The Ontario Salmon Run

TRANSCRIPT

Every fall, Canadian rivers are filled with zombies.

Here, they gather from across Lake Ontario into an unstoppable horde, pouring into estuaries and churning upriver. 

A rotting, roiling, single-minded swarm racing with uncanny precision.

It’s one of the most singular events in Canadian wildlife. And while British Columbia might be more famous for it, it’s happening at the same time in Southern Ontario’s backyards: the salmon run.

These salmon were born in these very same spots, years and years ago. And they spend the first stages of their lives here, growing and feeding on invertebrates until they finally graduate from their river nurseries into the wide open waters of Lake Ontario.

This is where they mature over several more years into full-fledged adults. And at a certain point, thanks to a mix of temperature, hormones, growth, age and energy accumulation, they get an overwhelming biological message: it’s time to return home to spawn.

When they do, they have an incredible ability to hone back in on the specific river or stream they were born in. 

And we’re still not a hundred percent sure how exactly they do this. But our best theory is that it’s a combination of two navigational systems, one broad and one specific.

To first get the salmon from that very wide open lake to the area of their natal rivers, we think they rely on the earth’s magnetic field.

In 1988, researchers actually found single domain magnetite in the skulls of sockeye salmon. After lots of hypothesizing, this finally provided the mechanical link for how the fish could interact with the magnetic field. 

In brief: the magnetite is a tiny, permanent magnet, like a compass needle. When it tries to physically align with the earth’s magnetic field it creates pressure that can be picked up by pressure-sensitive cells in the salmon and sent as signals to the brain. So the salmon can have a constant internal compass, a sense as reliable as sight or smell.

And speaking of smell: once the salmon reach the area of their natal rivers, we think precision navigation relies on scent. 

Individual rivers and streams have a unique make-up of organic matter and minerals which can differentiate their smells, and salmon could use this scent-trail to get them all the way back to the exact spot where they were born.

But as impressive as that navigational miracle is, it is not even close to the only challenge they face.

This will be the last, most arduous journey of their lives. Literally and figuratively swimming upstream, they’ll fight rocks, rapids, predators and waterfalls, just to name a few. We’ve recorded salmon making vertical leaps as high as 3.6 meters to clear obstacles.

Overcoming all of that means putting every ounce of energy they’ve got into moving forward. And that brings us back… to zombie salmon.

Most salmon species are semalparous (seh-MAL-per-us). That means they reproduce once and then die. And that means when they’re on their way to the spawning grounds, getting there is the only thing that matters: they’re dead either way.

Because of that, they’ve evolved programmed senescence: intentional immunosuppression and biological changes to prioritize forward movement at all costs.

Those changes can show up physically as hooked jaws or pronounced humps. Internally there’s a shift in swimming muscles to favour bursts of speed as the salmon’s organs shut down to divert more energy to locomotion.

This, along with the immunosuppression, makes them much more susceptible to infection and fungal growth. They can start to literally rot from the inside out, eventually showing open sores, decaying fins and eyes falling out of their heads. Zombie salmon.

If and when these exhausted, living-dead fish finally do reach their spawning ground, females use their last ounce of strength to dig a nest called a ‘redd’ by kicking up sand and gravel with their tails and depositing around 5,000 eggs. 

And males use their last ounce of strength to fertilize those eggs and then defend the redd from predators and other salmon until they just physically can’t anymore.

And then… that’s it. 

Only… it’s not. Not even close. The lives of these salmon may be over, but their impact is just beginning.

So by one perspective, these rivers have just been filled with rotting corpses.

But by another perspective, they’ve just been filled with an absolute smorgasbord of free food for the whole ecosystem.

And maybe most visible signs of this vitalizing force are the flocks of gulls who arrive en masse with the salmon.

Watch them individually and you’ll see them ‘surface plunging’, making repeated little dives around the salmon bodies.

You might think they’re doing this to snatch off pieces of salmon. But there’s actually a problem there.

See, salmon skin is quite tough. And it takes a powerful beak to break through it. Smaller gulls would have a very hard time even getting to the meat inside.

So why swarm the rivers for food they can’t get to? Well, they can - and do - wait for salmon to decay, but fortunately for them they don’t even have to. 

There’s a clue in the fact that when the gulls do feed directly on the salmon, they seem to overwhelmingly target females.

Why? Because there’s another super-concentrated source of nutrition just below the surface: salmon roe.

Eggs are a nutritional bombshell. They have to be! Their whole purpose is to keep an embryo nourished until it can fend for itself. So they’re a hot ticket for predators everywhere, all the way up to us. Some species have evolved to specialize in going after eggs.

And here, the water’s full of them. They can be jostled out of redds by the current, other salmon trying to use the same area to nest, or by the gulls themselves probing for them.

So these smaller gull species don’t even need to rely on the salmon bodies themselves - they can make a feast out of devouring roe. It’s estimated that, in part because of voracious gulls, only 10-30% of salmon eggs will actually make it to hatching. 

But even these flocks of gulls are just the tip of the ecological iceberg.

As the salmon carcasses at the shore start to decompose they become perfect nurseries for invertebrates. One study counted 60 species that make use of them to lay eggs and nourish larvae - to the point where those salmon can become seething masses of newborn life. At least 50,000 maggots per carcass, who can reduce them to bones within five days.

And zooming out, it seems like nearly every single animal in the area benefits. 

For example there’s evidence that small fish species are, on average, larger in salmon rivers.

Areas with salmon runs also have higher numbers of bald eagles.

Minks may actually time their breeding based on when the salmon show up.

And of course bears might be the most famous beneficiaries. But they play a much larger role than just getting a decent meal.

Bears haul salmon out of the river and into the forest, ensuring an even wider swath of the ecosystem benefits - to the tune of up to 4000kg of salmon per hectare.

Bears tend to be selective about what they eat. It’s estimated they leave fully half of the salmon they catch on the forest floor. 

That makes them what we call ‘ecosystem engineers’. One study counted 3,611 salmon - 63% of an entire salmon run - that had been transferred into the riparian and forest ecosystem by just 3-8 black bears, to the benefit of every other living thing.

Including maybe the most surprising of all: plants.

Thanks to the bears, plants as far as 500m away from the river can benefit from the high nitrogen content in the salmon. In some riparian ecosystems they can provide almost a quarter of all available nitrogen.

Trees actually grow significantly, measurably taller along the shores of salmon rivers.

So, the wild life cycle of the salmon themselves would be impressive enough on its own, from their initial journey out of the rivers into open water, to their remarkable navigation back years later, to the absolutely harrowing fight to get up those rivers as their bodies break down around them. 

But as we’ve seen, that ain’t even the half of it. And man, their timing could not be better. The salmon return to their rivers in late fall - just when pretty much every other animal is desperate to stock up on energy and fat reserves to migrate, hibernate or just power through the oncoming winter. 

In death these salmon are an absolutely massive resource transfer from the open water of the lake to the river to the riparian habitat at the shore to the forests beyond. On the way, that influx is estimated to benefit at least 137 species - giving them a good meal, a nursery, letting them grow tall and strong or survive a harsh winter. 

The salmon’s strange life cycle and biologically encoded, semalparous death in turn brings new life all around them.