You probably won’t have heard of the Urr or of the great “greybacks” that used to run up it at the back end of the year when the last of the swallows were leaving and the oaks were turning gold. It is not the sort of grand river that ruddy-faced sportsmen stay up late eulogising in Clubland. But local people still talk of a time when anyone who spent a couple of hours casting a fly across the fast-running water, after a night of heavy Dumfriesshire rain, was in with a good chance of catching a salmon for their supper.

Almost a decade ago, I sat on the east bank with my godfather, drinking a can of McEwan’s Export after a fruitless morning of hard fishing. The only snippet of conversation I remember was him asking what I was going to do when I grew up. Even now I still don’t know but, on the basis that I’d recently written a piece for the school magazine about the fifth-form trip to Stratford, which I was told went some way towards making up for my behaviour during King Lear, I said something vague about journalism.

My godfather, who has farmed Atlantic salmon all his days, was disappointed, telling me that journalists misrepresent his industry and lie about its supposed environmental impact. On draining the dregs of my McEwan’s, I waded woozily downriver but by mid-afternoon, we still hadn’t seen the faintest flicker of a fin, so we packed away our rods.

As Scotland’s biggest food export, farmed salmon would be economically vital if the SNP do ever get their independence day

While people have been tinkering with aquaculture for thousands of years, it was only in the 1970s that salmon farming really boomed. In the beginning, there were just a couple of sites in Scotland but people soon realised there was money to be made and, 50 years on, 200 fish farms are dotted around the coast, collectively producing 150,000 tonnes of salmon a year. Some of the fish goes into the one million salmon-based meals that are consumed in the UK every day, with the rest being sent abroad.

As Scotland’s biggest food export, farmed salmon would be economically vital if the SNP do ever get their independence day, resulting in the sector having powerful friends in Holyrood. In 1997, the production of farmed salmon surpassed the total wild harvest, which was pivotal in the fish transitioning from being an occasional luxury to a cheap three-times-a-week sandwich filling. The truth, however, is that out of sight the planet is paying through the nose.

Due to depleted stocks, there are now no commercial wild salmon fisheries in Scotland and the tiny number of netsmen hanging on in England are fast being legislated out of existence, meaning almost all salmon currently bought in Britain will have been raised in tightly packed cages.

In the early days of the industry, its pioneers were praised for taking the pressure off the wild population but we now know it was a false dawn. The well-documented consequences of salmon farming range from commonly-used chemicals poisoning the surrounding sea, to farmed escapees hybridising with wild fish, which dilutes gene pools and creates offspring that don’t make it to adulthood.

It would be reductive to suggest we didn’t see any fish that day on the Urr because of aquaculture but the west coast of Scotland is plagued with salmon farms and it has been shown that where the industry operates, wild stocks are up to 50 per cent lower than they should be. But it is important to recognise that farming is far from the only reason that salmon are swimming towards extinction.

Back in the early nineteenth century, numbers were already plummeting due to overfishing. Between 1815 and 1820, on the famous River Tweed, an average of 120,000 fish were caught annually. Those that escaped the nets had to contend with chemicals being pumped into the river by textile mills. The picture was a similarly destructive one on many salmon rivers throughout Britain.

Viewed in the context of our relationship with salmon down the centuries, it becomes clear that the industry is not so much the problem as it is a destructive attempt at a solution. By the mid-twentieth century, we had trashed our rivers and plundered the salmon that lived in them, so we thought we’d just make more but our attempts are fast becoming the final chapter in a very sorry tale.

I recently posted a picture of some woodpigeons, a serious farm pest, on social media. In the first picture I was about to pluck them and in the second, they’d just come off the barbecue. Most people said kind things about them looking delicious but there were a few bolts from the blue wishing me all sorts of topical misery.

Yet I’ve never seen threats of physical violence heaped upon he who dares tweet a picture of salmon en croûte, and the canapés at any given Christmas party make it clear that we’re guiltlessly fond of the smoked stuff.

This sorry paradox is no great mystery. It’s just another of modern man’s ethical blind spots born out of how far we’ve drifted from the realities of food production and the wondrous intricacies of the wild world.