Blooms are often associated with low dissolved oxygen events and warm ocean water temperatures and weather changes – all of which we are, or have been, experiencing," said Kiemele. Farm owner Cermaq says it has deployed fish protection countermeasures that have already caused conditions to improve.

Fifty-three thousand salmon died while being transferred out of a damaged marine pen at a fish farming facility in the West Fjords.

Researcher Sirpa Lehtinen from the Finnish Environment Institute (Syke) said that cyanobacteria exist in seawater all year round, but intense heat causes them to multiply quickly.

Scientists now say that the harmful alga will survive the winter and that it will probably turn green in the Oslo fjord next year as well.

Data collected over 90 years shows that Norwegian coastal water temperatures have increased by up to 2°C, a shift that is impacting marine ecosystems and local fishing practices.