Grieg Seafood is being ordered by the Norwegian Food Safety Authority to slaughter around a million young salmon in the north of the country.
The directive follows the discovery of a rare parasite known as Spironucleus salmonicida, which causes systemic infections in salmonid fish.
The outbreak affects eight individual cages at two locations in Finnmark containing up to a million fish weighing an average of 0.7 kg.
According to reports from the area, the parasite has also been detected in individual cages at other locations with fish from the same generation. However, the majority of cages in the area so far do not show any signs of the disease.
Grieg said in a Stock Exchange announcement that the source of the parasite is thought to be the water intake of the freshwater plant which started last autumn and continued into the spring of this year. There have been no detections of the current generation at the facility.
The company said it is in close contact with the authority, adding that further measure are being looked at.
Spironucleus salmonicida is a microscopic, two-celled organism in the group known as diplomonads, which also includes some parasites known to infect humans. It is a rare parasite in farmed fish for which there is no recognised treatment. The infection, which is associated with abscesses in muscles and internal organs, spreads to all parts of the body.
The Norwegian Food Safety Authority said: “Based on historical experience with the disease and the development of the disease at the locations in recent months, the Norwegian Food Safety Authority assesses that the health and welfare situation cannot be expected to improve in the eight cages.”
The cost to the company still has to be assessed.