Mowi lost 6,110 Atlantic salmon at its Salvågvika site in Flekkefjord on 2 March, according to a notification filed with the Norwegian Food Safety Authority (Mattilsynet).
The site held 154,038 fish at the time.
The filing, submitted by Mowi Seawater Norway, attributes the mortality to an algae and jellyfish attack. Two of four cages were affected, with elevated mortality recorded throughout the period. No end date was specified.
Mowi responded with health inspections, removal of dead fish, and partial slaughter and destruction of stock.
Separately, the Norwegian Institute of Marine Research (HI) warned on 17 March of a bloom of the potentially harmful alga Pseudochattonella along the Skagerrak coast.