Half-century trends of radioactivity in fish from Danish areas of the North Sea, Kattegat, and Baltic Sea.

This work reports comprehensive time-series datasets over the past 50 years for natural (210 Po) and anthropogenic (134 Cs and 137 Cs) radionuclides in three fish species (cod, herring and plaice) from Danish marine areas covering the North Sea, Kattegat, and Baltic Sea. Impact from the global fallout of atmospheric nuclear weapons testing, radioactive discharges from the European nuclear reprocessing plants and release from Chernobyl accident are clearly detected in the fish samples. While 210 Po concentrations in each fish species demonstrated comparable levels across the three regions without notable temporal trends, significantly higher median 210 Po concentration was observed in the lower trophic level fish, namely herring and plaice, compared to cod. In contrast, 137 Cs concentrations in all three species steadily decrease over time after the Chernobyl-attributed peaks in late 1980s in the entire study area, whereas 137 Cs always demonstrated higher concentrations in cod than herring and plaice. Our calculated concentration factors (CFs) for 137 Cs in this work indicate that the mean CFs for 137 Cs over the past 50 years are significantly different across the three species, following the order of cod < herring < plaice. Based on the time-series data, ecological half-lives (Teco ) of 137 Cs in fish from Danish marine areas were estimated to evaluate the long-term impact of anthropogenic radioactive contamination in different regions. Our results indicate no significant difference in Teco across different fish species, whereas the weighted mean Teco for fish in the Baltic Sea (29.3 ± 3.9) is significantly longer than those of the North Sea (9.8 ± 0.9) and Kattegat (11.7 ± 1.2), reflecting the strong 'memory effect' of the Baltic Sea due to its slow water renewal. However, the dose assessment demonstrates that the contribution of the natural radionuclide 210 Po to ingestion dose from fish consumption is 1-2 order of magnitude higher compared to that of 137 Cs.