I don't know where you get the bit about a third of Japan's rice being grown in Fukushima province. A lot of the rice eaten in Japan is imported, for one thing. For another thing the major part of the contamination from the Fukushima reactors was deposited in mountainous terrain to the north-west of the plant. Nearly all rice-growing in Japan is done on coastal flatlands such as the Kansai region, a looong way from Fukushima.
I get it from various sources... I don't recall exactly where I read that tidbit.. but it's easy enough to find sites like this one. http://www.pref.fukushima.jp/kokusai/IADwebsite/aboutfuku/aboutfuku8.htm Where it specifically mentions all the agricultural products that come out of that area and articles like this one: http://www.msnbc.msn.com/id/45337452/ns/world_news-disaster_in_japan/t/japan-bans-rice-grown-near-crippled-fukushima-nuclear-plant/#.TxuQOyOZNCI Interestingly.. those 'safe levels of radiation' mentioned in the article have been something of a moving target lately. Governments worldwide have been changing regulations to drastically increase what's considered safe with regard to food specifically from Japan. Google it yourself if you doubt what I'm saying.
This is basically the same situation for the agricultural areas contaminated with fallout although decontamination there might be easier as less soil needs removing and treating.
One has to look at the decon methods they are using.. and how effective they are likely to be. I've seen estimates that to really decon it right you have to remove 4" of top soil. That would be devastating to the environment there and utterly impossible on the scale we are talking. That's not how they are de-contaminating though. In many cases they are removing less depth or simply ploughing the soil to turn it over. They are also dumping radioactive soil into Tokyo Bay where it's sure to end up in marine life.. so not really getting rid of it.. just moving it around making it harder to find. The latest numbers I've seen were about 1400 square miles needs to be de contaminated. That's not just farm land.. that's cities.. woods.. private houses etc. Crops grown in contaminated soil can uptake hot particles and pass them on to the people or animals that consume them.
As for radioactivity levels, I do hope you are aware that seafood swims in radioactivity? Seawater has about 10Bq/litre of radioactivity due to the presence of potassium-40 (K-40). A rough BOTE calculation says there are 50 million tonnes of this radioactive isotope in the world's oceans continuously emitting beta particles and gamma rays. The few kilogrammes of cesium-134 and -137 deposited in the sea by the Fukushima explosions are a spit in the bucket by comparison.
Seriously? I guess that would be ok if your fish are all pulled from that 'average' ocean.. but of course that's where math and the real world part ways. Fishing is done near coast lines where the continuing discharge of radioactive water is going to create a hot zone to the east of Japan. It makes no sense to talk about worldwide averages when the danger is coming from a single point near a major food source. Big fish eat little fish.. and dilute contamination gets concentrated.. that's what I meant by working its way up the food chain.
The fact is.. the numbers you see in the press releases are based on estimates.. wild guesses actually. No one really knows how much material got released.. what it was or how far it might have gone. Ocean and land surveys are ongoing and as the numbers start to get updated.. we are finding out it was much worse than anyone thought initially. This is not hyperbole.. it's a realistic examination of the disaster that happened last year.. and the continuing disaster of bad policy and bad cleanup methods.