There is a shocking moment at the beginning of Riverwoods, a new wildlife documentary from SCOTLAND: The Big Picture. A jump cut from a thickly wooded river system in Alaska to the bare hills surrounding a Scottish river hammers home quite how naked the Highlands are. And this, says the documentary, is a crucial factor for Scotland’s vanishing salmon. What salmon need, counter-intuitively, is trees.

Ninety per cent of Atlantic salmon are juveniles, and while what is impacting on adult salmon out at sea might be outside of our control, there is plenty that can be done for them before they leave the rivers. Trees provide shade as river temperatures rise. Their fallen leaves bring nutrients, as do the insects they attract. Log jams protect the young salmon from predators and when the river is in spate. Forests slow water run-off, reducing the impacts of floods that can wash away fish and eggs. And trees encourage beavers, who continue to engineer habitat that is ideal for young fish.

Remarkably, trees need salmon as much as the salmon need trees. As the fish return from the oceans to the rivers to spawn, they carry in their bodies the nutrients they have accumulated at sea. When they die, their corpses feed predators and scavengers, and ultimately these nutrients leach out into the soil, nourishing the trees. It is an astonishing, perfect circle, and one that has frayed to the point of rupture.

Riverwoods is a beautifully shot film, three years in the making, and its core message, that ecosystems must be intact to flourish, is one that needs to be heard. Such holistic approaches to wildlife conservation are increasingly being recognised as essential. Yet this makes it all the more disappointing that the film fails to interrogate the politics and economics that are also driving the salmon to extinction. There is no mention of salmon aquaculture, the UK’s lar-gest food export and an industry set to double in size by 2030, responsible for decimating wild juvenile populations via the transfer of parasites. And apart from one reference to grouse, there is nothing about how Scotland’s system of land management and vast estates maintain these naked hills for sheep and sport. ‘People’, we are told, have done this to Scotland. But that is not you or I. It is the landowners and fish farm executives who cannot see the woods for the lack of trees. Certainly we must reforest, but we must also tackle the root causes that perpetuate these shattered ecosystems.

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Adam Weymouth is a writer and journalist whose first book, Kings of the Yukon, won the Sunday Times Young Writer of the Year Award.