by David Forsythe

FARMED salmon genes have been found in wild stock in a West Cork river, a new study has found.

The first nationwide study of its kind, led by University College Cork (UCC), discovered that genes from escaped farmed salmon were found in wild fish in the Glengarriff River.

The West Cork river was the only one in the Cork fisheries district where farmed ancestry was detected. The level found in Glengarriff was classed as low, with farmed ancestry estimated at 1.5%. An older sample taken from the same river in 2006 recorded a slightly higher level of 2%. Wild salmon in several other West Cork rivers showed no evidence of an impact from farmed fish.

No farmed ancestry was detected in the Bandon, Argideen, Ilen, Mealagh, Ouvane, Coomhola or Adrigole populations assessed. The River Lee population also showed no evidence of an impact.

A sample from the Leamawaddra River could not be fully assessed because only two fish were available for analysis.

The report does not identify the source of the farmed genes found in Glengarriff or say when the interbreeding occurred. Researchers said the farmed ancestry recorded across Ireland resulted from incidents that happened at least four years ago and probably longer ago.

The study was led by UCC in collaboration with Inland Fisheries Ireland and Teagasc. It found evidence of farmed salmon genes in one third of the wild populations tested across the country.

Wild Atlantic salmon fry.

A total of 41 of the 123 populations that could be fully assessed were affected. Thirty-three had a low level of farmed ancestry, seven had a moderate level and one was classed as having a high impact. Researchers examined 6,322 young salmon collected from 166 sites across 133 river populations between 2023 and 2025. Older samples collected from 61 rivers between 2003 and 2008 were also studied.

The report found that rivers close to marine salmon farms and freshwater salmon-rearing facilities were generally more heavily affected. However, it stopped short of linking the findings in individual rivers to particular farms or escape incidents.

Inland Fisheries Ireland senior research officer Dr Michael Millane said escaped farmed salmon were an added threat to already vulnerable wild stocks. ‘When farmed salmon interbreed with our wild salmon, they dilute the unique genetic integrity that adapts wild populations to their river of origin,’ he said.

‘This genetic disruption can ultimately lower offspring survival rates and reduce the overall resilience of wild populations to other pressures that impact them.’

The researchers have called for an ongoing national monitoring programme, with about one third of Ireland’s salmon rivers sampled every year. That would allow every river to be tested once every three years, while rivers at risk following a known escape would be examined immediately. The project has received additional funding and more salmon samples are due to be collected during the summer of 2026.