Bio accumulation of radioactive caesium in marine mammals in the Baltic Sea – Reconstruction of a historical time series

•30years after the Chernobyl accident activity concentrations of radionuclides in Baltic marine mammals are still elevated.•Mean activity concentrations of Cs-137 in ringed seals (Phoca hispida) declined from 75 Bq/kg w.w. to 30 Bq/kg w.w.•Mean activity concentrations of Cs-137 in grey seals (Halichoerus grypus) has declined from 165 Bq/kg w.w. to about 30 Bq/kg w.w.•Cs-137 biomagnifies in the Baltic Sea, seals have 3.5-9 times the activity concentrations in their main prey, herring.•Activity concentrations of Cs-137 in Baltic marine mammals are still elevated compared to before the Chernobyl accident. [Display omitted] Radionuclides from the Chernobyl accident in 1986 still circulate in the Baltic marine ecosystem and activity levels in water, sediments and fish species such as herring and perch are monitored annually. However, the activity levels of radionuclides in marine mammals have only been sporadically reported. Tissue samples from a museum collection were analysed in two species of seals, and the trends over time in activity level of radioactive caesium (Cs-137) after the Chernobyl accident were reconstructed. We also performed a literature review summarizing activity levels in marine mammals world-wide. We found activity concentrations of Cs-137 in Baltic ringed seals and grey seals to be elevated also in the most recent samples, and during the entire study period measurements ranged between 19 and 248 Bq/kg wet weight. A declining trend in time over the last 30 years follow the general trend of decline in activity levels in other Baltic biota. Accumulation was found to be species specific in the two seal species studied, with 9 times higher activity concentration in grey seals compared to herring, and 3.5 times higher in ringed seals compared to herring. We discuss potential paths and rates of bioaccumulation of radioactive caesium in the Baltic Sea including species specific prey choice of the two seal species and estimate life time exposure. The study contributes one important piece of information to predictive models in risk assessments for nuclear accidents.