Their waters are clear and pristine. And still it is not easy to see salmon in the rivers of northeast Iceland.

As in Scotland, Norway and even England and Spain there are far fewer of the “king of fish” now than ever since records began.

The Atlantic salmon is disappearing, and nobody knows why. Now scientists hope they can find out by researching the simplest of all the ecosystems where the fish, and its cousin, the trout, live: Iceland.

Stocks are in decline all over northern Europe and especially in UK waters. Only 3-5% of salmon hatched in British rivers return to breed compared with 25% two decades ago.

In Scotland, the estimated number of salmon returning to the Scottish coast has dropped by more than half in the past two decades.

In 2017, catches troughed. The number of rod catches in Scottish rivers was 49,444 – the fourth-lowest figure since anybody started tallying them all up in 1952.

That is 20% below a five-year average and the fourth-lowest figure on record. And that is despite nine out of 10 caught fish being thrown back in to rivers in a bid to help stocks.

There have long been projects counting fish. Now scientists want to go beyond the numbers game. They are testing a whole new way of managing salmon in an eco-system in Iceland.

They are able to do so thanks to Sir Jim Ratcliffe, the tax-exiled billionaire and chairman of refining giant Ineos, which runs Grangemouth oil refinery.

Ratcliffe has sparked controversy by buying up whole rivers in Iceland – where foreign or absentee ownership is even more controversial than in Scotland. He now holds an estimated 1% of the country.

But the keen angler is eager to show he is not moving in to make a profit. Any surplus he makes from his holdings – land and river – is being ploughed back into salmon conservation. Ratcliffe has teamed up with Imperial College London and the Icelandic Marine Freshwater and Research Institute (MFRI) to see where salmon decline can be stopped or reversed. It builds on an existing project on six rivers he is involved with in Iceland.

He said: “This work is vital to understanding what is happening to the Atlantic salmon and what more can be done to protect it.

“Once we have this information, we can start to put in place measures that will help the salmon not only survive but thrive.”

Ratcliffe has been a long-term advocate for the Atlantic salmon and argues that success here will provide learnings that can be used across the world. The philanthropist added: “I am determined to do everything I can to help protect this magnificent fish which is getting perilously close to extinction. I believe that the Six Rivers Conservation Project informed by the Imperial College and MFRI research will help salmon thrive in these waters, not just in the short term but for generations to come.”

Ratcliffe is now the biggest owner of the biggest salmon fishery in northeast Iceland. Money is being invested in new ladders to let the fish get through waterfalls, in planting vegetation on rivers banks to ensure the immature fish have food – from earth worms to insects – and in programmes to artificially spawn up river.

But at least another half a million pounds is going to wider research by Imperial and MFRI.

One lead researcher is Guy Woodward, professor of ecology and deputy head of life sciences at Imperial College

He said: “At present, the cause for the declines in the Atlantic salmon is not entirely known. There are various factors that might be behind this which we will investigate. The idea is to find one or more smoking guns that might be causing this decline.”

Iceland is not the same as Scotland. There is less food in those pristine crystal-clear rivers than in Scotland’s more fertile waterways. We have more vegetation and a far more complex eco-system. Icelandic fish take longer to mature and spend longer in the river than Scottish ones. The cold-blooded creatures adjust their biological clocks to temperature.

Woodward said a salmon in Iceland can spend seven years as a juvenile in a river; an English one can grow in a year.

Woodward is looking carefully at climate change and how rising sea and river temperatures could be interfering with these balances.

As The Herald reported last week, there is concern that warmer seas will bring trophic mismatch for some fish: so that their larvae will hatch too early or too late for the bloom in zooplankton. Young fish are “born” eating, but only if there is something for them to eat.

Work by Marine Scotland has focused on the diet of the sand eel, the main food for another iconic species in Scotland and Iceland: the puffin.

Woodward, however, reckons that building knowledge from the simpler systems of Iceland could provide clues for the more complex ecologies of Scotland.

Ratcliffe has been criticised in Iceland. “To portray Ratcliffe as a saviour of the salmon rivers in the northeast [of Iceland] is at best misinformed, at worst profoundly patronising to the farmers who have lived and bred salmon all their lives and whose livelihoods have partly depended on the silvery fish,”said Sigrun Davidsdottir, Icelandic TV’s London correspondent, in her Icelog blog. “But the fact that Ratcliffe has the funds to follow his passion cannot be disputed.”

Friends of Ratcliffe insist his passion and financial power are real enough. “It is absolutely not green-washing,” said Peter Williams, Ineos group technology director. “It is hard to say that without sounding defensive. But this project comes from a genuine interest in conservation.”

His boss, he said, is simply fascinated by the salmon and the people who support its eco-system.

Under Ratcliffe, farmers in Iceland – a country better known for abandoned smallholdings than working ones – are returning to traditional methods.

“The Atlantic salmon population has fallen to one-quarter of its 1970s level,” Williams said. “Most species with this level of decline would be categorised as endangered. This is an internationally important research and conservation programme, and the largest ever, never having been undertaken on this scale before.

“With the high level of science being applied here and the significance to nature conservation in general, we hope the governments of the countries concerned will also support the project.

“By undertaking this project Iceland will become a global centre of excellence for salmon conservation. However, the findings from this research and applied conservation initiatives are being networked across the science and conservation communities to help recover salmon populations in the UK, US, Canada and Scandinavia.”

Williams is hopeful the Icelandic government – which has blocked other foreign buyers – will get behind it. The work in Iceland, he said, was “holistic”, as much about the land as the rivers, adding that getting buy-in from the community and local and national authorities was essential.

He recognised that land ownership was a “topical subject” but added: “We have no plans to expand beyond northeast Iceland. Our project needs the support of Iceland. We’ll be sad if we don’t get it.”