Inland Fisheries Ireland is asking anglers in Mayo and the general public to report any sightings of Pacific pink salmon.
Also known as "humpback salmon", pink salmon were very rare in Irish waters until 2017, and are believed to have originated from stocking programmes in Russia.
Scientists at Inland Fisheries are concerned that, if there are large numbers of non-native species in Irish rivers, this may have negative impacts on Ireland's salmon and trout populations in the future.
The first reported catch of a Pacific pink salmon in Ireland this year was in the Ridge Pool in the Moy Fishery in Ballina on 27th June.
Inland Fisheries Ireland has published a guide on its website to help the public identify the Pacific pink salmon, which has large oval black spots on its tail.
Any sightings should be reported to the 24-hour confidential hotline on 1890 34 74 24.
Anyone who catches one of these fish should keep it, and not release it back into the water - even in rivers that are only open for catch and release angling.
Record the date and location of the catch, and the length and weight of the fish, take a photo of it, tag it and report it to Inland Fisheries Ireland as soon as possible, who will arrange collection of the Pacific pink salmon for further analysis.