Farmed salmon DNA has been detected in wild Atlantic salmon populations in around one-third of the Irish rivers examined in a nationwide genetic study.
The research, led by University College Cork in collaboration with Inland Fisheries Ireland and Teagasc, analysed more than 8,000 salmon samples collected between 2003 and 2025 to assess the extent of genetic mixing between escaped farmed salmon and wild populations.
Using single nucleotide polymorphism (SNP) markers, researchers identified evidence of genetic introgression in 41 of 123 river populations sampled between 2023 and 2025, representing 33.3% of the surveyed rivers. Most affected populations showed low levels of introgression, while seven were classified as moderate and only one as high.
The study found affected rivers across Ireland, including regions far from aquaculture sites. However, the highest levels of farmed salmon ancestry were generally recorded in rivers located near marine net-pen farms and freshwater smolt-rearing facilities.
Researchers analysed 6,322 juvenile Atlantic salmon from 123 river populations sampled between 2023 and 2025, alongside 1,755 historical samples collected from 61 rivers between 2003 and 2008.
Although genetic introgression was widespread, the report notes that its overall level in Ireland remains lower than that reported in Norway, particularly for rivers classified as having a high impact. Localised hotspots were identified in areas including Connemara, Ballinakill, Galway, Ballyshannon and Letterkenny.
The study found no evidence that escaped farmed salmon have established self-sustaining feral populations in rivers where wild salmon were previously absent.
A comparison of historical and contemporary samples showed no consistent national trend. Farmed salmon ancestry increased in eight rivers, decreased in six and remained largely unchanged in 28.
According to the report, the findings provide a baseline for future monitoring and underline the need for continued genetic surveillance of wild Atlantic salmon populations. The authors also recommend further research to assess the long-term effects of farm-origin gene flow on the genetic integrity and productivity of wild salmon stocks.
