Surveillance of Total Mercury and Methylmercury Concentrations in Retail Fish

Overview

This Japanese surveillance study measured total mercury and methylmercury in retail fish samples across 19 fish species.

Key numbers

Sample frame: 210 fresh fish products, 19 species organized into 21 groups (bluefin tuna and yellowtail split into natural vs farm-raised; yellowtail also has a young/inada group), n=10 per group. Purchased at retail in Tokyo and Kanagawa Prefecture, 2014–2015. Muscle tissue analysed.

Table 2 — mean / median / minimum / maximum, mg/kg wet weight, n=10 per group:

Tr = trace level (below LOQ); ND = not detected (below LOD).

Provisional MRL compliance (140 samples subject to Kannyu Notification #99 limits of 0.4 mg/kg tHg and 0.3 mg/kg MeHg): 6 natural-yellowtail + 1 flounder + 1 sea bream exceeded the tHg ceiling; 8 natural-yellowtail + 2 sea bream + 1 flounder + 1 amberjack exceeded the MeHg ceiling. Tuna species, deep-sea fish, and inland-river fish are excluded from the provisional MRL.

Correlation between tHg and methylmercury concentrations: r = 0.99 for both PMRL-subject and PMRL-exempt species. Linear regression slope 0.846 for PMRL-subject (y = 0.8456x − 0.0056, R² = 0.987); slope 0.869 for PMRL-exempt (y = 0.8691x − 0.0425, R² = 0.981). For samples ≥ 0.1 mg/kg tHg, the MeHg-to-tHg ratio (after molecular-weight conversion) sits between 0.6 and 0.9; the authors propose tHg < 0.3 mg/kg as a screening threshold that guarantees MeHg < 0.3 mg/kg without need for the more elaborate MeHg assay.

Farm-raised vs natural: in both bluefin tuna and yellowtail, farm-raised samples had significantly lower tHg and MeHg than wild-caught samples (p < 0.05); for yellowtail the difference was roughly four-fold on the mean.

Methods

Total mercury: heated-vaporization atomic absorption with the MA-2000 instrument (Nippon Instruments). Sample weight 50 mg or 100 mg, calibrated 1–20 ng/mL against the L-cysteine-diluted mercury standard solution. Reference material CRM7402-a (cod muscle, AIST/NMIJ; certified 0.61 ± 0.02 mg/kg tHg). Method performance: trueness 94%, repeatability 1.1–2.3%, intra-laboratory reproducibility 1.3–2.3%, LOQ 0.002 mg/kg, LOD 0.0007 mg/kg (at 100 mg sample weight).

Methylmercury: GC-MS after phenyl derivatization (Watanabe et al. 2015 method; 6890N GC and 5975 MSD, Agilent). Performance evaluated against five fish-muscle CRMs (certified 0.58–5.50 mg/kg) and two spike-recovery panels (0.3 mg/kg): trueness 85–98%, repeatability 0.8–6.7%, intra-laboratory reproducibility 1.6–8.1%, LOQ 0.01 mg/kg, LOD 0.003 mg/kg.

Sample preparation: muscle tissue separated from head, viscera, skin, and bone (except sashimi-grade products taken as purchased), homogenized in a Retsch GM200, centrifuged in a Kubota model 6200.

Implications

This is the most detailed Japanese retail-fish surveillance pairing total mercury and methylmercury at the species-group level in the corpus. Swordfish stands out as the highest-mercury group on every distributional moment (mean, median, minimum, maximum); large predatory tunas occupy the next tier. Farm-raised bluefin tuna and yellowtail show meaningfully lower mercury than their wild-caught counterparts, which is relevant to product-form distinctions for aquaculture-dominant supply. The 0.3 mg/kg tHg screening threshold proposed by the authors is a Japanese-context inspection efficiency device, not a regulatory limit; the underlying Japanese provisional MRLs remain 0.4 mg/kg tHg and 0.3 mg/kg MeHg for non-exempt species.

Related evidence

• Swordfish
• Fish
• Seafood
• Mercury, Total
• Methylmercury

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