Up to one million fish have died in Norway due to a harmful algal bloom, Norway’s Directorate of Fisheries reported on Monday. The algae bloom in production areas 9 and 10 has led to fish mortality among several farmers in Nordland and Troms.
The highest mortality has been reported from May 10-12 in production area 10, particularly in the area around Astafjorden. Four companies are affected, but to varying degrees. There have been challenges with handling dead fish, but all farmers have stated that they have the situation under control.
“It is currently estimated by the farmers that there are up to 1 million dead fish, of which 250,000 are smolts. Clean-up is still underway in cages where fish have sunk to the bottom, and the number of dead fish may be higher,” said Arnt Inge Berget, section manager for the aquaculture supervision and guidance section at the Directorate of Fisheries, on Monday.
“Farmers are asked to monitor signs such as changed behavior in the fish, reduced visibility in the water and increased mortality. It is important to notify us quickly if this is the case,” said Berget.
Why fish die from algae
Two types of algae are linked to the incident: Phaeocystis and Chrysochromulina. “Both species are naturally present along the coast, but in some areas, such as Vesterålen, they have dominated in recent weeks. This has resulted in a dangerous effect for farmed fish,” said researcher at the Norwegian Institute of Marine Research and algae expert, Lars-Johan Naustvoll.
If the density becomes high enough, algae can harm fish in various ways. Some can cause mechanical damage to the gills. These algae have “hard” outside structures that can damage the gills and thus cause problems with oxygen uptake. Other algae can cause clogging of the gills either because they secrete mucus or they occur as larger jelly colonies. Another group’s harmfulness is due to a toxin with hemolytic effects, affecting the gills.
“In general, harmful algae affect the gills of fish and cause reduced oxygen uptake,” said Naustvoll.
When fish have been affected by potentially harmful algae, they show clear behavioral changes. In the first phase, they avoid the algae themselves, and then behavioral changes are related to any damage. “Fish that are affected by algae often have bleeding in their gills and stand apathetic at the surface. In the end, they suffocate. Fish that are less affected stand deep in the cage and are stressed. In some cases of irritation in the gills, the fish will gasp for air at the surface,” said Lars Helge Stien, researcher at the Norwegian Institute of Marine Research. Once a fish farm is affected by a harmful algal bloom, there are few effective measures, mainly based on reducing contact between harmful algae and fish.
Every spring, there is a major algae bloom along the Norwegian coast. This is called a spring bloom. During the ongoing situation in Northern Norway, mortality has moved somewhat chaotically between facilities, as if it were different local outbreaks.
“The reason for the somewhat chaotic picture is probably a combination of spring bloom, the presence of Chrysochromulina and how the water flow is right now,” said Naustvoll.
