Here's why:
* Fish don't smell bad when they're alive: A fresh fish doesn't have a strong odor.
* Spoilage causes the smell: The smell we associate with fish comes from bacteria breaking down the flesh after death. This happens faster in warm temperatures.
* The head can decompose faster: The head has more delicate tissues and can decompose faster than the rest of the fish.
So, the saying is a bit of a joke, but it does have a grain of truth in that the head of a decomposing fish will likely smell worse than the rest of the body.