If fish were contaminated by radioactivity, would it spread to other things if it sat out?

If the fish: was irradiated, it would not retain any radioactivity. If the fish ingested a radioactive substance, it could potentially irradiate or contaminate someone. It would depend on the specific radioactive material ingested.

Not likely: the contamination would have to seep out of the fish. Just wash it off and bury it to get rid of it. if it sat out you can't eat it anyway. If you think it is contaminated, leave it alone and dispose of it.

Radioactive food?: If the fish had ingested something radioactive ie another fish or plankton contaminated with seafood - the radiation in in the fish's tissues - it won't leak out or spread to other food items but could degrade the food simply by being in contact (think of it as something hot - it will make things around it warm but if you move them away they are cool again