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Impacts of salmon lice emanating from salmon farms on wild Atlantic salmon and sea trout

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.

The report, produced by the Norwegian Institute for Nature Research (NINA) and commissioned by Salmon & Trout Conservation Scotland (S&TC Scotland) has summarised available and up to date research on the impact of salmon lice (sea lice) from salmon farms. 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 report, Impacts of salmon lice emanating from salmon farms on wild Atlantic salmon and sea trout, is published by the Norwegian Institute for Nature Research (NINA), Norway’s leading institution for applied ecological research. It authors Prof. Eva B. Thorstad and Dr. Bengt Finstad, are internationally recognised for their expertise in sea lice biology and the interactions between salmon farming and wild fish.

The report draws the following conclusions (pg. 15):


Scientific studies indicate that salmon farming increases the abundance of salmon lice in the marine habitat and that salmon lice in the most intensively farmed areas have negatively impacted wild Atlantic salmon and sea trout populations. The effects of salmon lice on Atlantic salmon and sea trout include increased marine mortality, reduced marine growth, and for sea trout as changes in migratory behaviour. Published studies range from those investigating the effects of salmon lice on individual fish, both in the laboratory and the field, to analyses of their impacts on wild populations.

Conclusions are based on comprehensive studies of the effects by salmon lice, which include:

1) Studies of individual fish in laboratory and field studies documenting (i) tissue damage, (ii) problems with salt regulation and other physiological stress responses, (iii) reduced growth, and (iv) increased susceptibility to secondary infections and reduced disease resistance. One or more of these effects have frequently been reported to occur as the result of heavy salmon lice infestations.

2) Studies documenting premature migratory return to freshwater of sea trout with high levels of salmon lice. Premature migratory return may facilitate individual survival and recovery from infestation, but reduce growth and thereby future reproduction by reducing the time spent feeding at sea. Sea trout with excessive skin lesions also may be more vulnerable to fungal and bacterial infection in fresh-water.

3) Studies based on catch statistics and routine population monitoring utilizing in-river traps that have indicated salmon lice-induced changes in population abundance, age structure and altered life history characteristics.

4) Monitoring of salmon lice levels on wild fish.

5) Comparisons of salmon lice levels in farm-intensive and less farm-intensive (or farm-free) areas.

6) Indications of population-level effects on sea trout based on monitoring of salmon lice levels on wild fish in relation to experimentally determined threshold levels known to induce physiological compromise and mortality of individual fish.

7) Quantification of Atlantic salmon mortality due to salmon lice in large-scale field studies by comparing growth and marine survival of salmon treated with prophylactic chemicals against salmon lice with un-protected control groups.

In sum, the combined knowledge from scientific studies provides evidence of a general and pervasive negative effect of salmon lice on salmonid populations in intensively farmed areas of Ireland, Norway and Scotland. Premature migratory return, increased marine mortality and reduced growth of survivors that are induced by elevated salmon lice levels inevitably imply a reduction in numbers and body size of fish returning to freshwater for spawning, and hence in number of fish available to fisheries. Levels of additional mortality by salmon lice as indicated in several scientific studies may result in salmon stocks not achieving river specific conservation limits and, if sustained over time, could result in significant cumulative reductions in adult salmon recruitment.

The report was published on 10th January 2018 and is available at or at

In June 2017, the Scottish Parliament’s Rural Economy and Connectivity Committee, in response to a formal Petition lodged in the Scottish Parliament in February 2016 by S&TC Scotland seeking protection for wild salmonids from sea lice from Scottish salmon farms, agreed to launch an Inquiry (scheduled for 2018) into salmon farming in Scotland.

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