Western Norway hit by strongest marine heatwave in three decades

Western Norway experienced its most intense marine heatwave in at least 30 years this summer, according to researchers at the country’s Institute of Marine Research and the Bjerknes Centre for Climate Research. 

The heatwave, which began on 9 July and lasted for 29 days, stretched along the entire western coastline. A marine heatwave is defined as a period of unusually high sea temperatures lasting at least five consecutive days. 

“We classify them into four levels: moderate, strong, severe, and extreme. The marine heatwave this year was at the second-highest level,” said Silvana Gonzalez, a researcher at the two institutes, in a press release. 

Based on available data, Gonzalez said it appeared to be the most powerful marine heatwave to affect the region since 1990. Earlier heatwaves in the same period were mostly moderate, with only two previously classified as strong.

“We’ve analysed data from 1990 onward, as we have more reliable measurements from the period after satellite observations became available in the 1980s,” Gonzalez explained.

The Institute of Marine Research reported that its Bud station at Hustadvika registered a July water temperature of 18°C, the highest recorded since the station became operational. Gonzalez said the usual temperature for this time of year is between 12°C and 14°C. 

Satellite and model data also indicated record temperatures along the coastline, peaking at 20.4°C just north of Bud. 

These heightened temperatures could damage marine life, Gonzalez warned. “During such temperature increases, kelp and seagrass beds are particularly vulnerable. The same goes for animals with limited mobility, such as farmed fish, and stationary organisms like oysters and mussels,” she said. 

Rising sea temperatures have also been linked to an increase in sea lice. The warmer water speeds up the sea lice life cycle, causing them to mature more quickly, reproduce faster and attach more easily to fish, the researchers said. 

“In recent weeks, many sites have struggled to keep lice levels below the regulatory threshold,” said Anne Dagrun Sandvik, a researcher and sea lice specialist at the Institute. 

She added that the situation was particularly severe in Northwest Norway and Trøndelag. “In North West Norway and in Trøndelag (PO5 and PO6) the number of adult female lice on farmed salmon in August 2025 was higher than ever recorded,” Sandvik said, noting that the previous summer also saw unusually high lice levels. “During the heatwave in 2024 there were also record-high lice levels in these regions, but at that time Northern Norway was hit even harder,” she said. 

The status of sea lice levels in Norway's different aquaculture production areas is updated weekly on the Marine Research Institute website.