Iodine-131's half life is 8 days; that means in 90 days, it will have diminished to 1/2000 of its previous amount, and will be essentially undetectable in 90 more. You'll lose the season's crops for human consumption, but you might can salvage some value by converting to feed for the animals you'll have to postpone slaughtering for about the same time.

As for wildlife, which ones does Japan have that live long enough to get cancer that might be in the area? Tortises and maybe dolphins, anything unreplaceable?