Later this month an alarming statistic will be published by a quango north of the border, Marine Scotland Science. It will tell us how many wild salmon were caught by anglers in Scottish rivers in 2014. It is likely to show that the number is the lowest since records began.

Wholly unnoticed by the public at large in the fantastic political hurly-burly of the referendum last year, something catastrophic seems to have happened to stocks of the great migratory fish, which is as much a part of Scotland’s wild identity as the Munro, the stag, the heather and the golden eagle.

The five-year average for the annual rod catch of salmon in Scottish rivers between 2009 and 2013 was 84,897. But the figure for 2013 itself was 67,468, and according to detailed soundings by the Salmon and Trout Association (Scotland) the rod catch for 2014 was between 40,000 and 45,000, which, if borne out by the official figures to be published on April 24, will be the lowest since records began in 1952.

We are talking an astounding 50 per cent decline, more or less, in just five years, and if we look at the 2014 figures that have already been published for one of Scotland’s greatest salmon rivers, the Tweed, we see something more astounding still. They show that 7,767 salmon were caught by anglers last year, compared with 14,794 in 2013, a scarcely believable fall of 47 per cent in just 12 months.

It is starting to look like the beginnings of a collapse, and this is despite the fact that around 80 per cent of salmon caught by anglers in Scotland are now released rather than killed (and on some rivers this figure is 100 per cent). Someone keeping a close eye on the situation said to me yesterday: “The writing appears to be on the wall for the Scottish salmon.”

There may be an ominous reason for why this is happening. Two major problems have already been highlighted. One is the extensive salmon farming, along the west coast of Scotland especially, where the farmed fish are affected by plagues of sea lice, which in turn can badly affect the wild fish swimming past.

The other is the indiscriminate take by netsmen of fish returning to spawn in their native rivers after their migration to the seas off Greenland.

But some experts think the main problem lies elsewhere: that it is a question of “marine survival”. Fifty years ago, for every 100 smolts, or young salmon, which went down to the sea in Scotland, about 25 to 30 came back to the rivers of their birth. Now that number is more like five. And the reason may well be climate change, as the young salmon’s prey species in the Atlantic ocean are moving north and out of their reach because of the warming seas.

The Scottish Government is seized of the situation: from next year you will be obliged to buy a licence to kill any salmon north of the border, with a rod in a river or a net in the sea. But the Holyrood Parliament cannot do much about global warming. Could this be one of the first major effects of climate change on the natural world in Britain?

The North Atlantic salmon, known as the “king of fish” for its power and beauty, is not only one of the most charismatic of all wild creatures; it is an icon of aquatic purity. It can only flourish in water with a high dissolved oxygen content, meaning that if you have salmon in your river, the ecosystem is probably in good shape: it’s a key environmental indicator.

It is also a magnificent cultural icon: a major component of the image of unspoiled wild Scotland. To lose it would be unthinkable. But something is clearly happening now, which means that the unthinkable may come to pass.