Tuna carry Fukushima isotopes to California

May 30, 2012 | Source: Physics World

Bluefin tuna (credit: NOAA)

Pacific bluefin tuna off the California coast have been found to contain levels of radioactive cesium isotopes that are around 10 times higher than expected.

It is believed that the fish ingested the cesium following a discharge of radioactive material into the ocean near the earthquake- and tsunami-damaged Fukushima Daiichi nuclear power plant in March 2011.

Stanford University and Stony Brook University scientists found cesium-134 and cesium-137 in 15 two-year-old Pacific bluefin tuna (Thunnus orientalis) that were caught near San Diego in August 2011.

The fish spawns in the western Pacific; some juveniles stay in Japanese waters, while others swim east to the California Current Large Marine Ecosystem, generally when they are around a year old. That means all two-year-old Pacific bluefin caught during summer in the eastern Pacific must have migrated recently.

Ten times higher, but safe

The team found radioactivity levels of around 4 becquerels per kilogram (Bq/kg) dry weight of 134Cs and roughly 6 Bq/kg of 137Cs in the white-muscle tissue of fish sampled in late summer 2011. Pacific bluefin caught before Fukushima and yellowfin tuna, which tend to stay in Californian waters, contained no 134Cs and only background concentrations of 137Cs.

The total cesium concentrations in the post-Fukushima fish were roughly 10 times higher than before the leaks. But the levels were low compared with naturally occurring radioactive isotopes of potassium and polonium, and an order of magnitude less than the Japanese safety limit of about 400 Bq/kg dry weight for human consumption.

The results suggest that these tracers may be of use, for a finite period of time, to study migratory patterns of marine animals that use the waters around Japan and then migrate large distances, the researchers concluded.

They also believe that loggerhead turtles, salmon sharks, sooty shearwaters, pinnipeds, whales and billfish may also have transported radiation from Fukushima.

Ref.: Daniel J. Madigan, Zofia Baumann, Nicholas S. Fisher, Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California, PNAS, 2012, DOI: 10.1073/pnas.1204859109 (open access)