The pre-Chernobyl values used to derive the threshold value have been calculated based on data in the HELCOM MORS database. The data used for defining the threshold value for 137Cs concentrations in herring, flatfish (plaice and flounder) and surface waters were collected between 1984 and 1985. The mean pre-Chernobyl 137Cs concentrations have been used as threshold values.
The evaluation of whether the threshold is achieved or not is carried out by calculating the mean value for all samples during the assessment period 2011 - 2016 in each assessment unit and comparing this against the threshold values.
This core indicator evaluates the activity concentrations of 137Cs using HELCOM assessment unit scale 2 (division of the Baltic Sea into 17 sub-basins). The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.