Stephanie Long measures radioactivity in porpoise at the RPII’s radioanalytical laboratory.

It is clearly important to monitor radioactive contamination in the marine environment and its transfer through the food chain. A joint study was carried out by the Radiological Protection Institute of Ireland and the Department of Zoology and Animal Ecology, UCC, to assess radionuclide contamination in cetaceans (porpoises, dolphins and whales) from Irish and British coastal waters. Cetaceans are top-level predators and thus are vulnerable to contaminants. The study focused on harbour porpoises (Phocoena phocoena), which are probably the most abundant of the Irish Sea cetaceans.

A wide range of radionuclides is present in the marine environment. The majority of these are naturally occurring radionuclides such as potassium-40 (40K), uranium and polonium-210 (210Po). Man-made sources include nuclear weapons testing, the Chernobyl accident and discharges from nuclear fuel reprocessing plants. In the Irish Sea, discharges from the British Nuclear Fuels’ reprocessing plant at Sellafield are the main source of artificial radioactivity. Of the radionuclides discharged from Sellafield, caesium-137 (137Cs) is the most important in terms of the dose to both man and marine organisms.

Muscle samples from twenty-five porpoises stranded on the coasts of Britain and Ireland or by-caught in fishing nets in Irish coastal waters were analysed for artificial 137Cs and naturally occurring 40K. The average levels of 137Cs measured were some ten times higher in porpoises from the Irish Sea (17.8 Becquerels per kilogram - Bq.kg-1) compared to those from the Atlantic (1.9 Bq.kg-1), Celtic (1.7 Bq.kg-1) and North Seas (2.4 Bq.kg-1). These results are consistent with monitoring studies which show that levels of 137Cs are higher in marine life from the Irish Sea than from the Atlantic, Celtic and North Seas. Average levels of 40K (94.7 Bq.kg-1) in porpoises were much greater than those of 137Cs and did not vary between the different sampling locations.

The average level of 137Cs measured in fish landed at Irish Sea ports during the same period was about 3.0 Bq.kg-1. Porpoises consume a range of fish species and so it is clear that the levels of 137Cs are increasing with trophic level (the different feeding levels within an ecosystem). The dose to porpoises from radioactivity in the marine environment was calculated and found to be insignificant. Thus, despite the elevated levels of 137Cs in Irish Sea porpoises relative to those from the Atlantic, Celtic and North Seas, the resulting radiation dose is unlikely to have had a detrimental effect on their health.