THE SWIMMING DEAD

From USFWS

They are in fact in real life animated rotting corpses which engage in homicidal frenzy and are obsessed with only one thing. It is not, however, eating brains--it is sex. 

Salmon from the Pacific migrate from the ocean into the stream in which they were born in order to spawn. The journey is not a pleasant one. The male salmon's jaw grows into a tooth-filled hooked weapon, called a kype, so that he can go medieval on other male salmon. Their color changes from a bright ocean silver to red or random streaks of purple or black. Their flesh and muscles deteriorate and fall off as they encounter rocks and waterfalls. By the time a salmon makes it to its spawning area (if it gets that far), much of its body will be moldy and rotten and the spines will be sticking out of its fins. These gangrenous tattered beings are piscatorial versions of the ghouls so vividly depicted in movies and TV, and their decay rivals the special effects from Hollywood. They usually do not survive past this first spawning.

It is for good reason that the U.S. Fish and Wildlife Service describes them in this video as "Zombie Fish--The Swimming Dead."

From USFWS