By Jay T. Cullen

The purpose of this post is to report on a recently published, peer-reviewed study documenting the contamination of whales and dolphins in northern Japan following the Fukushima Dai-ichi Nuclear Power Plant disaster in March 2011. This post is the most recent in an ongoing series that documents scientific research into the impacts of the FDNPP disaster on the health of the marine environment. The paper by Nakamura and colleagues investigated the levels of artificial radionuclides ¹³⁴Cs (half life ~ 2 years) and ¹³⁷Cs (half life ~30 years) and naturally occurring ⁴⁰K (half life 1.25 x 10⁹ years) in stranded whales and dolphins in 2011 and 2012 following the disaster. While there was little radiocesium present in the seawater around the northern island of Hokkaido after the disaster some of the animals had detectable levels of radiocesium from the FDNPP in the months following the disaster. By 2012 most stranded animals did not have detectable levels of FDNPP derived radiocesium. According to the authors, the sudden rise in radiocesium levels in the animals following the disaster suggests that the contamination in the animals reflected the seawater activities of the radionuclides through which they swam north rather than bioconcentration through the marine food web. Levels of artificial radionuclides were about 10-fold lower than naturally occurring isotopes in the organisms and are not likely to be causing negative health impacts but may be useful for helping to better understand the migration routes of these animals.

Nakamura and colleagues collected muscle samples from 47 cetaceans stranded on the Hokkaido coast of Japan between April 2011 and September 2012 at the following locations:

Note that 4 of the 47 samples were purchased. Two (Dall’s porpoise and a Baird’s beaked whale from the North Pacific ) were purchased in Miyagi Prefecture in 2003 and 2004 before the disaster and 2 (common minke whale caught off Kuroshio) in June and October 2011 after the disaster and obtained through the highly controversial Japanese Research Whaling Program.

The table below summarizes information relating the samples including the results of gamma spectrometry of the animal tissue:

Results

FDNPP derived radiocesium, as identified by the presence of the short lived isotope ¹³⁴Cs, was detected in 16 of the 47 cetaceans. Most of the cetaceans contaminated with ¹³⁴Cs and ¹³⁷Cs and ¹³⁷Cs only were stranded on the North Pacific coast of Hokkaido. Animals stranded on the Sea of Okhotsk or Japan Sea coast of the island did not have detectable contamination except for one harbour porpoise which had low but detectable ¹³⁷Cs levels (1.1 Bq kg^-1, January 12, 2012). Most of the cetaceans that had detectable levels of radiocesium were stranded between June and October 2011 whereas after January of 2012 stranded animals were all below detection limit except for one common minke whale that stranded near the end of May 2012. Levels of naturally occurring ⁴⁰K were about a factor of 10 greater than combined radiocesium activities in the animals.

The following figure shows how levels of artificial radionculides in the animals diminished from 2011 to 2012.

The spatial pattern of animals showing contamination, rapid rise of radiocesium immediately following the FDNPP disaster and drop below detection by early 2012 is consistent with the animals picking up contamination from the water rather than their position in the marine food web according to the authors. It is likely that the animals showing measurable ¹³⁴Cs activities traveled through the contaminated plume of seawater off of Fukushima shortly after the disaster when release rates from the site were at a maximum. Release rates to the Pacific diminished rapidly after April 2011 and continue to this day but at much lower rates. It appears these rates lead to seawater activities that do not cause detectable contamination levels in the cetaceans living in Japanese waters.

What does this mean for North American cetaceans?

Given the degree to which decay and mixing have diminished the levels of FDNPP derived radionuclides in the plume as it approaches the northeastern Pacific and coastal waters of North America it is unlikely that animals here will have measurable levels of artificial radionuclides owing to Fukushima. Indeed, measurements carried out a stranded grey whale near Tofino British Columbia, Canada by our monitoring program Fukushima InFORM did not detect artificial radiocesium in the whale muscle or blubber. Given the Japanese results above, it is quite unlikely that radioisotopes from Fukushima will reach levels in our cetaceans here off North America that would cause toxicity or measurable detrimental impacts to their health. We’ll continue to make measurements on cetaceans if samples are made available to the program and I will report results as they are generated.