Sea lice and winter ulcers: Norwegian farmed salmon industry is struggling with mass mortality

The farmed fish industry is the fastest growing food sector in the world. Fish farming seems to be a sustainable solution to combat overfishing. But the sector is struggling with mass deaths among farmed fish. In Norway, the largest farmed salmon producer in the world, more than 65 million farmed fish died prematurely last year. Farmed fish is very important for the Norwegian economy, but the mortality leaves a bitter aftertaste – and fuels the discussion about this method of intensive fish farming.

Salmon farmer Jørgen Viksund nimbly jumps from his motorboat at his fish farm, a kilometer off the Norwegian west coast near Bergen. Next to the house at the nursery are six large round basins that serve as fish cages. “We have about 100,000 salmon in each net,” says Viksund. “The first year they are grown in the laboratory, then they are taken to the sea and they swim around here for another year and a half, until they weigh 5 kilos.”

Paths have been constructed around the basins so that growers can access everything. They have to carry out random checks every week, checking ten fish per cage for sea lice. That tiny parasite feeds on the flesh and blood of live salmon. “You should especially look at the underside, near the tail. That’s where the sea lice prefer to settle.”

Sea lice major threat

Sea lice is one of the greatest threats to farmed fish. This used to be controlled chemically, but the parasite has become resistant. That is why, in the event of a sea lice outbreak, breeders now use a large ship to pump all the fish up from a cage, delouse them on the ship and pump them back again.

“That whole process is very stressful for the farmed fish,” says Ingunn Sommerset of the Veterinary Institute in Bergen. “Many fish are injured while pumping up. For example, they hit their heads or tops against the wall of the pump.”

‘Water amusement park without traffic lights’

Also according to Lars Heige Stien of the Norwegian Sea Research Institute, the method is difficult for the farmed fish. “You can compare it to a water amusement park without traffic lights, where tens of thousands of fish collide in a continuous stream.”

In addition, delousing itself on the ship is an attack on the health of the fish. One of the methods is to place the fish in warm water for a few seconds, which releases the lice. Another way is to rinse them with water or let them swim in fresh water. “It’s all stressful for the fish,” says Sommerset.

At the salmon farm where Viksund works they now have their own ‘delousing ship’. “Then there is at least less pressure and you can take things more slowly, which is better for the fish.”

The sea lice is not only a threat to farmed salmon, but also to wild salmon, the ‘panda of Norway’. The number of wild salmon is considered an indicator of the state of Norwegian nature. Young wild salmon that swim past the nets containing farmed fish become infected with sea lice. Farmed fish also escape from the cages, after which they contaminate the wild salmon.

Researchers from the Veterinary Institute in Norway recently raised the alarm. The mass mortality of farmed fish is growing steadily; in 2023, more than 65 million farmed fish died prematurely. In 70 percent of cases this can be attributed to infections, wounds and bruises. In addition, the delousing process leads to a weaker immune system of the fish, which means, for example, that wounds heal less quickly.

‘Intervention needed’

To make matters worse, a large proportion of the fish were plagued this winter by so-called winter ulcers, which create oval and deep wounds in the fish. Fish also suffered from apolemia uvaria, a type of jellyfish that moves through the water in a long ribbon. These jellyfish do not occur naturally in Norway, but accidentally came to the coast through the sea current and thus reached the cages. When these jellyfish hit salmon, they inject venom. Some fish die immediately, others lose consciousness.

The mass death of farmed fish is a hot topic in Norway, with a lot at stake. Farmed fish is the largest export product after oil and gas. “The mass mortality is too great and the industry must intervene,” the Norwegian Minister of Fisheries and Marine Affairs recently said. At the same time, Norway has set itself the goal of expanding farmed salmon production to 5 million tons of farmed fish by 2050.