When salmon make their challenging way upstream to reach their spawning grounds, they use up all their energy, and their bodies start to shut down. This process results in fish that are rotting alive, hence the term zombie salmon.
Salmon enter fast-flowing freshwater systems and swim up their natal rivers to find a suitable spawning location. The rivers’ running water and habitat provide both plenty of oxygen and shelter for the eggs and juvenile salmon, which increases their rate of survival.
All Pacific salmon and a majority of Atlantic salmon die after they have spawned. They basically stop eating and use up all of their energy when swimming upstream against the strong current. Once they are done spawning, their bodies keep deteriorating until death.
Salmon are really big predatory fish and as such, they have plenty of sharp teeth. While dwelling in saltwater, their teeth are deeply embedded in their fleshly lips, but as they enter freshwater territory, their bodies slowly deteriorate and the full size of their teeth is being exposed.