Inferring ecological separation from regional differences in radioactive caesium in harbour porpoises Phocoena phocoena

Geographic variation in the level of radioactive caesium-137 (¹³⁷Cs) was examined from the axial muscle of 36 harbour porpoises Phocoena phocoena obtained as bycatch from 5 locations along the coast of Norway, ranging from southern (North Sea) to northern (Barents Sea) Norway. Levels of ¹³⁷Cs in seawater and sediments have been found to differ along the coast of Norway due to distance from point sources, such as the Sellafield nuclear fuel reprocessing plant in Cumbria, UK (discharging into the Irish Sea) and outflow from the Baltic Sea, which contains significant amounts of ¹³⁷Cs that originated in Chernobyl. Radioactive caesium is accumulated in muscle in a manner related to ambient levels in the environment through exposure and/or ingestion of contaminated prey items. Hence, we hypothesised that if harbour porpoises are philopatric, their muscle tissue would reflect concentrations of ¹³⁷Cs in their area of residence. The levels found in harbour porpoise muscle were consistent with that found in the area from which they were caught. Porpoises from the Barents Sea had significantly lower levels of ¹³⁷Cs than porpoises from the west and south coasts of Norway, reflecting the concentration patterns of ¹³⁷Cs in the waters of those areas. This suggests ecological separation of harbour porpoises in these regions, and that porpoises are limited in their movements along the coast of Norway.