), then size-standardized comparisons across habitats and foraging guilds, then watershed-scale relativized THg estimates accounting for species/length/habitat. Final tier compared ecoregions. Getis-Ord G* hotspot analysis (ArcGIS v10) identified spatial clusters of high/low Hg.
Detection limits: Analytical detection limits varied considerably across the merged datasets (0.001-0.1 µg/g ww). Only 91/96,310 values (0.09%) were below reported DL; these were included as reported.
Speciation: THg in fish muscle is ≥95% MeHg per established literature (Bloom 1992). The paper treats THg as a proxy for MeHg, consistent with EPA’s Fish Tissue Residue Criterion approach. The paper acknowledges that this proxy is not appropriate when applied to non-muscle tissues (whole body, liver, gut), where the MeHg/THg ratio is lower.
Implications
Certification: For HMTc Cat 1 fish-containing baby foods row + the broader fish-and-seafood row family:
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The Western US/Canada freshwater-fish dataset is the largest single Hg-in-fish-tissue evidence base ingested into the wiki to date. The 30% / 17% / 3.9% exceedance fractions are key reference points for HMTc threshold-setting on the fish row, with the explicit caveat that the FDA 1.0 µg/g action level is for COMMERCIAL seafood and the EPA 0.30 µg/g criterion is the relevant standard for individual-consumption health protection.
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MeHg ≈ THg in fish muscle is the working assumption for HMTc threshold-derivation, consistent with EPA practice. Sources that report whole-body or visceral THg cannot be used as MeHg proxies at the same conversion factor; the methodology supplement (per CLAUDE.md Part 14 speciation rule) should flag this.
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The lack of correlation between sediment Hg and fish Hg at the watershed scale is methodologically important: HMTc certification cannot use sediment-Hg measurements as a supplier-screening proxy for fish-product Hg. Supplier audits must measure fish-tissue directly.
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Ecoregion effect dominates: HMTc Cat 1 fish thresholds for products sourced from semi-arid/arid Western regions (Great Basin, Desert SW) should anticipate higher tail-of-distribution Hg than products from temperate Pacific Northwest sources. The synthesis paper explicitly demonstrates this ecoregion-level heterogeneity.
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Piscivore species (e.g., largemouth bass, walleye, lake trout) carry the highest fish-Hg burdens via biomagnification; the lower trophic-level species (e.g., bottom feeders, planktivores) carry lower burdens. HMTc Cat 1 fish-containing baby food should preferentially source from lower-trophic-level fish to control MeHg exposure.
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Children’s exposure context: while the dataset is freshwater (sport-caught, subsistence), commercial fish baby food often uses marine species. The Western US ecoregion findings extrapolate cautiously: marine fish food chains have higher overall MeHg loads because of methylation in deep-ocean waters and species-specific bioaccumulation. The Cat 2 master page (children-personal-care isn’t the right place — fish-containing baby foods Cat 1 row is) should cross-reference this dataset.
Courses: Excellent case study for spatial epidemiology + mixed-effects modeling. The 96,310-record database + 10-ecoregion stratification is the canonical worked example.
App: For the consumer app’s per-meal MeHg risk scoring, this dataset establishes that 30% of randomly-sampled Western US freshwater fish meals exceed the EPA criterion. The app’s per-species + per-region risk weighting should reflect that ratio.
Microbiome: Not addressed. Coe 2023 (gut microbiome MeHg demethylation) is a complementary reference.
Wiki pages updated on ingest
- mercury-total
- mercury-methyl
- fish
- freshwater-fish
- fish-containing-baby-foods
- epa-fish-tissue-residue-criterion-mehg-03 (to be created)
- fda-seafood-mercury-action-level-1ppm (to be created)