The marine heatwave – which affected large parts of Norway's coastline in the summer of 2024 – led to an extreme increase in salmon lice, according to the report.
The more salmon lice in the pens, the more salmon die before they are supposed to. In recent years, mortality rates have reached record highs.
Authorities, researchers, and the industry are therefore searching for effective and quick solutions to the problem.
Lice skirts, closed pens, lice treatment in the feed, and electric fences are some of the measures being explored.
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During the launch of the report, deep farming was mentioned as a possible solution, among others, by former deputy director for fish health at the Norwegian Veterinary Institute, Brit Hjeltnes.
This method involves submerging the cages deeper into the ocean, anywhere from three metres to 50 metres down. At these depths, salmon lice struggle to thrive.
But how well does the salmon actually fare down there?
About the report
The risk report on Norwegian fish farming 2025 is produced by researchers at the Institute of Marine Research.
The researchers assess the sustainability of Norwegian fish farming in terms of animal welfare and environmental impact.
Among the biggest challenges are poor animal welfare and the negative effects of salmon lice on wild salmon.
The risk assessment is based on previous research, data, and models
Lars Helge Stien is a researcher at the Institute of Marine Research and has studied animal welfare in salmon.
"Will salmon have a better life in the depths?"
"The fish will benefit in the sense that it will have fewer lice and therefore require less handling," Stien tells sciencenorway.no.
Salmon can become ill from sea lice but do not die from them. However, they do die from the treatments used to remove lice – known as handling.
In one method, the fish are placed in warm water, which kills the lice but is painful for the fish. In another method, they are brushed or sprayed to remove the lice, which causes small cuts and wounds – so-called winter sores.
Once the fish have developed wounds from the lice treatment, they are more easily infected by bacteria in the sea.
Fewer lice in deeper waters mean the salmon can avoid this treatment.
Lars Stien is an animal welfare researcher at the Institute of Marine Research. He explains how salmon are faring along the Norwegian coast today.(Photo: Erlend Astad Lorentzen / Institute of Marine Research)
Trapped in the depths
At the same time, the fish would be trapped deep below the surface.
"It won't be able to swim up to the surface where it naturally wants to be," says Stien.
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Salmon follow temperature changes and normally move both up and down in the traditional pens, explains Stien.
If it is warmer in the depths, they will move downwards. But from time to time, they need to take in air for their swim bladder.
This is part the fish's natural behaviour.
Dependent on swallowing air
Because of this, deep-water pens must have an underwater air dome. This is a small area at the top where the fish can swim up and gulp air for their swim bladder.
"This is absolutely necessary for the fish to maintain equilibrium in the depths," says Stien.
If salmon do not get air, they can quite literally become seasick.
The fish will then have to swim faster to keep their balance, Stien explains. If they fail, they start swimming at an angle, with their tail down and head up.
Stien emphasises that an air dome must be stable, and there must always be a sufficient supply of air.
"They need to ensure that the air doesn't disappear, for example, during rough seas and storms. Everything must work perfectly all the time in such a pen," says Stien.
Risk of panic
Sometimes, salmon panic in deep water.
If this happens, they may start releasing the air they have taken in from the dome.
"This can harm them," says Stien.
Researchers do not yet fully understand why the fish panic. It could be due to flashes of light in the water or sounds from human activity.
What does the research show?
There has already been considerable research on deep farming in salmon aquaculture.
Studies show that salmon kept in submerged sea-cages had fewer lice compared to those in traditional pens.
However, a 2021 study found that animal welfare declined under these conditions.
Salmon in submerged sea-cages grew less, had poorer physical condition, experienced higher mortality rates, and generally suffered from lower welfare.
A 2024 study suggests that the surrounding environment of the pens plays a significant role in determining welfare.
Some salmon farming companies, such as Lerøy, have already started using deep farming techniques.
Can salmon lice adapt?
Rasmus Hansson from the Green Party was also present at the launch of the risk report. He remains sceptical about deep farming as a solution to the sea lice problem.
What if the lice adapt and follow the salmon into deeper waters?
"When you move a host animal to an area where a parasite has not been before, and the parasite follows, you need to have enough respect for evolution to be concerned about what that parasite might do next," Hansson tells sciencenorway.no.
"The lice could, for example, adapt," he says.
According to a study from the Institute of Marine Research, several families of salmon lice already prefer deeper waters – deeper than researchers previously believed. This suggests that the aquaculture industry must take this into account.
"As a solution, deep farming seems promising. But only if it actually works," says Hansson.
Rasmus Hansson attended the launch of the risk report in Bergen.(Photo: Bente Kjøllesdal / Institute of Marine Research)
Must function at all times
"Deep farming is probably not the ideal solution for farmed salmon," says Stien.
According to him, the optimal environment would be an open pen where water flows freely through the net, and salmon live are not an issue.
"But when it works, deep farming is an effective and relatively affordable solution. At the same time, it's a method that demands more. It becomes more challenging and complex," says Stien, adding:
"When animals are kept in captivity, everything has to function perfectly at all times. There is no room for error."
