New Scottish Government study confirms the severe damage being done to wild salmon populations by salmon farming

Scottish Government study confirms that many wild salmon populations in the west Highlands and Islands are severely compromised with farmed salmon genes

The levels of genetic introgression have serious implications for the “fitness” and thus the future survival prospects of already depleted wild salmon populations


A comprehensive new study by Scottish Government scientists – A national assessment of the influence of farmed salmon escapes on the genetic integrity of wild Scottish Atlantic salmon populations – shows that many wild salmon populations in the west Highlands and Islands are severely introgressed with “farmed” genes, as a consequence of interbreeding with escaped farmed salmon of Norwegian origin.

Fish samples from 237 sites across Scotland were classified. The study states that “signs of introgression were found in salmon at 55 (23.2%) of the sites with 182 (76.8%) sites classified as Good, 21 (8.9%) as Moderate, 20 (8.4%) as Poor, and 14 (5.9%) as Very Poor”. The Moderate, Poor and Very Poor categories are concentrated almost entirely in areas of Scotland where salmon farms, both marine and freshwater, are located.


Andrew Graham-Stewart, Director of Salmon and Trout Conservation Scotland (S&TCS), said:

“Farmed salmon, the great majority of Norwegian origin, are essentially domesticated animals, bred for the table. When they interbreed with our wild salmon, the offspring are inevitably unsuited and unfit to survive in the wild. The future viability of wild salmon is dependent on their genetic integrity not being compromised by domesticated strains.

“It is little wonder that this study has been published by Scottish Government without fanfare. It is a damning indictment of the insidious impact of salmon farming and the ongoing failure of the operators of salmon farms to contain their fish. It represents yet more evidence why open-net salmon farms should be closed down as soon as is practicable before further damage is done to the vital genetic integrity of our wild salmon populations.”


Guy Linley-Adams, solicitor to S&TCS, commented:

“The damage that has already been caused to wild salmon populations will get worse unless Scotland stops farming in open-net salmon farms at sea. Open-net salmon farms suffer escapes and leak fish however strict the technical standards may be. That is just the reality.

“Nor will it be any help to these introgressed wild fish populations if Scotland just decides to fine farmers for escapes. Such fines will just become a business risk that the multinational fish farm companies will be happy to absorb. Of course, the industry no doubt appreciates that there are wider and potentially very severe liability issues at play for it to consider.

“But perhaps it is their supermarket and restaurant customers who really need to consider the reputational damage they will also suffer if they are seen to turn a blind eye to what is going on here.”


Wild salmon populations have been affected by farmed salmon introgression in Clyde (eg River Leven), Argyll (eg River Awe), Lochaber (eg River Shiel), Wester Ross (eg River Balgy), West Sutherland, the Inner and Outer Hebrides, and the River Shin and the River Ness systems (the latter two are east coast but they host freshwater cages producing juvenile farmed salmon).

Picture 1

© Crown copyright 2021 Key. The proportion of wild and farmed (Norwegian) origin genetic material in fish samples analysed using a colour grading system:

• Green – Good condition: No genetic changes observed
• Yellow – Moderate condition: weak genetic changes indicated
• Orange – Poor condition: moderate genetic changes detected
• Red – Very Poor condition: major genetic changes detected
• Unclassified – Fish numbers too low to classify

Issued by For further information contact S&TCS Director Andrew Graham-Stewart.

The study “A national assessment of the influence of farmed salmon escapes on the genetic integrity of wild Scottish Atlantic salmon populations” (© Crown copyright 2021) is available here 


Extracts from the study

Executive summary
Interbreeding between escaped farmed Atlantic salmon and wild indigenous salmon (hybridisation) introduces genetic material from farmed stocks into wild populations (introgression) with resulting disruption of the adaptive genetic composition of individuals and populations. This can impact their fitness resulting in a significant negative pressure on the viability of wild populations. Recent advances in analytical and statistical techniques are able to differentiate between farmed salmon of Norwegian origin, native wild Scottish salmon and progeny resulting from interbreeding. By sampling a number of juvenile salmon from a particular location it also possible to estimate the proportion of foreign genetic material present in wild Scottish salmon populations and to identify whether this is due to recent or historical events….. This is the first time a survey to examine the genetic status of populations has been conducted across the geographical extent of Scotland. A bespoke panel of genetic markers, developed specifically to detect genetic changes in Scottish wild salmon, was used to screen tissue samples collected from juvenile fish from 252 sites across Scotland between 2018 and 2019 (n = 2,964 fish). ….these data represent the first national scale examination of the genetic integrity of wild juvenile Atlantic salmon in Scotland in relation to interbreeding with Norwegian farm origin salmon strains.

Of the 252 sites examined, 237 were classified. Of these, signs of introgression were found in salmon at 55 (23.2%) of the sites. Overall classification found 182 (76.8%) sites classified as Good, 21 (8.9%) as Moderate, 20 (8.4%) as Poor, and 14 (5.9%) as Very Poor.

The genetic integrity of populations observed across the country was not uniform. Rather, signs of introgression were concentrated in areas of marine aquaculture production and freshwater smolt rearing. Outside these areas, little to no genetic changes were detected. The available evidence indicates that introgression of genetic material from Norwegian farm salmon strains has altered the genetic composition of some populations within rivers near marine aquaculture production.

This study provides further evidence that escapes of juvenile salmon from freshwater smolt rearing facilities can also affect the genetic status of local salmon populations. Three systems were identified as having notable genetic changes in areas with no marine aquaculture. Two of these (Shin and Ness) contain freshwater smolt production.

Selective breeding programs, targeting important commercial traits… together with unintentional hatchery domestication…. have resulted in aquaculture strains that are genetically and phenotypically very different from wild stocks…. These genetic changes make such fish substantially less fit when released into the wild compared to their wild conspecifics…. In addition, around 90% of aquaculture salmon production in Scotland uses strains of Norwegian origin…. which are genetically distinct from Scottish stocks… and interbreeding of fish from such divergent, genetic groups results in outbreeding depression and associated reduced fitness…Interbreeding of escaped farmed fish of Norwegian origin with wild Scottish fish thus has the potential to negatively impact wild populations. Such effects can persist across subsequent generations and can be increasingly detrimental if the interbreeding is cumulative and frequent.

Summary of findings
Escapees of Norwegian origin salmon from aquaculture production facilities in Scotland have introduced genetic material from farm strains into some wild populations. This introgression is not uniform across the country and appears to be concentrated near areas of marine aquaculture production and freshwater smolt rearing. Outside of these areas any evidence for interbreeding was very limited.

The data analysed here clearly show there has been hybridisation and introgression of genetic material of Norwegian aquaculture strains into Scottish wild salmon populations associated with areas of marine or freshwater production. The regional differences in levels of introgression are striking, and are usually associated with the presence of marine or freshwater production in the immediate area/system. Previous studies indicate there are significant negative fitness consequences of farm/wild hybridisation and introgression of aquaculture origin genes into wild populations...