Salmon deaths: Mowi Scotland tackles rising temperatures and hidden threats

Over the past production cycle, in 2023, more than a million salmon have died at Mowi’s three Loch Seaforth sites, a significant loss the company attributes to increased jellyfish blooms and warmer seas. The jellyfish stings damage the salmon’s eyes, skin, and gills, posing serious health risks and often leading to death. 

Mowi’s response ​

Mowi has rejected claims​​ that these losses stem from mismanagement, profit-driven practices, or overcrowded conditions. In fact, the company notes that salmon mortality rates have dropped by two-thirds this year, correlating with a return to more typical temperature patterns.

To understand more about the salmon company’s strategies for managing environmental challenges, we spoke with Hervé Migaud, Mowi’s director of fish health, welfare, and biology.

While jellyfish blooms are not new in Scotland, he said that they are occurring more frequently, a trend potentially linked to periodic spikes in sea temperature.

The sea is not consistently warmer year-round, but local temperature increases of around 1.5°C in specific areas and times are creating favorable conditions for blooms of jellyfish and phytoplankton. "These higher temperatures," he explained, "aren’t harmful to the salmon directly." Rather, the blooms are.

Jellyfish blooms impact salmon health in multiple ways. "At night, they deplete oxygen levels, creating environmental stress for salmon. More notably, jellyfish swarm salmon enclosures and discharge stinging cells upon contact with the salmon's gills. These toxins, mostly uncharacterized, cause localized damage on the gills, impairing the fish’s ability to breathe and increasing mortality risk."

To manage this emerging threat, intensive monitoring programs are in place at Mowi’s salmon farms across Scotland. Migaud highlighted the daily tracking efforts, which involve species counts, water sampling, and temperature and oxygen monitoring at various depths. However, predicting blooms remains challenging. "While we can't forecast blooms with precision," he said, "we analyze trends daily to anticipate and mitigate conditions that might lead to harmful blooms."

Although technology for automated species identification is not yet advanced enough to replace manual monitoring, research initiatives aim to develop such tools. Meanwhile, Mowi Scotland relies on its trained personnel to monitor conditions.

In partnership with its global network, Mowi is also evaluating technology used in other regions, such as Canada and Chile, to adapt these solutions to Scotland’s challenges with blooms. And Migaud explains that while blooms can’t be removed, new techniques—such as bubble curtains—are being trialed to keep jellyfish out of salmon enclosures. "Initial results are very promising."

This level of intervention is critical as the scale of jellyfish blooms worsens. Many jellyfish causing issues are at the microscale and are invisible to the naked eye, making them difficult to detect. Mowi’s regular water sampling and microscopic analysis help identify blooms early, yet Migaud emphasizes that the long-term presence of these species in Scottish waters is tied to climate factors beyond aquaculture’s control.

Post-smolt transfer ​

Mowi Scotland is adopting other innovative strategies to protect farmed salmon from environmental threats and disease. One approach focuses on transferring larger, post-smolt salmon to sea farms, reducing the time they spend in open waters where they are more vulnerable to health risks.

Traditionally, young salmon are kept in sea farms for up to 18 months or more, but the salmon company is aiming to shorten this period to under a year by raising post-smolts that reach weights of up to 400g before transfer. This initiative was launched this year and builds on successful programs in other regions.

Unlike many countries that rely on land-based facilities for post-smolt rearing, Mowi is leveraging Scotland’s brackish-water lochs, which provide a sheltered, lower-risk environment. These lochs are naturally protected from jellyfish and harmful algae, offering salmon a safer start before they are moved to sea, explained Migaud. The company plans to "further refine and expand" this strategy in the coming years, with the possibility of rearing the smolts in semi-contained systems under review, he reported.

Vaccination and disease prevention ​

Mowi is prioritizing preventive health strategies as well, focusing on vaccines. The program is based on autogenous vaccines, derived from bacterial isolates directly taken from the salmon population to be immunized.

This year, the company introduced a new vaccine specifically designed to protect salmon against Yersinia ruckeri, a bacterium responsible for enteric red mouth disease (ERM) in salmonid species. ERM has historically caused significant outbreaks and mortality among farmed salmon along Scotland’s west coast.

The development of the ERM vaccine began in 2022 with an extensive collection and characterization of bacterial isolates from Mowi’s farms. After rigorous testing in controlled conditions throughout 2023, the vaccine was introduced commercially in 2024, with initial batches vaccinated in the spring. Mowi expects to assess the vaccine’s impact in 2025. The vaccine strategy includes a dip vaccination during early freshwater stages, followed by an intraperitoneal injection before the fish are transferred to the sea.

Mowi’s collaboration with Ridgeway Biologicals has been instrumental in vaccine development. In 2021, the two companies launched a vaccine to guard against Pasteurella skyensis, fully implemented across Mowi’s stock by late 2022. Since then, while the bacterium has been detected in the environment, no clinical outbreaks have occurred.

Migaud said the partnership has "yielded valuable insights into the evolution of bacterial communities in farmed salmon, utilizing advanced biotyping and serotyping technologies."

This focus on early threat monitoring and targeted vaccine development has significantly enhanced salmon health and welfare while reducing antibiotic use. “From 2021 to 2023, Mowi Scotland cut its antibiotic usage by 79%.”

The promise of phage therapy  ​

The salmon producer is also working to get regulatory approval for phage therapy technology in the UK.

“Phages are highly effective tools against various bacterial strains, and many companies are developing bacteriophage solutions for a wide range of pathogens. However, regulatory restrictions are significantly slowing progress.

“Until we receive approval to use even a single targeted application, this promising technology remains out of reach. Many scientists and companies are ready to advance in this area; we currently have experimental and foundational projects underway. However, without the ability to conduct testing in the UK, these efforts are still limited to the lab stage,” remarked Migaud.

Mowi is actively collaborating with research institutions across the UK to address various environmental and bacterial challenges. One example of this support is Mowi backing a PhD project on bacteriophages at the University of Exeter. This highlights the company’s commitment to advancing innovative research in this critical area, said the biologist.

Nutritional strategies  ​

Mowi has also been testing and gradually refining salmon diets to boost resilience against environmental challenges.

In Scotland, the company has been developing functional feeds to address seasonal needs and bolster fish health. These adjustments focus on strengthening immune systems and enhancing healing capabilities through carefully tailored nutrient formulations. “We are using ingredients like vitamins, minerals, antioxidants, pigments, probiotics, and immunostimulants, along with essential oils and plant extracts.”

The company’s global feed team, based in Scotland and Norway, continually assess experimentally new ingredients and nutrient combinations to optimize health benefits. 

However, as Migaud said, there are limits with what can done through feeding, adding that developing a “one-size-fits-all” solution for salmon nutrition is complex, with challenges around balancing nutrients without over-supplementing. Even in humans, he noted, nutritional needs are still not fully understood, and over-supplementation can be ineffective. For fish, these issues are compounded by sustainability concerns; excessive use of certain costly or hard-to-source ingredients doesn’t always yield better outcomes and may have broader environmental impacts.

Mowi's new hatchery ​

Mowi Scotland’s recently launched genetic program is focused on improving the resilience of its salmon stocks to environmental conditions in Scottish seas. A major milestone will be reached with the completion of its new hatchery by spring 2025, which will have the capacity to produce up to 50 million “eyed” eggs annually for its production hatcheries.

Located at the Ardessie site, this facility will mark the first local production of Scottish salmon eggs from native broodstock in two decades, taking a significant step toward self-sufficiency.

The establishment of this local broodstock and egg facility addresses a critical need for Mowi, which had previously depended on external egg supplies. "The aim is to increase the robustness of the fish against some of the challenges that they are experiencing, and it's difficult to do if eggs are produced from broodstock elsewhere in a different environment," said Migaud.

Equipped with advanced recirculating aquaculture system (RAS) technology, the facility will create optimal breeding conditions, ensuring consistent egg quality and stronger fish. Mowi geneticists have also been deeply involved in the spawning process, using DNA analysis to rank male salmon based on growth potential, disease resistance, and flesh pigmentation. All combined, these efforts should contribute to a high standard for future generations of Scottish salmon, commented Migaud.