Zombie Fish
A Halloween episode about a particularly macabre natural phenomenon.
EPISODE NOTES
TRANSCRIPT
Today we’re taking a quick break from the Rideau for an episode in honour of spooky season but we’ll be back next time with more stories about the canal.
Every fall, right around Halloween, rivers across Ontario… are filled with zombies.
An army of rotting creatures swarms into the waterways from deep in the Great Lakes, mindlessly, obsessively focused. Innumerable, unstoppable. It’s one of the creepiest and most impressive phenomena in Canadian wildlife: the salmon run.
Salmon spend the majority of their lives in open water. But they return to freshwater rivers and streams to spawn. And that is when things start to get horrifying - thanks to programmed senescence.
In short, their bodies give up on absolutely everything except getting them up the river to the spawning grounds.
They barely eat. Their immune systems are suppressed. Their organs shut down. They start literally rotting from the inside out. They turn into zombies. On a good day that might mean just spreading, open sores. On a bad day, their eyes can fall out of their heads. Every ounce of their energy is dedicated to moving them upstream - and none to keeping them alive.
It’s a sacrifice they have to make - they’re not just fighting the current, they’re hitting rocks, rapids, and throwing themselves up waterfalls, desperately trying to reach the spawning grounds.
So no surprise, they tend not to last long after spawning - if they even make it that far.
But the horror does not end here. Because now, these rivers are filled with dead salmon. And the ecosystem proceeds to absolutely feast on their corpses.
You probably know bears rely on salmon for food. They are just the tip of this creepy iceberg. But they do serve a very important role - bringing the salmon out of the river and into the riverside ecosystem - a lot of salmon, sometimes up to 4000kg per hectare. They’ll often leave their meals half-finished, and that’s when the rest of the food chain goes to work. Invertebrates descend en masse, flies lay millions of eggs that hatch into millions of maggots. One study counted 137 species that benefit from these corpses.
But maybe the weirdest beneficiaries are the trees. Plants depend on nitrogen, and salmon have a lot of it. There’s a huge impact on trees near salmon rivers - sucking up the nitrogen from those salmon corpses lets them grow taller and faster - up to three times faster. Around salmon-rich rivers, the dead fish can provide 40-80% of the available nitrogen. And thanks to the bears, trees up to 500m away from the river can benefit.
So the next time you’re out on a beautiful fall hike next to a babbling brook, spare a moment of reflection for the beautiful cycle playing out all around them. An army of rotting zombie fish dying in countless number, being dragged from the water and half eaten, left for armies of maggots and finally having the nitrogen in their corpses slurped up by those stunning and thriving autumn trees. Happy Halloween.