France accounts for 62% of all Scottish salmon imports, with the fish representing the UK's largest food export by value and on course to reach £1bn this year.

Maxime Carsel is an investigative journalist and documentary filmmaker who has written a book, published this week, about the industry.

Titled Un poison commé saumon (A Poison Called Salmon, the title being a play on 'poisson', the French word for fish) it represents "a plea for a change of model and an urgent awareness of the future of aquatic ecosystems, coupled with a reflection on our relationship to consumption".

He hopes to eventually publish the book, the preface of which by Paul Watson calls the industry 'an ecological crime against humanity', in English in the UK.

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Mr Carsel tells The Herald: "We French are among the biggest salmon eaters on the planet, without being producers.

"We import 99% of our salmon and we are the largest customer of Scottish salmon farming. On average, each French person eats four kilos of salmon per year per inhabitant.

"In France, we've seen and heard everything about salmon, but no book ever explains how the salmon you eat is produced.

"A French scientist friend who lived in isolation in Durness and spent his time collecting plastic pollution from beaches from all over the world told me, 'pollution here is the background noise of humanity'.

"Among this pollution was that from salmon farms whose cages had been destroyed by storms. Pipes, ropes, various plastics...

"Salmon production companies spent no energy, no budget, cleaning the beaches of their own pollution. From there, I began collecting information and considering writing a book."

Figures released earlier this year showed fish farming companies had the second highest number of breaches of any industry regulated by SEPA.

Un poison nommé saumon, a new book about the salmon industry(Image: Rocher Eds Du)

While concerns have been raised about the environmental impact of fish farming, it's not just domestically that the impact is felt.

Farmed salmon are fed on a combination of fishmeal, oils and vegetable material, as well as an antioxidant, astaxanthin, to turn the flesh pink.

Astaxanthin is found naturally in the wild in the shrimp and small fish salmon consume, and were it not added to farmed salmon their flesh would be grey.

The fish they do consume, in the form of fishmeal, are often fished from the waters off Africa and South America, devastating the local industry.

France24 reported that Senegalese fishermen were fleeing their native land and heading to Europe due to the overfishing of the Round sardinella.

Gaoussou Gueye, president of the Confederation of Traditional Fishermen in West Africa, told the outlet: "The demand for fodder fish to feed salmon and carp has generated an extremely voracious industry that has monopolised fishing resources, because it takes five tonnes of sardinella to produce a single tonne of fish."

In total, around a quarter of wild fish caught globally is ground into meal. Salmon Scotland says no Scottish salmon farmers use marine ingredients sourced from West African fisheries. Campaigners such as Feedback say complex supply chains mean "there is very little transparency for consumers and independent observers such as Feedback to interrogate these kinds of claims".

Farmed salmon is the UK's top food export(Image: PA)

Mr Carsel says: "To produce three million tons of salmon each year worldwide, the global industry needs food, which is primarily forage fish.

"We measure the 'fish-in:fish-out' (FIFO) ratio, that is, the ratio between wild fish 'inputs' and farmed fish 'outputs'. If the FIFO ratio is greater than 1, it means that a certain amount of fish was caught to produce a smaller amount. This is not a sustainable model.

"Currently, for one kilo of salmon produced, it takes between two and four kilos of wild fish. This gives a sense of the scale of the disaster, especially since the countries suffering from this overfishing make their living from fishing.

"The small sardine called yaboye (in the local dialect) comes every year to reproduce and lay its eggs off the coasts of Mauritania, Senegal, and Gambia. Today, because this fish is overexploited, traditional fishermen bring in their nets empty and watch helplessly from afar as trawlers devour the sea before their eyes.

"The same scenario is occurring in Peru, where the city of Chimbote, once known as 'The Pearl of the Pacific' thanks to its wealth from anchovy fishing, is now a port where the seabed is covered in mud, depriving biodiversity of oxygen. This is due to the factories that process anchovies into salmon feed. As for the anchovies that swim offshore, they are in danger of extinction."

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Another key ingredient in fishmeal is soy beans, which is used to reduce or even replace wild caught fish as a protein source, given it's far cheaper.

Brazil is the world's largest exporter of soy, and earlier this year reversed a moratorium on its cultivation by deforestation of the Amazon rainforest, potentially opening up an area the size of Portugal to destruction by farmers. Salmon Scotland said the sector is committed to full traceability of all feed ingredients, including soy, and only uses fishmeal and fish oil from responsibly managed and certified fisheries.

Whatever the concerns may be, there's no question salmon farming is lucrative. Scottish exports were worth £844 million to the UK economy in 2024, over half of the 101,000 tonnes going to France.

In addition, with farms typically established in rural areas, they provide well-paid and highly-skilled jobs for communities facing the threat of depopulation.

Mr Carsel says: "What I discovered, but which I'm sure will come as no surprise to you, is the stranglehold multinationals have in areas where they 'produce' salmon.

"For example, in the Orkney and Shetland Islands, the Cooke company has become the backbone of society.

"The company pays for nursery equipment, provides kits for football teams, organises raffles to support local associations, builds playgrounds, sponsors marathons, and recruits heavily. It acts as a local monopoly, with all the benefits and dangers that entails. If it were ever to leave, everything would collapse."

Cooke's own numbers would appear to support this assessment.

A salmon farmer at the Strondoir Bay fish farm at Loch Fyne(Image: PA Archive)

Its jobs pay almost 25% more than the Orkney average and 8.6% above the Scottish average, and it invested £18.27m during 2016-2021 period in Orkney, as well as giving £267,714 in donations to music and culture; education; health and social care; and sport.

Mr Carsel says: "German economist Ulrich Beck rightly wrote that the presence of a single, powerful local company increases the risk of job loss and the danger of precariousness becomes omnipresent. This is the risk for society which, over time, is not viable."

It's not just at the local level that the salmon industry holds significant sway - it's been seen on a national level too.

Sea lice are a long-established issue in aquaculture.

The tiny, translucent copepods attach themselves to the skin of salmon and other fish, feeding on their skin and blood causing pain, distress and, in extreme cases, even death.

The industry uses pesticides to treat the problem, but there are concerns around the wider environmental impacts.

One of the products used to deal with sea lice is emamectin benzoate (EMB). In 2017 the Scottish Aquaculture Research Forum conducted research which found that high usage of EMB could reduce the numbers of mobile crustaceans in surrounding waters by up to 83%.

Initially SEPA proposed a complete ban on its use, but dropped that under pressure from the Scottish Government.

Instead it imposed an interim limit of 12 nanograms per kilogram (ng/kg), which applied only to applications to increase existing discharges or for new fish farms.

Following sustained pressure from the industry the limit has now been set at 272ng/kg, 23 times higher than the interim, and will not come into effect until 2028. Salmon Scotland said use of veterinary medicines is tightly regulated by SEPA under precautionary environmental standards based on the latest science.

Activists opposing salmon farming have also found themselves feeling the pressure of a multi-million pound industry.

Corin Smith and Don Staniford were the subject of a report produced by private intelligence firm Blue Square Global for the Scottish Salmon Company, now owned by Faroese company Bakkafrost.

Mr Carsel says: "When I visited Scotland I didn't get too close to the salmon farming activities because I knew it could lead to... complications.

"I contacted those who had visited the various production sites to film and gather evidence, such as the tireless Don Staniford.

"It's important to note that Scotland is one of the countries in Europe where opposition is strongest. But Scotland is also a place where intimidation is very prevalent.

"The case of Corin Smith, activist photographer and founder of Inside Scottish Salmon Feedlots, is quite unique. In December 2021, he posted photos of a GPS tracker stuck under his car. Internal documents from the company in question showed that a former director had requested an intelligence report on the activist.

"This report included analyses of Corin Smith's movements and behaviour, monitoring of his social media accounts, and photos of his home. It was a scenario straight out of a Netflix series."

Salmon Scotland said: "These allegations categorically do not apply to Scottish salmon farming.

"Scottish salmon is globally recognised for its world-leading standards, sustainability, and growing demand from chefs and consumers."