Methylmercury and Cesium-137 in fresh water fish

Methylmercury and Cesium-137 accumulate in predatory fish (pike, perch, pikeperch, burbot) in the same lakes. The older the fish, the more contaminants it has accumulated. By avoiding the consumption of large predatory fish caught in lakes, you avoid the accumulation of both mercury and radioactive Cesium in your body.

Elevated mercury levels have been found particularly in older pikes of forest lakes in Southern, Central and Eastern Finland. Fish imported from abroad, such as fresh tuna and swordfish, may contain equally high mercury levels. Canned tuna comes from a smaller tuna species that accumulates less mercury.

Mercury is found in both organic and inorganic form. More than 90% of the mercury contained in fish is of the more toxic methylmercury form. Levels vary from one fish species to the other, but increase with age and size. Methylmercury is a fat-soluble and persistent compound, which is enriched in fish and then in people who eat fish. It penetrates the placenta and damages the central nervous system of the foetus.

The regulatory maximum level of mercury in fish and fish products is 0.5 - 1 mg/kg. The Provisional Tolerable Daily Intake, or PTDI, of methylmercury is 0.23 µg/kg of body weight and that of inorganic mercury is 0.6 µg/kg of body weight. In Finland, the average intake of methylmercury from food is 0.11 µg/day for both adults and children, i.e. considerably less than the tolerable intake.

137Cs levels vary considerably within quite small areas: 3 - 1850 Bq/kg. The maximum level of 137Cs recommended by EU for wild products placed for sale on the market is 600 Bq/kg, which is the limit for safe intake (2003/274/Euratom).

Consumers can contact the municipal food control authorities for information about the status of the environment in their own region, including the mercury and cesium levels found in fish.

Fish that may show high mercury levels (0.5 -1 mg/kg)

  • Pike (both sea and inland water)
  • Perch (inland water)
  • Burbot (large, inland water)
  • Pikeperch (large, inland water) and
    Imported fish:
  • Swordfish
  • Shark
  • Fresh tuna


  • A toxic heavy metal found in the environment, particularly in soil humus and the bottom sediments of water bodies
  • Also released into the environment from industry, burning of waste and fossil fuels, and oil refining processes.

Cesium-137 (137Cs)

  • Radioactive substance
  • Deposited in Finnish water bodies and forests as a result of the Chernobyl accident in 1986.
  • Levels are on the decrease, but certain fish and mushrooms, particularly in fallout areas, may still contain levels that exceed the safe limit (600 Bq/kg).