Grieg Seafood has been ordered by the the Norwegian Food Safety Authority to cull 1 million fish after the discovery of the parasite Spironucleus salmonicida on two farms in Norway's northern Finnmark region.
The fish weighed an average of 0.7 kilograms.
The parasite has also been detected in individual pens on other farms with fish from the same generation, although salmon in the majority of those pens have shown no signs of sickness, the company said.
