1) Radioactive contamination from the Fukushima Daiichi nuclear accident spread to the marine environment through atmospheric fallout and discharge of contaminated water into the sea. 2) Monitoring shows contamination levels decreasing in sea water near the plant but remaining relatively constant further offshore due to ongoing water discharge. 3) Sediment near discharge areas shows higher radioactivity, indicating particle adsorption and removal from the water column. Traces of radionuclides may disperse across the northern Pacific ocean over the coming years.