The major earthquake and subsequent tsunami that struck Japan in March led to the meltdown of the Fukushima Daiichi nuclear power plant. For months the nation battled to control the meltdown and reduce radiation exposure which was unfortunately prevalent in the region, both in the air and water. Not only did radiation enter the nation’s drinking water, it also entered the sea.
High levels of radiation were found well out to sea near Japan in the following weeks of the meltdown. Radiation was entering the sea directly in large quantities according to sources at the time, so dire was the situation that radiation was also found in the rain. Although many scientists concluded that the radiation would be diluted by the ocean, even small levels can be poisonous to ecosystems and individual species.
Radiation in the air was detected thousands of miles from Japan at the height of the disaster but the true spread of the radiation in the ocean is still being studied. Researchers from the Woods Hole Oceanographic Institution studied the spread of radiation from the disaster in Japan in the months following the tsunami.
Normal levels of radiation exist in the oceans, measured in becquerels (Bq), one decay per second. The level of radiation considered “normal” in the oceans is 1-2 Bq per cubic meter of water. During the tests, the researchers found levels of upwards of 3,900 Bq per cubic meter close to the shores of Japan. At 372 miles from shore, the tests concluded levels of 325 Bq per cubic meter.
Tests on zooplankton and fish found levels between 10-15 Bq per kilogram, with levels higher in the zooplankton. The allowable factor in seafood in Japan is 500 Bq per kilogram. All species tested were determined to have radioactive materials of cesium-137 and cesium-134 in their tissues.
Larger fish are considered less likely to retain the radioactive material due to their ability to metabolize the materials and rid their bodies of the problem. However, researchers have recently discovered that bluefin tuna originating off of Japan are showing heightened levels of cesium- 134 and cesium- 137 as they migrate to the California coast, approximately 6,000 miles away. The size of the fish led scientists to believe it was likely their levels would have been near normal after their journey.
However, in the study recently published in the Proceedings of the National Academy of Sciences, researchers noted radioactive levels ten times higher in the bluefin tuna compared to normal levels. Comparison tests on yellowfin tuna which survive in the eastern Pacific determined that the radiation was not caused by radiation traveling the oceans but by exposure for the bluefin off of Japan.
The levels of radiation in the bluefin tuna in previous months were likely higher as their bodies were able to purge some of the radiation over time. The researchers plan to study the result of radiation exposure on bluefin tuna which were exposed to the radiation for longer periods of time as the tuna in the current study began their migration shortly after exposure.