Revered for centuries as the king of fishes, in more recent years Atlantic salmon has become better known as the battery chicken of the sea. The culprit? Salmon farms: once a cottage industry, confined to a few wooden pens run by Scottish crofters, now the country’s fastest growing food production business, responsible for £580m of trade in 2023 alone.

Nearly 30 per cent of all fish consumed in the UK is farmed salmon, and we’re not alone in our appreciation: Scottish salmon is the country’s top food export globally. Yet as with any animal protein, with great demand comes great challenges around ethics and sustainability, and as chefs and consumers have become ever more interested in where their food comes from, the issues have become more pressing.

The problem – as salmon farming’s critics see it – is one of scale. In 1980, Scottish farmers landed 600 tonnes of salmon; by 2023, this had reached 187,725 tonnes. Ireland’s production pales into comparison – around 15,000 tonnes annually – but has still grown exponentially in the past 30 years. In both countries, salmon is farmed in “open” pens – a method of farming outlawed in several countries, but has so far had free reign in the UK.

Anyone who has flown over the west coasts of Scotland and Ireland may well have spotted the ring-shaped pens along sea lochs and shorelines. At first glance, they seem unobtrusive; yet beneath the surface lies a different story.

Every one of these pens will hold between 30,000 to one million adult fish in open nets that allow water in, but also allow the flow of waste, sea lice and chemicals used to treat the lice, out into the surrounding environment with occasionally disastrous consequences.

“Any large-scale food production comes with an environmental impact. It doesn’t matter whether it’s broad beans, cows, chickens or salmon,” says Adam Hughes, a marine ecologist and Professor of Innovation in the so-called “Blue Economy” (the term for the economic activities associated with the oceans and seas).

In the case of salmon, these impacts include faecal matter and uneaten food deposited beneath the pens, sea lice and chemicals used to treat sea lice. “It is carefully monitored by salmon farms, and limits are set by regulatory agencies but the natural environment is relied upon to accumulate that waste,” says Hughes – which, if you think about it, “is no different to terrestrial farming.”

Yet the impacts of terrestrial farming are more readily felt and seen. What consumers haven’t realised – up until relatively recently – is the extent to which the waste produced by salmon farming is affecting not just the surrounding environment, but the salmon themselves.

Lice might sound innocuous, but in fact, they cause huge welfare issues for farmed salmon, effectively eating the fish alive. En masse, lice can kill a fish, and salmon farms are all about mass. In such crowded pens, lice spread as effectively – but far more lethally – as head lice do in schools. The mortality rate in itself is cause for alarm – in February this year, the Fish Health Inspectorate revealed that mortalities on Scottish salmon farms reached 17.4 million in 2023, exceeding those the year before.

But it’s the impact of salmon farming on wild salmon and other species that are a particular source of controversy. As the number of farmed salmon has risen exponentially in the past 50 years, wild salmon populations have dwindled to the point where even seeing one, let alone catching one, is a rarity.

Where once the leaping of wild salmon up the river to their birthplace to breed was a highlight of the Scottish culinary calendar, with fish so numerous that locals could feast for weeks, now many rivers have catch and release policies, or limit the number of salmon that can be kept.

There are even heavier restrictions in England and Wales, where it is illegal to sell salmon caught in the wild. Just last week, figures from the Environment Agency and Cefas showed the number of wild Atlantic salmon caught by rod and line in England and Wales had dropped to a record low – down to 5,399 when as recently as 2017 it was 20,000.

In September, an annual salmon count recorded the lowest ever number of juveniles – that is, young salmon heading out to sea – in Frome, one of the country’s most important rivers for the species. In Ireland, where wild salmon can – with strict regulation – be caught to sell for consumption, wild salmon numbers have declined by almost 80 per cent in the past 20 years.

Admittedly the causes are complex, and by no means confined to salmon farms. “The Scottish Government has identified 13 high-level pressures and more than 40 specific pressures facing wild salmon – and the main issues in Scotland are water quality (sewage and agricultural pollution) and habitat loss,” says Salmon Scotland, the trade body representing Scotland’s salmon industry. It argues that decline in wild salmon numbers has been recorded across the globe, “including on the east coast of Scotland, England and across Europe, where there have never been any salmon farms.”

Indeed, scientists investigating the situation in Frome cited sewage and agricultural run-off as key factors. Other causes cited by the International Union for Conservation of Nature include climate change, which reduces the availability of prey, as well as dams and other barriers in the rivers blocking access to spawning and feeding grounds.

Yet the life cycle of a wild salmon remains so poorly understood, it’s impossible to rule salmon farms out as a factor – especially given their proximity to wild salmon’s migratory routes. From their birthplace in the riverbed to their feeding grounds in Greenland and back again, during their lifetime salmon travel over 2000 miles, through saline water and fresh – and no one knows how they navigate. Regardless of where you stand on farmed salmon, it is extraordinary that humans can even try to simulate this trajectory artificially and at scale.

It is also hard to believe there aren’t consequences for the wild fish. Most farmed salmon are from a limited gene pool and have been selectively bred for growth rate, disease resistance and age of maturation; traits which are profitable for salmon farmers, but disastrous for the few wild Atlantic salmon left in our waters. Both storms and seals have been known to rip holes in the pens, permitting farmed fish to escape; as recently as August, thousands of farmed salmon escaped in Connemara, western Ireland, and were subsequently found in the rivers in which wild Atlantic salmon swim and spawn.

Nor is it just wild salmon that are suffering. Though most livestock farms are working to decrease their dependence on chemicals, the salmon farming industry has done the reverse in its bid to cope with soaring mortality rates. Excessive antibiotic usage is feared to increase antimicrobial resistance among humans, but “salmon farms are the only livestock industry to have increased their antibiotic usage in recent years” says Matt Palmer, once a veterinary surgeon on an intensive Scottish salmon farm, now Campaign Manager at Wild Fish; a conservation charity dedicated to reversing the decline in wild fish populations. Meanwhile, pesticides, also used to combat disease, seep into the surrounding water and can affect crustaceans up to 40km (25 miles) away.

Organic farmed salmon, although stocking density is lower, natural sea lice control methods are used and antibiotics only under authorised control, are not immune from contamination either. “It’s just a net in the sea. However minimal the farm is, there is no way that by-products and lice [from non-organic farms] don’t get into the surrounding area,” says Ed Smith; a food writer and one of a growing number of high-profile chefs to have sworn off salmon in recent years in an effort to highlight the issues around farming.

In fact, the last straw for Smith was not the lice, escapees or chemical pollution – all of which the Scottish Environment Agency has made concerted efforts to improve in recent years. “A lot is being done to reduce lice numbers, and the amount of medicines which farms are allowed to release has been reduced greatly,” says Hughes. Antibiotic use has plummeted, new pens must meet strict criteria to limit escapees and build-up of organic matter, and freshwater baths or symbiotic cleaner fish like wrasse are being used to combat lice, instead of harsh chemicals.

Yet no farm or government agency has yet been able to address the biggest barrier to long-term sustainability, which is the amount of wild fish that must be caught and killed to feed the farmed salmon. The Foraged (Wild) Fish Dependency Ratio (FFDR) is used to measure the amount of wild fish ingredients in the feed, and their yield ratio. On average, the global ratio for farmed salmon is 1.5 – meaning more fish are killed to feed the salmon than salmon is yielded to feed humans. “That crystallised the problem, for me,” Smith says.

“They [salmon] are apex predators, at the top of the food chain. To catch fish like mackerel, wild herring and anchovies – fish that humans can [and do] eat – to produce salmon is a totally inefficient way of farming and eating,” he continues. Substituting a proportion of this with plants – as many organic salmon farms do – hasn’t helped much, because of the environmental cost of growing, transporting and processing the plants into something that offers the nutrients required for a carnivorous species.

There is one company, though, that may have found a way past this seemingly insurmountable hurdle. Loch Duart, which sells via the online fish market Pesky Fish describes its salmon as “the very first farmed salmon that we can qualify as regenerative”, according to Pesky Fish’s founder, Ben King. “The term regenerative is overused, but we judge against three criteria, no matter what the species.”

Loch Duart has reduced its dependency on wild stocks by sourcing its marine content from industry trimmings from MSC certified fisheries. “There is such large volume in fish processing, so there is a huge amount of by-product potentially going to landfill,” he explains. “Using that to feed fish reduces dependence on wild-caught fish.”

King says they have also engaged with local fishing associations to measure stock levels of existing species and marine vegetation around the farm to ensure no signs of degradation.

“One of the ways we gauge the effect of a farm on the surrounding environment is by taking a bit of the seabed underneath the pen and measuring its contents. Conventional salmon farms have to keep moving the pens because they are destroying the seabed beneath. It is dead, not thriving. But Loch Duart is not seeing the same effects.”

Loch Duart is “not perfect,” King concedes. The ethical and environmental implications of sourcing and transporting wrasse to salmon farms is still a bone of contention, as are Loch Duart’s methods of deterring the seals who are constantly finding new ways to break into the pens. Yet “in a murky environment, they are a single beacon” – and striving to improve.

There are others. In Iceland, salmon is now being reared on closed net farms on land, in a controlled environment – a model that may be tricky to replicate sustainably in the UK (though there are plans to try it in Yorkshire) but works well in Iceland, where there is more land, and abundant renewable energy. In Norway, Hughes mentions “technologies being developed where salmon cages are almost fully enclosed; still in the sea but enclosed to the extent that issues around lice and waste material are negated. They are being trialled in Norway, and companies want to bring them into Scotland,” he explains.

So what can we as consumers do in the meantime? Buying Loch Duart seems an obvious choice, but it’s an expensive one. “It’s much more expensive than the local supermarket,” says Harry Lobek, director of London Shell Co fishmonger and restaurant in north London. “Fish isn’t something people should be scrimping on, and the correlation is generally that the higher the quality the higher the price,” he continues – but if £6 a fillet [the Loch Duart price] makes your eyes water, there are more, cheaper fish in the sea. “Gurnard, grey mullet, Whiting, megrim sole etc… I would prefer to sell over salmon. They are cheaper too. But sadly they aren’t as popular and seem to be more of a connoisseur’s choice,” continues Lobek.

There’s too much money at stake for salmon farming not to up its game – and between farming fish and farming meat, the former remains the least carbon-intensive option. “There are definitely ways in which the industry can become more sustainable,” says Hughes, “but it will come at a cost, and someone has to pick up that cost.”

If we want our salmon farms to transition toward a more sustainable way of operating, we must accept paying more for a product which, up to this point, has come cheap at the expense of our sea.