Bigeye tuna: mercury

Levels measured in Japanese government surveys and studies

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.

How much to eat (official advice)

• The guidance applies to “women who are pregnant or may become pregnant” (MHLW, revised June 2010). Children, women who are breastfeeding, and adults other than pregnant women are not covered.
• Bigeye tuna (mebachi-maguro): up to once a week (about 80 g/week).
• One serving is calculated as about 80 g (one serving of sashimi, or one slice of fish). One piece of sushi or sashimi is about 15 g.
• If you eat two kinds of “up to once a week” fish in the same week, halve the amount of each. If you eat one “up to once a week” fish and one “up to twice a week” fish in the same week, also halve the amount of each.
• Even if you eat too much in one week, eating at one meal or over one week does not greatly change the mercury level in your body, and it is explained that you can simply eat less the following week.

Limits (Japan, international, EU)

Good to know

• Bigeye tuna was added to the guidance amounts in November 2005, together with Pacific bluefin and southern bluefin. One stated reason is that an analysis of the National Nutrition Survey found that many people eat 60–100 g of tuna at a time as sashimi, sushi, or tekkadon (tuna rice bowl). → How much fish is OK during pregnancy?
• The average (total mercury 0.832 ppm) exceeds the provisional regulatory limit of 0.4, but tuna-type fish are exempt from the limit. Instead there is a guidance amount for pregnant women.
• Japan says “up to once a week (about 80 g)” and the US says “avoid” — different advice for the same bigeye tuna. The difference in concentration is small: in a US survey the average total mercury was 0.689 ppm (21 samples), and in the Japanese compilation it was 0.832 ppm (113 samples). We have not found documents from either country explaining why the lines are drawn differently.
• The methylmercury in one serving (about 80 g) of bigeye tuna (average 0.539 ppm) is about 43 μg. That is about 39% 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).
• Individual variation is large in this fish: the maximum total mercury was 9.9 ppm (1 sample) and the maximum methylmercury was 2.3 ppm. In the MAFF survey (fiscal 2007–2010, 120 samples), the maximums were 2.3 ppm total mercury and 2.0 ppm methylmercury.
• There are also data on Japanese pregnant women. In the Ministry of the Environment’s Japan Environment and Children’s Study (JECS), the median methylmercury in cord blood from 3,822 women was 7.39 ng/mL, and no significant association was found with developmental tests at ages 2 and 4 (3,083 children). Among fish, “katsuo and maguro” (bonito and tuna) correlated most strongly with mercury in cord blood. The authors write that further research is needed before recommending a guideline for a safe intake.
• Cadmium is low in muscle: bigeye tuna was 0.02 (5 samples) in the MAFF survey. The fish survey values are from a compilation through fiscal 2008 and may differ from fish on the market today.

Related explainers

• How Much Fish Is OK During Pregnancy? Guidance for Tuna, Canned Tuna, and Kinmedai, with Mercury Numbers
• Why Do Heavy Metal Standards Differ Between Japan and Other Countries? Japanese, Codex, and EU Standards and Tolerable Intakes Compared

Pages for each metal

• Mercury

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)”
• US FDA/EPA, “Advice about Eating Fish” (October 2021)
• US FDA, “Mercury Levels in Commercial Fish and Shellfish (1990-2012)”
• Kuraoka et al. Sci Total Environ 2024 (JECS: cord blood methylmercury and development)（PMID 39528210）
• European Commission, Regulation (EU) 2022/617 (maximum levels for mercury in food)

Last checked 2026.10.03 (against the original government documents and papers, collected and cross-checked in Gemini Notebook).

Also in “Fish”

• Pacific bluefin tuna and southern bluefin tuna
• Yellowfin tuna, albacore, and young Pacific bluefin
• Canned tuna
• Skipjack tuna
• Swordfish and striped marlin
• Splendid alfonsino
• Whales and dolphins
• Salmon
• Mackerel
• Sardines, horse mackerel, and Pacific saury