If it's indeed leftovers from the tests, not leaks in containment, this is a good thing. Let the clams collect the waste, harvest and dispose of them safely, and replace them with seed stock farmed elsewhere.
Obviously not having done the damage in the first place would be better, but that ship sailed in the 50s.
Is it possible the damage caused by these animals being irradiated is less impactful than the damage caused by over harvesting? That is, if the radiation is scary enough to prevent people from harvesting them.
Gives one ideas about a novel with a plot based on an insane ecoterrorist group that poisons huge stretches of the earth with radioactive isotopes in a bet it causes less damage than the humans would have.
Giant clams have an average lifespan of 100 years. They are filter feeders. The nuclear testing was done in the 50s and 60s. They were sucking up the fallout for decades.
Obviously not having done the damage in the first place would be better, but that ship sailed in the 50s.