THE largest mass die-off of farmed salmon in Scotland in more than a decade has occurred, with more than a million fish dying in just a year and a half.

Two adjacent sites run by Mowi Scotland on Loch Seaforth in the Outer Hebrides, which are licenced as one farm, reported more than a million deaths during a year and a half production cycle that began in spring 2023.

Data from the Scottish Government was analysed by the Coastal Communities Network (CCN) and Free Salmon to reach the figure.

Mowi confirmed to The Guardian that 1.05 million fish had died in the period, which it said was a combined figure for the two sites Seaforth and Noster.

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“This is the first time since 2014 [when regular reporting began] that more than a million farmed salmon deaths have been reported at a single farm site in one production cycle,” said John Aitchison of the CCN.

“We expect to see more salmon deaths in Scotland because farms are becoming even larger.”

The activist group Animal Rising also claims to have filmed salmon at Seaforth during the same productions cycle, with footage appearing to show fish with raw patches of flesh and swollen or burst eyeballs.

Mowi supplies farmed salmon to many retailers, including Tesco, Asda, Ocado and Sainsbury’s.

Mowi Scotland’s chief operating officer, Ben Hadfield, said any suggestions the mass die-off was related to low welfare were “false”.

He said the deaths were due to an unprecedented rise in sea temperatures, which resulted in jellyfish blooms — a problem impacting fish mortality at farms across Scotland.

“[Any] suggestion that this is caused by bad farming, fixation with profits [or] overstocking is … very false and misleading,” said Hadfield.

Hadfield also said the footage taken by Animal Rising “does not show the majority of the population.”

“What the video shows is fish with eye damage after, you would think, jellyfish stings or wounds that are healing after jellyfish blooms,” he added.

Its farms are certified by the RSPCA, which claims to guarantee higher animal welfare standards.

However, last month Queen guitarist Brian May stepped down as vice president of the RSPCA citing “damning evidence” of its animal welfare failings.

The industry has also faced increasing scrutiny after footage from the campaign group Green Britain Foundation appeared to show tonnes of dead salmon being buried on a Scottish beach. 

A spokesperson for Mowi Scotland said: 

"The welfare of our fish is at the heart of what we do. It is also our business model.

"Healthy fish that grow to optimum levels are highly sought after all over the world. This is why we are constantly investing in research, new technology and new techniques to ensure that our salmon have the best possible care.

"We are always looking ahead, analysing environmental conditions such as water temperatures, or taking daily water samples to check for jellyfish, this allows us to be proactive and take mitigation measures to avoid negative impacts on our salmon.

"In the case of Loch Seaforth, the environmental conditions proved too challenging. Specifically, El Niño, caused a marine heat wave in the North Atlantic. El Niño is part of the natural climate phenomenon called the El Niño Southern Oscillation (ENSO) and leads to warmer than average sea surface temperatures.

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"This marine heat wave led to warmer seawater temperatures causing exceptionally high levels of jellyfish. The impact of the jellyfish was to weaken the salmon and cause them to suffer from gill disease.

"This was an environmental event that the company worked tirelessly to mitigate and ensure surviving stock could be brought to market under the highest welfare conditions possible. 

"The RSPCA understands the challenges presented to all farmers. In the case of salmon farmers, our biggest challenges are often caused by the environment and salmon farmers need to demonstrate the measures we have in place to deal with those challenges and aim to minimise potential harm to the fish in our care.

"The RSPCA Assured accreditation remained in place throughout this period and is still in place today.

"It is worth noting that in August 2024, Mowi Scotland recorded its lowest monthly mortality for over eight years and achieved record high feeding and growth rates in the cooler coastal waters."