Fish and mercury: what you need to know

Carlos Soler

Graduado en Nutrición Humana y Dietética, Universidad de Valencia. Máster en Fisiología del Deporte, Universidad de Barcelona. Certificación ISAK nivel 1, The International Society for the Advancement of Kinanthropometry.

Fish and mercury: should we be concerned? There is no need for alarm, but species and individual circumstances matter. Spain’s food safety agency AESAN, formerly AECOSAN, provides recommendations to help us enjoy fish while limiting mercury exposure.

Mercury and its relationship with fish

Mercury is a toxic environmental contaminant. Methylmercury can affect the developing nervous system and cross the placenta, making exposure particularly relevant during pregnancy and childhood.

How does it reach fish? Mercury released into the environment enters aquatic food chains. Larger, longer-lived predatory species can accumulate higher concentrations.

With suitable choices, fish can be a safe and nutritious part of the diet. Mercury risks have been known for decades, and European food legislation sets mandatory maximum levels.

Here we explain the species of concern, vulnerable groups, intake recommendations and benefits of including fish.

Fish classified by mercury content

Species with high mercury content include swordfish, bluefin tuna (Thunnus thynnus), shark species such as dogfish, mako, spiny dogfish, small-spotted catshark and blue shark, and pike.

Species with low mercury content include pollack, anchovy, herring, cod, blue whiting, cockles, mackerel, squid, shrimp, crab, purple dye murex, saithe or coley, carp, small squid, clams, cuttlefish, langoustines, wedge clams, gilthead sea bream, sprat, prawns, horse mackerel, spiny lobster, king prawns, European sole, dab, sea bass, mussels, whiting, hake, razor clams, oysters, pomfret, flounder, flying squid and octopus, small shrimp, Atlantic salmon, Pacific salmon, sardines, round sardinella, Madeiran sardinella, plaice and trout. AESAN treats species not specifically listed as having medium mercury content.

IMPORTANT: For the general population, AESAN recommends 3–4 fish servings a week, varying species and alternating white and oily fish. This does not mean eating high-mercury species at every serving.

For context:

• White fish: cod, wedge sole, blue whiting, red sea bream, megrim, hake, monkfish, sole, turbot and pollack.
• Moderately fatty fish: gilthead sea bream, sea bass, red mullet and trout; fat content varies with season and other factors.
• Oily fish: examples include sardine, tuna, salmon, bonito, mackerel, anchovy, herring, eel, carp and horse mackerel. Categories overlap and do not replace mercury guidance.

Fish intake in vulnerable groups

People who are pregnant, planning pregnancy or breastfeeding, and children aged 0–10 should avoid the high-mercury species listed above. They can eat 3–4 weekly servings of low- and medium-mercury fish, varying white and oily species. For example, grilled cod with mashed potato and courgette soup, lemon sea bass with black pepper and griddled asparagus, or thoroughly cooked prawns. Leave out bluefin tuna, shark, pike and swordfish for these groups.

Children aged 10–14 should limit those high-mercury species to a total of 120 grams per month, roughly 1–2 small servings, while following the same advice for other fish.

Benefits of eating fish

AESAN and EFSA recommend fish within a varied, balanced diet.

1. It supplies high-quality protein with essential amino acids that must come from food. It is an important protein source in the Mediterranean dietary pattern.

2. Depending on the species and edible parts, it contributes iodine, selenium, calcium, phosphorus, iron, vitamin A and vitamin D.

3. Many fish provide protein for relatively few calories, although preparation and fat content matter.

4. Fat content varies considerably. The original 5% and 55% figures should not be treated as general fat-by-weight values for white and oily fish. Oily species provide omega-3 fatty acids. Regular fish consumption, including around 2–3 servings of oily fish in the studies discussed, has been associated with cardiovascular benefits; recommendations must still respect mercury guidance.

5. In sports nutrition, omega-3 fats have been studied for possible effects on muscle adaptation, vascular function and inflammation. Evidence does not justify promising improved performance or recovery from eating fish alone.

6. Suitable low-mercury fish during pregnancy contributes nutrients involved in fetal neurological development.

7. Research comparing the benefits and risks of around 300 g weekly, or 2–3 servings, should not be read as advice for vulnerable groups to eat high-mercury species. Follow the specific species and age guidance above.

Three tips for choosing fish

Look at the eyes: fresh fish generally has bright, clear, full-looking eyes.

Check the smell: it should be mild, without an ammonia-like or spoiled odour.

Check the skin and scales: the surface should look fresh and shiny, with scales attached. These clues do not replace proper storage or safety controls.

Seasonality is another consideration. This Vitónica post provides a seasonal fish calendar.

How should fish be stored?

Keep raw fish and seafood at 4.4°C or colder, preferably in the coldest part of a fridge kept at 4°C or below. Use or freeze within 1–2 days and respect use-by dates. Refrigerate cooked fish promptly and use within 3–4 days or sooner if required by the product guidance.

Conclusion

For most people, fish is a useful part of a varied diet: aim for the recommended 3–4 weekly servings while varying species. Vulnerable groups should avoid or limit high-mercury species as described, choosing other fish instead.

Here are 3 fish recipes to try:

Teriyaki tuna with asparagus, carrot, edamame and wakame

Ingredients

• Wild asparagus 150 g
• Garlic 2 g
• Table salt 1 g
• Lime 40 g
• Tuna 125 g
• Vinegar 20 g
• Black pepper 1 g
• Edamame 50 g
• Wakame seaweed 20 g
• Carrot 80 g
• Teriyaki sauce 30 g

Method

1. Finely chop the garlic.
2. Mix the teriyaki sauce, lime juice, vinegar, garlic and black pepper in a bowl.
3. Add the tuna loin and marinate in the fridge for 5–10 minutes. Choose a suitable species and follow the mercury advice above.
4. Cook on a hot griddle or frying pan. If leaving the centre rare, use fish appropriately frozen for parasite control and suitable for raw consumption; vulnerable groups should choose fully cooked fish.
5. Put the marinade in a pan and bring it to a thorough simmer before reducing.
6. Add the trimmed asparagus, peeled and chopped carrot and edamame.
7. Cook until tender and serve with the tuna, adding the prepared wakame as appropriate.

Original figures per serving

Calories: 365
Protein: 45% | Carbohydrates: 33% | Fat: 2%. These percentages total 80%, so the source panel requires recalculation before being used as an accurate macro breakdown.

Baked salmon with artichokes

Ingredients

• Salmon 150 g
• Artichoke 100 g
• Olive oil 5 g
• Lemon with peel 70 g
• Table salt 1 g
• Parsley 2 g
• Water 400 g
• Black pepper 2 g

Method

1. Prepare a bowl of water and lemon juice.
2. Trim the artichokes, removing tough leaves until the softer ones appear. Carefully trim and peel the stalk, quarter the artichokes and put them in the lemon water. Gloves can help prevent staining.
3. Lightly oil an oven tray and bake the artichokes for an initial 5 minutes. Add the salmon with a little oil and salt and bake for around 10 minutes more, adjusting the oven temperature and time until both fish and artichokes are properly cooked.
4. Serve with lemon and parsley to taste.

Figures per serving

Calories: 347
Protein: 39% | Carbohydrates: 22% | Fat: 39%

Baked cod with potatoes, green beans, onion and carrots

Ingredients

• Atlantic cod 150 g
• Potato 100 g
• Baby carrots 50 g
• Red onion 30 g
• Green beans 60 g
• Coriander 3 g
• Table salt 1 g
• Cooking wine 100 g
• Olive oil 5 g

Method

1. Preheat the oven to 180°C.
2. Peel and slice the potato and thinly slice the onion.
3. Lightly oil an ovenproof dish to prevent sticking.
4. Layer the potatoes, onion, green beans and carrots in the dish, with the cod on top.
5. Add the cooking wine and salt.
6. Bake for around 12–15 minutes, adjusting for the oven, thickness and ingredients. Thinly slice or precook the potatoes as needed so they are tender when the fish is cooked through.
7. Plate up, add coriander to taste and serve.

Figures per serving

Calories: 302
Protein: 41% | Carbohydrates: 41% | Fat: 18%

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REFERENCES

Agostoni C, Canani RB, Fairweather-Tait S, Heinonen M, Korhonen H, La Vieille S, et al. Scientific Opinion on health benefits of seafood (fish and shellfish) consumption in relation to health risks associated with exposure to methylmercury. EFSA Journal 2014;12(7):3761.

Alexander J, Benford D, Chaudhry Q, Griffin J, Hardy A, Jeger MJ, et al. Statement on the benefits of fish/seafood consumption compared to the risks of methylmercury in fish/seafood. EFSA Journal. 2015;13(1):3982.

Raimann X, Rodríguez L, Chávez P, Torrejón C. Mercurio en pescados y su importancia en la salud. Rev Med Chile. 2014;142(9):1–7.