According to the Annual Review of Marine Sciences, when the Tohoku tsunami struck Japan in March 2011, large amounts of radioactive material was vented from the Fukushima Daiichi nuclear reactors. 80% of that radioactive material was thought to have fallen in the ocean along with more being washed in that initially landed on land. The main source of radiation released by Fukushima is cesium-137, other radioactive isotopes were either too rare to pose a major threat from the start, or, like iodine-131, have such a short half-life they have basically dissipated, which has eliminated their threat to ocean life.
Most estimates place the cesium-137 released to equal 15 to 25 x 1015 Becquerel, (a Becquerel, Bq, is one nuclear decay per second), which is about 1/5th of the radiation that was released in the Chernobyl accident of 1986. The amount of radiation released by this disaster may seem large, but the sheer size of the Pacific Ocean allows the radiation to disperse and dilute. Measurements taken offshore from April to July 2011 observed a 50 percent decrease in cesium-137 at the ocean surface every seven days. Due to the fact that cesium-137 has a 30-year half-life this reduction represents the radioactive atoms sinking or being dispersed, rather than decay.
In early April 2011, 68 million Becquerels per cubic meter was recorded not far offshore of the power plant, but this plunged by a factor of a thousand within a month, even with the continued leaks in the area. Cesium from Fukushima was first detected off the coast of Canada in June 2013. Measurements in the eastern pacific are still on the rise, but the highest so far has been – 10 Bq/m3 – which is far below levels that would be considered unsafe.
Before the disaster struck, Japan considered fish with radioactivity levels of more than 500 Becquerels per kilogram to be unsafe, but this was lowered to 100Bq/kg as a result of public concerns after the disaster. “In 2011 approximately half the fish sampled in coastal waters off Fukushima Prefecture had radiocesium levels above 100 Bq/kg,” the paper reports. By 2015 only 1 percent exceeded this level. Fish caught within the Fukushima harbor remain highly radioactive, but this is attributed to the continued leakage from reactors and radiation that is trapped within the sediments.