The thing is that the particles will have dispersed so much before they reach the US (mainland, not Hawaii and Alaska which are a bit closer) that you'll need a very sensitive detector to pick any of it up. The same goes for the fish, the amount of particles in a single fish will be very low (if any at all) and even if you get a few particles ingested, the dose will be very low. Add to this that the short-lived isotopes will be virtually gone in something like 50-80 days and your left isotopes with longer half-life. If you ingest these in a small amount, they will not do that much damage, as their activity is not that high. They will pass through your body with little effect.
There is no threshold dose for developing cancer from radiation, but the probability is considered to have a linear dependence of the dose. Therefore, depending on the radiation exposure, you can calculate how many cancers should have been created by the released radiation. Still, there's always a (very) small chance that you'll develope a cancer from any radiation exposure. The problem lies in all the other factors which humans are exposed to. Smoking, chemicals, background radiation, particles etc. Once the dose is under 100 mSv, it's virtually impossible to draw any conclusions.
To sum it up, the risk of cancer is a lot higher from other sources than from radiation exposure from nuclear fall-out or background radiation.