Heavy metals in food databaseFishChecked 2026.10.03
Skipjack tuna: mercury
Skipjack (katsuo) is a relative of tuna, but it is not in the MHLW’s table of guidance amounts for pregnant women. In the national compilation, average total mercury was 0.154 ppm (79 samples), and in the MAFF survey (120 samples), average methylmercury was 0.13 ppm. We found no Japanese official survey figures for arsenic.
Levels measured in Japanese government surveys and studies
| Metal | Form / state | Mean | Range / max | Samples | Survey |
|---|---|---|---|---|---|
| Mercury | Total mercury | 0.154 | 0.020–0.390 | 79 samples | MHLW, compiled through fiscal 2008 |
| Mercury | Total mercury | 0.15 (median 0.16) | <0.01–0.39 | 120 samples | MAFF, fiscal 2007–2010 |
| Mercury | Methylmercury | 0.13 (median 0.14) | <0.01–0.35 | 120 samples (4 below the limit of quantification) | MAFF, fiscal 2007–2010 |
Units are mg/kg (= ppm). Values are for the edible portion of fish on the market and appear to be wet weight (the sources do not say). Total mercury and methylmercury were not necessarily measured in the same samples. The MAFF rows (fiscal 2007–2010) give the median next to the mean. The MHLW compilation has no methylmercury value for skipjack.
Limits (Japan, international, EU)
| Set by | Limit | Note |
|---|---|---|
| Japan (mercury) | Total mercury 0.4 ppm, methylmercury 0.3 ppm (provisional regulatory limit for fish and shellfish) | Skipjack counts as a tuna-type fish and is exempt. We found no official document explaining the reason for the exemption. |
| Codex (mercury) | Methylmercury 0.5 mg/kg (guideline level for fish). For predatory fish such as shark, swordfish, and tuna, 1 mg/kg | A 1991 guideline of the Codex Alimentarius Commission. |
| EU (mercury) | 1.0 mg/kg (muscle of tuna, skipjack, shark, swordfish, and others) | The value from Regulation 2022/617, carried over into Regulation (EU) 2023/915. Wet weight. |
| US FDA/EPA advice (2021) | Canned light tuna (including skipjack): Best Choices (2–3 times a week) | Advice for people who are pregnant or breastfeeding and for children aged 1 to 11. One serving is about 113 g (4 ounces). |
Good to know
- Skipjack is not listed in the national table of guidance amounts. For fish and shellfish not in the table, the MHLW explains that most have low mercury levels and are not at a level that would affect the health of pregnant women who eat them. → How much fish is OK during pregnancy?
- The mercury in one serving (about 80 g) is about 12 μg when calculated from the average total mercury of 0.154 ppm (79 samples), and about 10 μg from the MAFF average methylmercury of 0.13 ppm (120 samples). That is about 11% and 9% of the FSCJ’s tolerable weekly intake (2.0 μg/kg body weight per week; 111 μg per week for a 55.5 kg person) (Urayomi’s calculation).
- We found no Japanese official survey figures for arsenic. Total arsenic in fish and shellfish is high, but most of it is organic arsenic such as arsenobetaine, and the FSCJ summarizes inorganic arsenic in fish muscle as 0–4% of total arsenic. However, knowledge of the toxicity of organic arsenic is limited, and more data are said to be needed. → Arsenic page
- In data on Japanese pregnant women, among the 3,822 cord blood samples of the JECS, “katsuo and maguro” (bonito and tuna) correlated most strongly with mercury among fish. No significant association was found with developmental tests at ages 2 and 4.
- Cadmium is low in muscle: skipjack was 0.01 (maximum 0.04, 15 samples) in the MAFF survey. By contrast, shiokara made from skipjack (salted and fermented; 10 commercial products, fiscal 2002) averaged 0.64 (maximum 1.1), higher than the muscle.
- In a questionnaire across 14 regions of Japan, 20.4% of people said they eat skipjack “often” (salmon 62.0%, tuna 47.9%). The fish survey values are from a compilation through fiscal 2008 and may differ from fish on the market today.
Related explainers
Pages for each metal
Sources
- MHLW, “Notes on Eating Fish and Shellfish and Mercury for Pregnant Women” (revised June 2010)
- MHLW, “Notes on Eating Fish and Shellfish and Mercury for Pregnant Women,” Q&A
- MHLW, Document 2-4, “Survey Results on Mercury in Fish and Shellfish (Summary)” (May 2010)
- MAFF, “Data Collection of Survey Results on Hazardous Chemicals in Food (Fiscal 2003–2010)”
- MAFF, “Provisional Regulatory Limits for Mercury in Fish and Shellfish”
- MAFF, “Survey Data on Cadmium in Domestic Agricultural, Livestock, and Fishery Products: Fish and Shellfish” (fiscal 1997–2002)
- Kuraoka et al. Sci Total Environ 2024 (JECS: cord blood methylmercury and development)（PMID 39528210）
- US FDA/EPA, “Advice about Eating Fish” (October 2021)
Last checked 2026.10.03 (against the original government documents and papers, collected and cross-checked in Gemini Notebook).
Heavy metals occur naturally in soil and the sea, and every food contains small amounts. This page lists facts from government surveys, limits and assessments and from published papers; it does not judge any food as good or bad. Where there is official advice — such as on eating fish during pregnancy — follow it.