Around 80,000 farmed salmon are estimated to have been lost following a large toxic algae bloom event around Bantry Bay, off the south-west coast of Ireland.
The Irish marine authorities are working on the possibility that sea warming due to climate change could be responsible.
Mowi is one of the companies hardest hit although the company is still trying to assess the full extent of the damage.
But the company confirmed that its Ahabeg and Roancarrig sites in Bantry Bay have both been affected.
It has notified the Department of Agriculture, Food and the Marine (DAFM) of the incident.
Mowi Ireland has farms in a number of counties along the west and south west coast of the country and its products are exported to Europe, the UK and the US.
The overall cost of losses suffered by fish farms in the region is thought to be at least £2m.
Algal blooms are a natural phenomenon and notoriously difficult to control. They usually come with little warning, sucking up oxygen in the water and suffocating the fish in farm cages.
Chile and Norway are just two salmon farming countries to have been badly hit by this menace in recent years.
Ireland’s Marine Institute said it is still working to establish the exact cause of the incident, but it warned earlier this year that harmful algal bloom patterns in marine waters are changing because of climate change.
According to the Irish Examiner, the average water temperature in Bantry Bay is normally around 11.5C but it has been hovering close to 13C in recent days.
Sea warming is thought to be one of the causes behind a large algae attack in Norway in the spring of 2019.