Zombie Salmon Literally Rot Alive After Spawning

After they spawn, Pacific salmon turn into zombies. They remain alive, but their skin falls off as it rots and their bodies begin to slowly shut down. But while they appear to be the "living dead," these rotting fish actually provide huge benefits to the surrounding ecosystem.

After birth in freshwater streams, rivers and lakes, Pacific salmon journey to the North Pacific waters around the U.S. and Canada, where they spend most of their lives.

Every year, a biological clock tells the fish it is time to return to the freshwater to spawn. This will be the last journey they ever make. Once they spawn, the salmon die. And this process takes a relatively long time, causing them to swim around as their bodies slowly deteriorate.

Katrina Liebich, Fisheries Outreach Coordinator at the U.S. Fish and Wildlife Service, told Newsweek that other fish species experience mortality after spawning, but only salmon enter this "zombie-like decline." She said "every single one" goes through this process after spawning, making the natural phenomenon particularly unique to the species.

Zombie salmon
A picture shows a zombie salmon in Alaska beginning to show signs of decay. Katrina Liebich

As salmon journey back to the streams, they go through dramatic changes. The fish begin to change color—this happens as the pink pigments in their fillets move towards the surface of their skin.

"The salmon you get at the store are fillets from salmon caught at sea—so they've still got that chrome skin and the pigment hasn't moved from the fillets to the skin," Liebich said.

They then begin to use up all their energy. Males use it to grow extra-large teeth to compete against each other for a mate, while females will use up all their fat to produce the eggs. They go through this process until they literally have no energy left.

Once their eggs are laid, salmon stop eating and no longer have the stamina to return to the ocean. They continue to breathe and swim, as their internal organs begin to deteriorate, and their skin begins to rot and fester.

"They'll continue to decline and get moldy on their skin as time goes on after they spawn," Liebich said.

Zombie salmon
Zombie salmon keep swimming as their bodies deteriorate USFWS/Jan Boyer

After they lay their eggs, the zombie fish only stay alive for a few days or a few weeks at the most.

The swimming dead provide huge benefits the ecosystem of the Pacific Northwest.

Liebich said that when the zombie salmon eventually die, they provide food for scavengers. Depending on where the bodies end up, they can provide an easy meal for bears and even insects if they wash ashore. Some carcasses may even float back to the ocean, and give back the nutrients they used to fuel their last journey home.

These zombie salmon are not harmful humans and Liebich said they would even be safe to eat.

"People do catch and eat salmon when they return to freshwater, and the fillets are fine ... The fillets are highest quality when caught at sea—bright red or pink depending on the species." she said. "[Zombie salmon] will have burned all their fat reserves so they just won't be as good, and maybe a little more watery/less firm."

Zombie Salmon
Before they die, salmon go through drastic changes. Males grow large teeth before their bodies give in. USFWS/Katrina Liebich