(geek hat on)
The problem with radioactivity is, it depends on which isotope. Iodine 131 is the stuff that goes to the thyroid. This stuff has a half life of 8 days. So even if it did get into fish, it would decay rapidly to inert (stable) elements.

On the other hand, you have strontium 90, which has a half-life of 28.8 years; and cesium 137, about 30 years. Sr 90 is the isotope which is like calcium and goes into the bones; and Cs 137 is in the sodium group. These are what made the milk dangerous after Chernobyl. It is the longer half-life isotopes that are the worst, especially since they like to get into the system and hang around, causing great amounts of damage.

(geek hat off)