Why Tuna and Deep-Sea Fish Have High Mercury — Biomagnification

The ocean naturally contains trace mercury — volcanic activity, rock weathering, plus human sources like coal combustion and gold mining waste.

Inorganic mercury → methylmercury

Anaerobic bacteria on ocean floor (especially sulfate-reducing bacteria) methylate inorganic mercury: Hg²⁺ → CH₃Hg⁺. This methylmercury is the real problem.

Why is methylmercury more dangerous

• Dissolves in lipids and proteins, crosses cell membranes easily

• 95% gut absorption (inorganic mercury 7~15%)

• Biological half-life 70~80 days (inorganic is 1~2 months but barely absorbed anyway)

• Crosses blood-brain barrier and placenta

Once in, it's hard to get out.

Concentration up the food chain

Small plankton absorb methylmercury. Small fish eat plankton. Bigger fish eat smaller fish. Even bigger fish eat those.

Each step concentrates roughly 10×. After 4~5 steps, concentration is over 10,000× seawater.

Seawater: ~0.1 ng/L

Shrimp/sardines: ~0.01~0.05 ppm

Small tuna: ~0.1~0.3 ppm

Large tuna (bluefin): ~0.3~1.0 ppm

Swordfish/shark: ~0.7~1.5 ppm

Why deep-sea fish especially

• Long lifespan — many deep-sea fish live 50+ years. Lifelong mercury buildup

• Cold, low-oxygen environment — slow metabolism, slow excretion

• Often top predators — high in food chain too

Notable high-risk deep-sea fish: orange roughy, escolar, Chilean sea bass, some deep-sea sharks.

So should we avoid tuna entirely

Not all tuna is equal.

• Skipjack (light tuna): ~0.1 ppm. Relatively safe

• Albacore: ~0.3 ppm. Recommend once a week

• Bluefin / swordfish: 0.7~1.5 ppm. Pregnant women and children should avoid

US FDA recommends pregnant women, nursing mothers, and children get protein from low-mercury fish (salmon, sardines, shrimp) 2~3 times a week.