Piranha Aggro Range

I'm not sure if this is a bug or not, but the piranhas aggro onto the player from ridiculous distances. My render distance is set to ten, and I've seen piranhas five chunks away aggro onto me. This means every piranha in the area will immediately swarm me, to the point of just being annoying. 

As a side note, it's a bit weird that they spawn in the ocean. Especially the frozen ocean.  