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Key Message

This core indicator evaluates the state of the environment using the concentration of the radioactive isotope cesium-137 (137Cs) in herring, flatfish (flounder) and surface waters. Good status is achieved when concentrations reach levels representative of those measured before the Chernobyl accident in 1986, the point at which the biota of the Baltic Sea received the most significant contribution to their level of artificial radionuclides, predominantly in the form of 137Cs.


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Key message figure 1. Status assessment results based on the evaluation of concentrations of cesium-137 in biota (herring and flatfish -flounder) and in seawater. The One-Out-All-Out approach is used to summarise status (main figure) and the assessments are shown for biota (upper insert) and water (lower insert). The assessment is carried out using Scale 2 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4). Click here to access interactive maps at the HELCOM Map and Data Service: Radioactive substances in herring and flatfish and radioactive substances in seawater.

The indicator presents a status evaluation using data from 2011-2016.


In general, the activity concentrations of the radioactive isotope cesium-137 (137Cs) in herring, flatfish and surface waters were still above the pre-Chernobyl levels that constitute the boundary for good status – threshold values.

For herring, good status is only achieved in the Arkona Basin, the Bay of Mecklenburg, the Kiel Bay and the Kattegat (Key message figure 1). For flatfish, good status is also achieved in the Arkona Basin, the Kiel Bay and the Kattegat (Key message figure 1). All other assessed sub-basins are characterized by not-good status. For surface waters, good status is not achieved in any of the sub-basins, where data is available (Key message figure 2).

Time series analyses show that the 137Cs activity concentrations in herring, flatfish and surface waters in the Baltic Sea basins are decreasing and approaching pre-Chernobyl levels. It is expected that good status may be reached in the Baltic Sea by circa 2020 - 2025.

The confidence of the indicator status evaluation is considered to be high.

The indicator is applicable in the waters of all countries bordering the Baltic Sea.


Relevance of the core indicator

The radionuclide cesium-137 (137Cs) is the greatest contributor to the level of artificial radionuclides in the Baltic Sea, where the level of 137Cs contamination is still higher than in any other ocean of the world. The main source of 137Cs deposited to the Baltic Sea stems from the accident at the Chernobyl nuclear power plant in 1986. 137Cs was introduced into the Baltic Sea water by atmospheric deposition and, to a lesser extent, through riverine input. 137Cs introduced to the Baltic Sea is bioaccumulated in marine flora and fauna and is eventually deposited in the marine sediments. Therefore 137Cs activity concentrations are the key factor in an assessment of the radiological hazard to marine organisms and humans (Nielsen et al. 1999). Both are highly interconnected as the dominating exposure pathway of humans from man-made radioactivity in the Baltic Sea is related to the ingestion of 137Cs in fish.


Policy relevance of the core indicator

 BSAP segment and objectivesMSFD Descriptor and criteria
Primary link

Hazardous substances

  • Radioactivity at pre-Chernobyl level.

D8 Concentrations of contaminants

D8C1 Within coastal, territorial and areas beyond territorial waters the concentration of contaminants do not exceed the threshold values.

Secondary link

D9 Contaminants in fish and seafood

D9C1 The level of contaminants in edible tissues of seafood caught or harvested in the wild does not exceed maximum levels which are the threshold values.


Cite this indicator

HELCOM (2018) Radioactive substances: Cesium-137 in fish and surface seawater. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full indicator report

Radioactive substances HELCOM core indicator 2018 (pdf)