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No evidence of eutrophication found

Emissions of dissolved nutrients by the aquaculture industry in Hardangerfjorden and Boknafjorden are probably not so high that they are causing eutrophication. In both of these fjords, the concentration of dissolved nutrients is within the limits of the categories Good and Very Good defined by the Climate and Pollution Agency (Klif). This is the conclusion of an expert group appointed by the Ministry of Fisheries and Coastal Affairs in consultation with the Ministry of the Environment.

Water circulation is high in both Hardangerfjorden and Boknafjorden, thanks to tides and density-driven currents which transport large quantities of nutrients into, and out of, the fjord systems. Neither winter nor summer concentrations of chemical nutrients were measured to be elevated in the two fjords and the emissions of the current fish production appears to be too low to cause overfertilisation. After assessing the available data from the two fjords, the expert group found no evidence of any increase in the phytoplankton biomass. Eutrophication is here defined as a 50 % increase in phytoplankton biomass (OSPAR).

In Hardangerfjorden, recent studies of macroalgae have been compared with equivalent studies carried out in the 1950s. The findings show that the number of species present in the fjord has increased, and also that species with warm water affinity have become more common. The abundance of habitat-forming species, such as brown algae, is unchanged since the 1950s. There is no evidence of any change in the lower depth limit of sugar kelp in either of the two fjords. In Hardangerfjorden, this limit is often determined by sea urchin grazing. In various places in Boknafjorden, sugar kelp grows at depths of up to 20-25 metres.

Some local impacts

Although no increase in nutrient levels has been detected in the pelagic zone in areas with intensive fish farming (except in the immediate vicinity of cages), there is a risk of local eutrophication in areas with poor water circulation. There may be impacts on the shore in the approximate zone up to 500-1000 metres from farms, although only minimal impacts were observed around farms sited in areas with good water circulation. Some impact on the shore communities can be seen in the local area around farms in the inner part of fjords, particularly if the farm is close to the shore.

Seaweeds are often used an as indicator when assessing eutrophication in coastal waters. One of the characteristics of healthy brown algae is that they are not overgrown with fast-growing, annual algae. However, at certain times of the year there were large quantities of these algae in both of the fjords studied. The expert group considers that it is hard to determine why brown algae are at times heavily overgrown with annual algae, as the phenomenon has also been observed in open coastal areas without any chemical nutrient emissions.

Monitoring programme is proposed

Given that Norwegian production of farmed fish is constantly increasing, the group argues that the situation in the fjords should be monitored closely.They therefore propose establishing a comprehensive monitoring network in both Hardangerfjorden and Boknafjorden. In combination with an extensive network of study sites, this will ensure that routine samples can be taken frequently. Establishing such a monitoring programme will allow us to detect possible future changes to the ecosystems in the fjords.


The sustainable level for an aquaculture location or area is defined as being the maximum quantity of farmed organisms that can be produced without the environmental impacts exceeding overall tolerance levels. The tolerance levels must be measurable, and they cannot be exceeded if the aquaculture industry is to remain sustainable.


The expert group

In January 2011, the Ministry of Fisheries and Coastal Affairs, in consultation with the Ministry of the Environment, appointed an expert group to study the issue of eutrophication along the Norwegian coast. The group’s mandate was to assess the situation along the coast, with a particular focus on Hardangerfjorden and Boknafjorden in western Norway, and to write a report describing the conditions and explaining potential causal relationships.

Members of the group:

  • Professor Stein Fredriksen (University of Oslo, Chair of the group)
  • Associate Professor Kjersti Sjøtun (University of Bergen)
  • Senior Researcher Hartvig Christie (Norwegian Institute for Water Research)
  • Senior Researcher Trine Dale (Norwegian Institute for Water Research)
  • Senior Researcher Hein Rune Skjoldal (The Institute of Marine Research) 
  • Senior Researcher Vivian Husa (The Institute of Marine Research)
  • Professor Yngvar Olsen (Norwegian University of Science and Technology in Trondheim)


Hein Rune Skjoldal
905 72 356
Vivian Husa
915 38 429
Beate Hoddevik
908 21 630