Transfer of freshwater-resident salmon from Sweden: Assessment of the risk to biodiversity and fish health in Norway


Report no: 2021:04

Published: 13.04.2021

Key message:

Using the gene bank model to reintroduce Atlantic salmon to the Trysilelva watershed from Sweden is associated with the lowest risk, although the method is associated with some risks. The risks primarily stem from hitchhiking organisms like the parasite Gyrodactylus salaris, crayfish plague and various types of bacteria and viruses.

This is the conclusion from the Norwegian Scientific Committee for Food and Environment (VKM) in a risk assessment of the negative impact on biodiversity and ecosystems in Norway from transfer of either adult salmon or eggs from the Klarälven river in Sweden to the Trysilelva watershed. The assessment was commissioned by the Norwegian Environment Agency.


Following the establishment of hydropower stations between the spawning grounds in the Trysil watershed and the feeding grounds in the lake Vänern, the Atlantic salmon has ceased to exist in the Norwegian part of the joint watershed. The aim of the project has been to evaluate the possibilities for re-establishing the salmon on the Norwegian side of the border. This assessment is needed after the loss of the salmon in Norway, since alien species, disease organisms and viruses have developed differently on the two sides of the border.

None of the methods are certain

VKM has assessed three different methods for introducing the salmon: 1) import of fertilised eggs to a local hatchery in Norway, 2) import of fertilised eggs and establishment of a broodstock in Norway using the gene bank model, 3) import of adult salmon spawners.

“Importing eggs and establishing a broodstock using the gene bank model is the method associated with the lowest risk. However, the risk of negative impact to biodiversity in Norway is still moderate for this method. The risk associated with the two other methods is moderate for import of eggs to a local hatchery, and high for importing adult spawners directly”, says Eva B. Thorstad, Scientific Leader of the project group.

“One can reduce the likelihood of introducing a number of the disease organisms and viruses by using the gene bank model, but none of the methods are certain”, Thorstad emphasises.

Importantly, none of the three methods alone will be sufficient to re-establish the salmon population in Norway. Thorstad points out that introduction of eggs or adult fish in the Norwegian part of the watershed will only lead to a one-way movement of fish to Sweden.

“In order to truly re-establish the population on the Norwegian side, the waterway needs to be opened to allow upstream migration, so the spawners can return. This kind of two-way migration between the spawning grounds in Norway and feeding grounds in Sweden necessarily increases the risk of spreading disease organisms. A situation like that will require a separate risk assessment”, Thorstad explains.

The parasite Gyrodactylus salaris

The most serious consequences following imports like these would be from the spread of the parasite G. salaris to the Norwegian part of the watershed. This parasite is very harmful for salmon and was detected in the Swedish part of the watershed in 2013.

“The parasite can spread further to other Norwegian watersheds if it reaches the Trysilelva watershed”, Thorstad points out.

Viral haemorrhagic septicaemia virus (VHSV) and Aphanomyces astaci, which causes crayfish plague, are two other organisms that can have a serious negative impact on biodiversity and the ecosystem if spread to Trysilelva.

Wide array of threats and high uncertainty

VKM has assessed the risk of negative impact for 26 different viruses and organisms, including alien species like the freshwater plant Canadian pondweed, signal crayfish and zebra mussel. The assessments are based on the evaluation of available scientific literature and qualitative considerations for each of the three methods of reintroducing salmon. Some of the assessments have high uncertainty. This is in part due to limited surveillance and lack of knowledge about disease organisms and viruses found in wild salmon and other freshwater fish species.

“There is also an overall uncertainty since VKM can only assess known threats, while the biggest threat might still be undiscovered”, Thorstad concludes.

The assessment is approved by the VKM Panel on Alien Organisms and Trade in Endangered Species (CITES) supplemented by members of the VKM panel on Animal Health and Welfare.


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