Farmed salmon DNA found in wild fish in a third of tested Irish rivers
Introgression was widespsread but impact was mostly low, report researchers
Around a third of wild salmon sampled by researchers in Ireland had traces in DNA from escaped farmed fish, a report has revealed.
Based on samples of over 8,000 contemporary fish (2023-2025) and historical samples (2003-2008), an approach based on genetic identifiers – single nucleotide polymorphisms, or SNPs - was used to quantify farm ancestry in individual river populations.
Results show evidence of introgression in approximately one third of populations sampled, with levels of geneflow considered as being generally low to moderate.
Affected populations were found nationwide, including in areas distant from major aquaculture activity, although higher levels were more common near marine net-pen farms and in rivers where freshwater smolt rearing facilities are located.
A baseline for monitoring
The study was led by University College Cork and undertaken in collaboration with Inland Fisheries Ireland, the government agency assigned to protect, manage and conserve Ireland's inland fisheries and sea angling resources, and Teagasc, Ireland’s Agriculture and Food Development Authority.
The findings of the report, called An Assessment of the Level of Genetic Introgression in Irish Wild Salmon Populations from Farmed Escaped Salmon (GeneFlow), provide a baseline for monitoring and highlight the importance of ongoing surveillance and management of salmon aquaculture installations to protect genetic integrity of wild stocks, write its authors.
The assessment was undertaken using a sample of 6,322 juvenile Atlantic salmon individuals collected contemporarily from 166 locations across 133 river populations throughout Ireland in 2023, 2024, and 2025. Sufficient sample size for a population level assessment was obtained for 152 locations across 123 river populations.
In addition, 1,755 historical juveniles from 61 rivers collected between 2003 and 2008 as part of the National Atlantic Salmon Genetic Stock Identification Programme were also genotyped.
Genetic introgression was found to be widespread across the country although the magnitude of the impacts varied and there was only one case that was classified as high impact.
Evidence of genetic introgression was detected in 41 of the 123 (33.3 %) contemporarily sampled populations. Of those 41 populations, 33 were classified as having low impact from introgression, seven as having moderate impact, and one as having high impact.
Rivers close to sea farms and to freshwater rearing facilities were more heavily affected, but there were other affected rivers where the intrusion was of unknown origin.
Overall introgression levels in Ireland were lower than reported in Norway, particularly in respect to the high impact category. Several rivers and districts showed evidence of elevated levels of gene flow adjacent to known salmon farming areas, indicating local introgression hotspots e.g. Connemara, Ballinakill, Galway, Ballyshannon and Letterkenny.
There were also several rivers that were not classified due to low sample sizes but that had introgression scores suggestive of impact, or at least one fish that strongly resembled a recent escape or the descendant of a recent escape.
No new populations
There was no evidence for the establishment of novel feral salmon populations arising from escapes in rivers sampled that had no salmon previously (i.e. those rivers close to marine salmon farms).
A comparison between contemporary and historical samples (early 2000s) did not show any consistent pattern in terms of increase or decrease in the relative level of farm ancestry. Eight of the 42 rivers compared showed an increase, while six showed a decrease and 28 showed no significant change.
Key conclusions from the study were that:
Rivers affected by genetic introgression from farmed salmon were found throughout the country.
Populations adjacent to areas of marine net-pen production commonly exhibited higher levels of genetic introgression.
Populations adjacent to juvenile salmon production facilities also commonly exhibited higher levels of genetic introgression.
Rivers located far from areas of Irish salmon farm production (i.e. Northeast, East and South coasts) showed unexpected, elevated levels of genetic introgression.
The report’s authors recommend that a programme of genetic surveillance is established to monitor wild Atlantic salmon populations on an ongoing basis, and that research is undertaken “to establish the biological implications of the observed and any potential future farm mediated geneflow on the immediate and long-term health (i.e. genetic integrity and productivity) of recipient wild populations”.
