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After years at sea, salmon are returning to spawn in the same forest streams in which they were hatched. The swollen rivers make it easy for them to swim deep into the forest. But the scent of home also draws them irresistibly towards danger. Predators make the most of this banquet of seafood. But none of them compare to the most formidable fish eater of all.

Grizzly bears, just look at this – there's a female here, about 40 metres in front of me, in the shallows, fishing for salmon. Behind her, on the bar over there, she's got three cubs. They're not struggling to catch the fish here. There's such a tremendous surfeit of tired salmon out there. All she really has to do is wander into those shallows until one comes close. And then she can grab it, much to the delight of her cubs. For these cubs, it's the first salmon run. They've got to learn how to catch fish by watching their mother.

Over just six weeks in autumn, tens of millions of Salmon are going to return to these rivers. And during the course of a day one adult bear like this can eat 40 kg of Salmon, and during the course of a Salmon run 1400 kg – that's just one bears intake.

But every salmon caught by these bears increases their chances of survival. They are incredibly important to these bears, particularly at this time of year, when it's essential that they bulk up as quickly as possible before they slip into hibernation with the winter coming. This club hasn't quite got the hang of it yet, but he hasn't got long – the salmon run has only got a couple of weeks to go. Until they learn, mum will have to work even harder. Every salmon caught makes a real difference. These bears, those that are close to a huge amount of salmon, grow 80% larger than those in other areas. They have 25% more cubs, and occur at densities 50 times greater. So Salmon, frankly, are great news for bears.

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Exciting as it is to watch, there's a lot more going on than simply bears catching fish. And the reason is thanks to what happens next.

And the only way to see it is with remote cameras, positioned deep in the forest.

These younger bears have carried fish 30m from the river. Because here they are less likely to be challenged by hungry adults. So they can eat in peace. There is so much fish available, they just eat the richest bits to lay down enough fat for hibernation. The rest appears to be wasted, abandoned on the forest floor! Along with our camera.

The aftermath of this feast is unbelievable. Up to 4 tonnes of carcasses are left in an area the size of a football pitch. but what have dead fish got to do with the forest preparing for winter?

Well, this is where it starts to get really intriguing, because the catching of Salmon is just a start of it. Bears aren't the only creatures attracted by such a feast. A Banana slug.

At 25 cm long, it's one of the largest slugs in the world. And masses of insects.

These flies won't survive the winter, but if they plant their eggs in the salmon's flesh, their offspring might. This flurry of activity eventually breaks the flesh down into simple nutrients that are absorbed into the soil.

The significance of all of this decaying fish goes far beyond it being just a feast for scavengers. Without all of these rotting salmon accumulating here every autumn, this forest would be a very different place. The salmon nutrients in the soil are taken up by the fungi. So this ancient forest is better equipped to face the almighty change that's fast approaching.