Whales and dolphins: mercury

Levels measured in Japanese government surveys and studies

Units are mg/kg (= ppm). Values are for the edible portion of products 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. For methylmercury, some species have only 4 to 5 samples.

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.
• Bottlenose dolphin: up to once every two months (about 10 g/week).
• Short-finned pilot whale: up to once every two weeks (about 40 g/week).
• Baird’s beaked whale and sperm whale: up to once a week (about 80 g/week).
• Dall’s porpoise: up to twice a week (about 160 g/week).
• Baleen whales such as the minke whale and Bryde’s whale are not in the table of guidance amounts (their mercury levels are said not to be high).
• 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

• The magnitudes are different. The methylmercury in one serving (about 80 g) of bottlenose dolphin is about 530 μg, about 4.8 times (477%) the weekly tolerable amount (111 μg per week for a 55.5 kg person). Short-finned pilot whale is about 119 μg, about 107%; Baird’s beaked whale and sperm whale about 56 μg, about 50%; and Dall’s porpoise about 30 μg, about 27% (Urayomi’s calculation).
• For many species the sample count is small, so the averages are not stable: bottlenose dolphin 5 samples, short-finned pilot whale 4, Baird’s beaked whale 5, Dall’s porpoise 4. We cannot say “this species is dangerous” or “safe” from the average alone. Some have many samples, such as the minke whale (874 for total mercury).
• Toothed whales and baleen whales differ. The MHLW explains that some toothed whales (Dall’s porpoise, bottlenose dolphin, Baird’s beaked whale, short-finned pilot whale, and sperm whale) have high mercury levels, and that mercury levels in baleen whales (minke whale and others) are not high.
• In a survey of Taiji, Wakayama Prefecture (summer 2009), the geometric mean hair mercury of 1,017 residents was 11.0 ppm for men and 6.63 ppm for women, markedly higher than in 14 regions of Japan (men 2.47, women 1.64), and a link to eating whales and dolphins was suggested. 32 people (3.1%) were above 50 ppm. The survey said that “no one suspected of possible methylmercury poisoning was found,” but because some people had very high hair mercury, continuing the survey was considered. It is a cross-sectional survey, so cause and long-term effects cannot be established.
• Epidemiological studies are split. In the Faroe Islands (pilot whale source; 1,022 births, 917 children examined at age 7), prenatal exposure was associated with lower language, attention, and memory scores. In the Seychelles (779 mother-child pairs, mothers ate fish an average of 12 times a week, age 9), the study did not support a neurodevelopmental risk. The FSCJ calculated its tolerable intake from both, and replied that it could not evaluate the effect of other components such as nutrients in fish.
• The MHLW calls on everyone, not only pregnant women, to avoid eating lopsided amounts of fish and shellfish particularly high in mercury, such as whales and dolphins, and to keep a well-balanced diet. In a questionnaire across 14 regions of Japan, 0.7% of people said they eat whale “often.”

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)
• Ministry of the Environment, National Institute for Minamata Disease, “Survey on Mercury and the Health of Residents in Taiji Town,” fiscal 2009 report (MHLW Document 2-2)
• FSCJ, Evaluation of the tolerable weekly intake of methylmercury (August 2005), document posted by the MHLW
• Grandjean P et al. Neurotoxicol Teratol 1997;19:417-428 (Faroe Islands)（PMID 9392777）
• Myers GJ et al. Lancet 2003;361:1686-1692 (Seychelles)（PMID 12767734）

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