The ocean currents surrounding the Japanese Archipelago: 1. Kuroshio 2.Kuroshio extension 3. Kuroshio countercurrent 4. The Tsushima Current 5. The Tsugaru Current 6. The Sōya Current 7. Oyashio 8. The Liman Current
http://en.wikipedia.org/wiki/Kuroshio_CurrentKinda looks like the Radioactivity will be pushed along the shore first and there are some countercurrents that may bring it back again.
Total aquaculture production in 2003 was estimated at 1 327 361 tonnes, worth US$ 4 428 962 000, corresponding to 22 percent of total national fish production and 31 percent of the total value of fish produced in Japan.
Marine aquaculture accounts for 96 percent of total aquaculture production, and 90 percent of the total value produced by aquaculture. The main cultured species are nori (27 percent of total aquaculture production, and 18 percent of the value produced), yesso scallop (20 and 7 percent), oyster (17 and 8 percent), Japanese amberjack (12 and 25 percent), red seabream (6 and 11 percent), and sea mustard (5 and 2 percent), eels (2 and 4 percent), ayu (1 and 2 percent), rainbow trout (1 and 1 percent), and pearls (0.002 and 5 percent). (Statistics Department, MAFF 2005a).
The graph below shows total aquaculture production in Japan according to FAO statistics:Chart
http://www.fao.org/fishery/countrysector/naso_japan/en#tcN900AD