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Far away to the West, the Raincoast forest is coming back to life. Thanks to the richness of the autumn salmon run, the bears have survived the winter. They've now returned to the forest, looking for something to eat. They'll survive on vegetation until the next salmon run. The emergence of the bears is a cue for scientists to conduct a rather risky experiment.

They need a large hungry, bear. It's the only way to measure the impact of almost salmon on this ecosystem and to understand why this vast, ancient forest has thrived for so long.

For such a big question the methods employed by senior researcher Chris Darimont seem a bit curious. He's equipped with a can of old salmon guts, effusing, probably, the most disgusting smell known to man. But he hopes the bears are going to love it.

He's made an aerial lure. So, the wind will carry this distinctive perfume deep into the forest. Now this around the area with barbed wire. And it's this that they hope will collect what they are so interested in – a single hair from a visiting bear.

Now the site is prepared, it's time to set some remote cameras, and beat a hasty retreat.

Personally, I'm very happy to watch from a safe distance – it's not the smell – some of those bears are huge! Look at the size of this bear! As a trap this is the perfect bait, it's working brilliantly. The bears have come in, and they're snagging themselves on the wire there.

And that's just what we want. Fresh out of hibernation, it seems they can't resist this pile of stinking salmon. This one is even rolling around in the stuff. No doubt it values the scent – I'm not sure we would. The bear's coat has been growing for nearly a year. But soon it will be moulted and lost. The more bears we can attract the better.

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Now the coast is clear, and it's time to retrieve any fur from the barbed wire. So, what on earth can a hair tell us about this forest ecosystem?

Well, hair is made of protein, sourced from whatever the bear has been eating over the last year. And by analysing this hair, science can reveal an astonishing level of detail about a bear's life. We can learn so much from a single bear's hair. So I know, for instance, that this one has come from a female grizzly. I know exactly what it's been eating even on a week-by-week basis, where that food has come from and even the impact on the quality of its life.

So this bear has been getting most of its protein, not from the forest around here, but actually from the deep ocean, via the salmon. And we know that throughout the course of the year 80% of that bear's protein has come from these salmon.

And that's surprising because, remember, they're only available to the bear for a few weeks during the autumn. And yet the impact is clearly lasting all year.

So, how come the bears appear to be so full of salmon?

Well, back in autumn, we saw the bears scattering fish carcasses all around the forest floor. As the protein in those rotting salmon broke down, nitrogen from it accumulated in the soil. And this salmon nitrogen is like fertiliser!

So in spring, nutrients all the way from the ocean gradually appear in all the vegetation growing here. Just in time for the hungry bears to eat as they emerge from hibernation. And hungry bears have huge appetites – they'll eat a third of their body weight every day! That's why their bodies appear to contain so much salmon.

But the impact of this ocean-born nitrogen extends far beyond the bears and their food. This particular form of nitrogen can be found in almost all of the animals and plants that appear here in the spring.

These Rufous hummingbirds have migrated to the forest to breed. They're drinking nectar from plants that have been fertilised by rotted fish. So, they'll carry the same salmon nutrients with them as they fly through the forest.

Many of the insects pollinating the plants now were incubated in that decaying flesh back in the autumn. As they themselves are eaten, the salmon nutrients are spread even further.

Thanks to the bears, the insects, and the birds, this salmon fertiliser spread deeper and deeper into the forest – sometimes as much as 800m from the river, and this pulse of nutrients then allows the organisms which define the forest itself to prosper – its trees.