Algal bloom 'will cause mortality'
South Norway fish farmers warned of Pseudochattonella spread
Norway’s Institute of Marine Research (IMR) has warned that a marked increase in the amount of the algae Pseudochattonella was registered at the beginning of this week in the country’s southeastern salmon production area, PO1.
The production area encompasses the coast of the Skagerrak - a strait between the North Jutlandic Island of Denmark, the southeast coast of Norway and the west coast of Sweden.
The algae is harmful to farmed fish when the concentration becomes high.
The highest concentrations of the species were recorded in the inner Skagerrak and only low densities further south, near Arendal.
“In recent days, however, we have registered a marked change in the density of Pseudochattonella on the Skagerrak coast. The densities are now above the threshold values that lead to mortality in farmed fish,” says researcher Lars-Johan Naustvoll.
Analyses of water samples taken on March 17 show densities that will cause behavioural changes or mortality in farmed fish, and there have been reports of increased mortality from the aquaculture industry on the Skagerrak coast, writes IMR.
Based on the results of the water analyses, a dispersion modelling has been done to investigate where harmful densities of Pseudochattonella may occur in the coming days.
The results indicate that the algae will spread close to land and in the fjords along the Skagerrak coast.
“The Institute of Marine Research has received and analysed samples from the Farsund area and the species is already in this area, in moderate concentration and significantly lower than at Arendal. Spread from southern areas, and from Arendal, shows that the bloom could potentially reach the outer parts of Boknafjorden during the coming week,” says Naustvoll.
According to the marine scientist, low densities of the algae have already been observed in PO2.
Naustvoll writes that the last bloom of the algae in this region was in April 2025, but that the bloom remained mainly out in the Skagerrak and did not cause mortality on the coast.
