Impacts of salmon lice on wild Atlantic salmon and sea trout

Publisert 10.01.2018

New report concludes: Considerable evidence exists that there is a link between farm-intensive areas and the spread of salmon lice to wild Atlantic salmon and sea trout. 

Foto: Bengt Finstad / NINA
Foto: Bengt Finstad / NINA

A new comprehensive study has summarized the available research on the impact of salmon lice. It concludes that the effects of salmon lice from fish farms on wild Atlantic salmon and sea trout populations can be severe - ultimately reducing the number of adult fish due to salmon lice induced mortality, resulting in reduced stocks and reduced opportunities for fisheries. Depending on the population size of wild fish, elevated salmon lice levels can also result in too few spawners to reach conservation limits.


– The impacts of salmon lice on Atlantic salmon and sea trout are well-studied compared to impacts of many other human activities on natural resources, says Prof. Eva B. Thorstad, NINA, who is one of the scientists summarizing the results from studies of salmon lice on wild salmonids. 

There are a large number of scientific studies on the impacts of salmon lice on salmonids, ranging from laboratory and field studies of the effects on individual fish, to analyses of impacts on wild populations. 

This report is built on earlier reviews and new publications from recent years. 

– New studies of salmon lice have confirmed and strengthened earlier conclusions on the impacts on wild salmonids, says Dr. Bengt Finstad, NINA, who is the other author of the report. 

Marine parasite

Salmon lice are external parasites on Atlantic salmon and trout at sea. They feed on fish’s mucus, skin and muscle, ultimately causing reduced growth and increased mortality of fish. Salmon farming increases the spread and abundance of salmon lice, and thereby the risk of infection and mortality among wild Atlantic salmon and sea trout in farm-intensive areas.

Reduced wild fish stocks

Wild salmon populations in farm-intensive areas can be reduced by an average of 12-29% fewer adult salmon. This is revealed by several large-scale release experiments in nature, protecting individually tagged fish against salmon lice with chemical treatment and comparing these to untreated fish. 

– In Norway, an annual loss of 50 000 adult wild Atlantic salmon because of salmon lice has been estimated, which corresponds to an overall loss of 10% of the wild salmon, says Prof. Thorstad. 

This estimate includes both farm-free and farm-intensive areas, based on data from the years 2010-2014. Salmon lice from fish farms are identified as one of the two largest threats to wild salmon in Norway.

Dr. Finstad points out that mortality of sea trout is likely larger than in Atlantic salmon, because unlike the ocean-migrating Atlantic salmon, sea trout remain in coastal waters, where fish farms are situated.

The work with the report was commissioned by Salmon and Trout Conservation Scotland.

Read the report: Impacts of salmon lice emanating from salmon farms on wild Atlantic salmon and sea trout. 

Dr. Bengt Finstad, Norwegian Institute for Nature Research
Phone +47 934 66 784 E-mail bengt.finstad@nina.no

Prof. Eva B. Thorstad, Norwegian Institute for Nature Research
Phone: 916 61 130 Email: eva.thorstad@nina.no 

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