Swordfish, Shark & Marlin
Swordfish, shark, marlin, and similar large predatory fish should be avoided during pregnancy due to very high mercury levels.
Swordfish, shark, marlin, and king mackerel should be avoided entirely during pregnancy. These large, long-lived predatory fish sit at the top of the marine food chain and accumulate mercury through a process called biomagnification — every time they eat a smaller fish, they absorb and retain the mercury that fish had accumulated, on top of their own. The result is mercury concentrations many times higher than in smaller fish like sardines or salmon. High mercury exposure during pregnancy can damage the developing nervous system of your unborn baby, affecting brain development, hearing, and vision. The NHS specifically advises pregnant women to avoid shark (often sold as flake or rock salmon), swordfish, and marlin. Tilefish (less common in the UK) is also in this category. This guidance applies regardless of how these fish are cooked — cooking does not reduce mercury content.
What to be aware of
- Avoid shark, swordfish, marlin, and tilefish entirely during pregnancy — this is firm NHS advice, not just a precaution.
- Shark is sometimes sold as 'flake', 'rock salmon', or 'huss' in UK chip shops — ask if unsure.
- Mercury is not reduced by cooking — there is no safe preparation method.
- Regular tuna (tinned) is a different, lower-mercury product and has its own guidance — see the tuna article.
What to eat instead
- Salmon — Rich in omega-3 and far lower in mercury — an excellent substitute for any oily fish dish.
- Cod or haddock — White fish with no mercury concerns, no portion limit, and suitable for most recipes calling for a firm fish fillet.
- Trout — A low-mercury oily fish that provides similar omega-3 benefits without the mercury risk.
- Sardines — Small, short-lived fish that are exceptionally low in mercury and very high in omega-3 and calcium.
NHS guidance: https://www.nhs.uk/pregnancy/keeping-well/foods-to-avoid/
US guidance
FDA and EPA guidance in the US is consistent — swordfish, shark, king mackerel, tilefish, orange roughy, and bigeye tuna are all on the 'avoid' list for pregnant women due to high mercury. The US guidance is arguably more detailed on specific species than NHS guidance.