Slower Decrease in Radioactive Concentrations in Some Fish Species After the Fukushima Daiichi Nuclear Power Plant Disaster

Abstract

Large quantities of radioactive material were released into the ocean as a result of the Fukushima Daiichi Nuclear Power Plant disaster. Because of the movement of the highly radioactive water mass, coastal seawater, seabed sediments, and many marine organisms were contaminated by radioactive material. Although radioactive concentrations in many species decreased over time, concentrations in some fish species decreased very slowly (e.g., Japanese rockfish and skate). In this study, the changes in radioactive cesium (Cs) concentrations in marine organisms are reported over the 4 years since the accident and possible reasons for the slower decrease in radioactive concentrations observed in some fish species are discussed. Marine organisms and seawater and seabed sediment samples were collected from May 2012 to May 2014. Radioactive Cs concentrations in the collected organisms and samples were measured using a germanium semiconductor detector. An ultrasonic pinger was inserted into the bodies of some fish, and fish movements were recorded by biotelemetry for 1 year. Japanese rockfish were cultured in a large water tank for 1 year, and the biological half-life of radioactive Cs was estimated in the fish. Radioactive Cs concentrations in benthic and omnivorous fish, such as Japanese rockfish (367 days) and a skate species (560 days), had relatively long ecological half-lives. Japanese rockfish remained within the same rocky area (within a radius of 200 m) in open sea areas throughout the year. The ecological half-life of radioactive Cs was approximately 270 days longer in rockfish than that in other marine fish. These results suggest that relatively high radioactive Cs concentrations remained in coastal benthic fish owing to slow Cs excretion rates.