Table of contents
- What is methylmercury and how does it get into tuna
- Why tuna accumulates more than other fish
- Mercury by tuna species table
- AESAN recommendations
- Pregnancy and children: special rules
- Canned vs. fresh tuna: is there a difference?
- How to reduce risk without stopping tuna consumption
- Frequently asked questions
What is methylmercury and why is it in fish?
Mercury is a heavy metal that exists naturally in the Earth's crust. Volcanic eruptions release it, but the main current source is human activity: coal-fired power plants, mining, and the chemical industry. This mercury reaches the oceans through rain and rivers.
In water, bacteria transform inorganic mercury into methylmercury (CH₃Hg⁺) — an organic form that is easily absorbed by living organisms and is neurotoxic. Phytoplankton absorb it, small fish eat the phytoplankton, medium fish eat the small fish, and large predators eat them all.
At each link in the food chain, the concentration of methylmercury multiplies. This is called bioamplification. A 200 kg bluefin tuna that has been eating other fish for 15 years has accumulated methylmercury throughout its life — because the body absorbs it much faster than it eliminates it. The half-life of methylmercury in the human body is 70-80 days.
Why tuna accumulates more mercury than other fish
Three factors explain this:
- Position in the food chain: Tuna is a super-predator — it eats medium-sized fish that have already accumulated mercury from small fish. Each link multiplies the concentration by 3-10 times.
- Longevity: A bluefin tuna can live 40+ years. More years = more accumulation. A sardine lives 5-7 years — not enough time to accumulate significant amounts.
- Size: The larger the size, the greater the bioaccumulation. A 300 kg bluefin tuna has more mercury per kilo than a 50 kg one.
Not all tuna are the same. Skipjack (the smallest and youngest when caught) has much less mercury than bluefin tuna (the largest and longest-lived).
Mercury by species table
| Species | Scientific name | Average mercury (ppm) | Risk level |
|---|---|---|---|
| Sardine | Sardina pilchardus | 0.013 | Minimum |
| Salmon | Salmo salar | 0.022 | Minimum |
| Anchovy/Boquerón | Engraulis encrasicolus | 0.017 | Minimum |
| Mussel | Mytilus galloprovincialis | 0.020 | Minimum |
| Cod | Gadus morhua | 0.066 | Low |
| Skipjack tuna (canned) | Katsuwonus pelamis | 0.128 | Medium-low |
| Yellowfin tuna | Thunnus albacares | 0.354 | Medium |
| Albacore tuna | Thunnus alalunga | 0.358 | Medium |
| Bluefin tuna | Thunnus thynnus | 0.386 | Medium-high |
| Swordfish | Xiphias gladius | 0.995 | High |
| Shark | Various species | 0.979 | High |
Source: FDA Mercury Levels in Commercial Fish and Shellfish (2024), average data. Individual levels vary by size, age, and catch area.
The legal mercury limit in the EU for predatory fish is 1.0 ppm. Most tuna is below this, but swordfish and shark are close.
Official AESAN recommendations (2024)
The Spanish Agency for Food Safety and Nutrition establishes these recommendations based on the tolerable weekly intake of methylmercury (1.3 µg/kg body weight/week according to EFSA):
General adult population (>70 kg):
- Canned tuna (skipjack): up to 4-5 cans/week without risk
- Fresh yellowfin tuna/albacore tuna: 2-3 servings of 150g/week
- Bluefin tuna/swordfish: maximum 1 serving of 150g/week
For a 70 kg adult, the tolerable weekly dose is 91 µg of methylmercury. A 150g serving of yellowfin tuna contains ~53 µg. A serving of swordfish: ~149 µg (already exceeds the weekly limit in a single serving).
Pregnancy and children: the rules change
Methylmercury crosses the placenta and the blood-brain barrier. The developing brain of fetuses and young children is particularly vulnerable.
Pregnant women, nursing mothers, and women planning pregnancy:
- Completely AVOID: Swordfish, shark, pike, bluefin tuna
- Limit: Yellowfin tuna/albacore tuna to a maximum of 1 serving/week
- No restriction: Sardines, salmon, hake, cod, mussels, anchovies
Children under 3 years old: Avoid the 4 high-mercury species. For the rest, no restriction within age-appropriate portions.
Children from 3 to 12 years old: Limit yellowfin tuna/albacore tuna to 2 servings of 50-70g/week. Avoid swordfish and shark.
These recommendations do not mean that tuna is "toxic" — they mean that at the mercury accumulation levels, consumption in vulnerable populations must be moderated. A healthy 70 kg adult can eat tuna several times a week without a problem.
Canned vs. fresh tuna: is there a difference in mercury?
Yes, and it is significant. Canned tuna is usually skipjack (Katsuwonus pelamis) — a small tuna (3-5 kg) with an average mercury of 0.128 ppm. Fresh tuna at the fish market is usually yellowfin tuna (0.354 ppm) or bluefin tuna (0.386 ppm).
That is to say: a can of tuna has 3 times less mercury than a fresh tuna fillet. Not because the canning process removes mercury (it doesn't), but because a smaller, younger species is canned.
Exception: canned albacore tuna is Thunnus alalunga, with 0.358 ppm — similar to fresh. Canned albacore tuna has more mercury than canned skipjack tuna.
If you are concerned about mercury but want quality canned fish, cod is an excellent alternative: only 0.066 ppm of mercury, rich in protein and omega-3. Explore our gourmet canned goods catalog and our artisan desalted cod.
How to reduce risk without stopping tuna consumption
- Diversify species: Don't just eat tuna. Alternate with sardines, salmon, cod, mussels. All the benefits of omega-3 without accumulating mercury.
- Choose smaller tuna: Canned tuna (skipjack) has 3x less mercury than fresh (yellowfin/bluefin). For frequent consumption, canned is safer.
- Limit swordfish and shark: Maximum 1 time per month. These are the fish with the highest mercury load.
- Selenium helps: Selenium binds to mercury forming mercury selenide (inert). Fish rich in selenium (tuna, mackerel, sardines) already contain some natural protection. Nuts (Brazil nuts) also help.
- Consume vitamin C and fiber: Studies suggest that vitamin C reduces intestinal absorption of methylmercury and fiber facilitates its excretion. Eat fruits and vegetables with fish meals.
- Don't give up fish: The nutritional benefits of fish (omega-3, protein, D, B12) far outweigh the risks of mercury for the vast majority of the population. Don't stop eating fish for fear of mercury — diversify and moderate the highest-risk species.
Keep exploring
Tuna properties · Canned sardines: properties · Canned tuna: comparison · Desalted cod
Frequently asked questions
How many cans of tuna can I eat per week?
A healthy adult of 70 kg can eat 4-5 cans of light tuna (skipjack) per week without exceeding the tolerable mercury intake. For canned albacore tuna (more mercury), limit to 2-3 cans. Pregnant women: maximum 1 can/week of any type of tuna.
Does canned tuna have less mercury than fresh tuna?
Yes. Canned tuna is usually skipjack (0.128 ppm) while fresh is yellowfin or bluefin (0.35-0.39 ppm). Canned tuna has 3 times less mercury, not because of the process but because a smaller species is used.
Can pregnant women eat tuna?
Limited. AESAN recommends a maximum of 1 serving/week of light tuna or albacore. Completely avoid bluefin tuna, swordfish, and shark. No restriction for sardines, salmon, hake, and mussels.
Is mercury in tuna eliminated by cooking it?
No. Methylmercury binds to fish proteins and is not destroyed by heat or freezing. Cooking tuna does not reduce its mercury content. The only way to reduce exposure is to moderate consumption and choose species with lower accumulation.
Which fish has the least mercury?
Sardines (0.013 ppm), anchovies (0.017 ppm), mussels (0.020 ppm), and salmon (0.022 ppm) are the safest. They are small/filter-feeding fish with a short lifespan and low in the food chain.
Does mercury accumulate in my body forever?
No. The half-life of methylmercury in the human body is 70-80 days. If you reduce the consumption of high-mercury fish, your levels will drop in 2-3 months. It's not like lead or cadmium, which accumulate in bones for decades.
Does the selenium in tuna counteract mercury?
Partially. Selenium forms complexes with mercury (mercury selenide) that are inert. Tuna contains more selenium than mercury, which offers some natural protection. But it's not a complete neutralization — it's still wise to moderate consumption.
