First Strike: Description of the Events at the First Salmon Farm Affected by the 2025 Algal Bloom in Northern Norway.

Harmful algal blooms (HABs) are a threat to fish welfare, occurring suddenly and unexpectedly causing significant consequences for fish and salmon farmers worldwide. Norwegian farmers have been facing this challenge at irregular intervals since the very beginning of the industry. This report describes the events on the first fish farm affected by the spring HAB of 2025 in northern Norway. Specifically at the production site Fornes, which at the time of the strike had more than 1 million Atlantic salmon (Salmo salar) with an average weight above 3 kg in its sea cages. First signs of the event were reduced fish appetite and cloudy water appearance, followed by changes in fish behaviour and acute mortality of 39.5%. Water samples showed dominance of the phytoplankton species Phaeocystis pouchetii, followed by Chrysochromulina leadbeateri. Necropsy of the fish and histopathological lesions on the gills and liver supported that the mortality was caused by the algae. The farm's contingency plan was enforced immediately when the acute high mortality was observed. Including, but not limited to, feed withdrawal followed by emergency harvest of the whole production site. The dramatic incidence at Fornes highlights the urgent need for monitoring and early-warning systems for HABs, as well as further development of suitable mitigation strategies and contingency plans to minimise the effect of HABs once they have been forecasted or detected.