Li et al. 2024 — Global fishing patterns and methylmercury exposure (PNAS)
This PNAS modeling study combined high-resolution global fisheries catch data for 1,774 species with an empirically constrained spatial model of seafood MeHg concentrations to estimate the mass and geographic distribution of MeHg extracted from the ocean. Catch-weighted MeHg concentrations varied over two orders of magnitude (range: 0.004–1.7 µg/g wet weight; geometric mean: 0.13 µg/g wet weight) and were highest in tropical and subtropical regions where large pelagic species (tuna, swordfish, sharks) are harvested. Tropical and subtropical fisheries account for over 70% of MeHg extracted from the ocean, driven by the combination of large harvest biomass of high-trophic-level species and higher bioavailable Hg at the base of tropical food webs. The study estimates that 84–99% of subsistence fishing entities globally likely exceed MeHg exposure thresholds based on typical subsistence fish consumption rates.
Key numbers
Central estimate of total MeHg extracted from the ocean annually: 6.1 Mg (IQR: 4.2–9.5 Mg), representing approximately 0.05% of the upper 1,000 m seawater MeHg reservoir.
- Catch-weighted MeHg concentrations by region: range 0.004–1.7 µg/g wet weight; geometric mean 0.13 µg/g wet weight.
- Large pelagic species (tuna, swordfish, billfish, sharks) contribute approximately 36% (IQR: 36–38%) of total MeHg fished and approximately 60% of fisheries biomass in the highest-MeHg regions.
- Spatial model validation on yellowfin tuna dataset (n=1,482 measurements across 24 marine regions): R² = 0.80; modeled values within empirical concentration range for common market sizes (5–20 kg fish).
- Subpolar and polar EEZs are the only coastal regions where catch-weighted omega-3:MeHg de minimus ratios are met, meaning most global fisheries provide insufficient omega-3 fatty acids to nutritionally offset MeHg risks under US EPA RfD (0.1 µg/kg BW/day).
- 57% (IQR: 46–71%) of global EEZs show Se:Hg molar ratios > 1; in tropical EEZs, only 30% (IQR: 24–59%) do.
Methods (brief)
Spatially resolved catch data: Sea Around Us database, 2001–2010, 0.5° × 0.5° horizontal resolution. Empirical MeHg synthesis from prior literature (n not stated; synthesis of all available species-level measurements). Spatial scaling of MeHg using foraging territory data and modeled seawater MeHg. Total Hg not reported; MeHg is the reported metric throughout. Wet weight basis. Global EEZ-level aggregation for micronutrient comparisons.
Implications
Certification: Establishes geographic sourcing context for MeHg risk across global fisheries; tropical and subtropical industrial fisheries for large pelagic species carry the highest MeHg loads. Relevant to HMT&C fish/seafood category guidance and sourcing geography flags.
Courses: Provides the best available global-scale quantitative framework for communicating MeHg in commercial seafood by species and harvest region.
App: Ingredient-level MeHg risk flags for marine fish should weight by catch origin latitude and species trophic level; this study provides the quantitative spatial basis.
Microbiome: MeHg from fish consumption is a primary human exposure pathway with relevance to gut-mercury-microbiome interactions documented on gut-mercury-axis.