Thanks for all the interest in this story. You can read more about how scientists are thinking about possible causes of this die-off here: http://www.earthfix.info/water/article/northwest-starfish-experiments-give-scientists-clu/
Scientists have not ruled anything out, but do not think this is related to radiation exposure from Fukushima because: First off the isotopes won't reach here until 2016 if in water. Secondly, there are constant air monitors in WA state as well as regular food fish testing for radioactivity. All have been at background levels only. Finally, the pattern of disease is patchy, not one big wave -- For example, Seattle's waters have been hit hard, but coastal Oregon has largely been spared.