Gyrodactylus salaris

Gyrodactylus salaris


Habitat: Gyrodactylus salaris is a parasite which infects the skin, gills and fins of salmon, trout and some other types of fish in freshwater.

Description: Gyrodactylus salaris is a parasite which infects the skin, gills and fins of salmon, trout and some other types of fish in freshwater. It is less than half a millimetre in size, so small that it is barely visible to the naked eye. Despite this, it can cause serious damage in some strains of Atlantic salmon.


Origin and Distribution: Gyrodactylus salaris occurs naturally in the Baltic rivers of Finland and Russia (possibly also eastern Sweden). The native fish of these rivers, including Baltic salmon, are tolerant of the parasite and normally the infection causes them no harm. Atlantic salmon in areas where the parasite does not naturally occur have little or no tolerance of it. Some years ago, Gyrodactylus salaris was accidentally transferred for the first time to some rivers of the west coast of Sweden, to Norway and more recently to some rivers in northern Finland and northern Russia.


Impacts: The effects of the disease are so serious that salmon stocks have now been lost completely from more than 20 Norwegian rivers, with the particular races of salmon in the affected rivers being lost forever. Gyrodactylus salaris does not occur in Irish or UK rivers but experiments carried out in Norway have shown that our salmon, like those of Norway, are killed by the parasite. Gyrodactylus salaris (GS) is probably the greatest threat to wild salmon in the UK and Ireland, it is therefore essential that the parasite is not introduced.

In Norway infected rivers lost 98% of their salmon within 5 years. Infected rivers must be poisoned (Rotenone) to remove all fish hosts (See image below), and barriers erected to stop salmon entering the river to spawn and generate more hosts. 

 How might it get here? Contamination of fishing equipment, fish bait and introduced fish. GS can survive for 5 to 7 days without a host in damp conditions (e.g. angling clothing, waders, wet reels, lines or landing nets).


Prevent Spread:

Download and keep the Environment Agency Gyrodactylus prevention flier

Follow the guidelines each and every time you return to Ireland regardless of the Gyrodactylus status in the country you visited. Remember: prevention is better than cure. It only takes one infected fish to start an epidemic. If Gyrodactylus becomes established in Ireland every fish capable of harbouring the parasite must be removed from the affected area. In Norway this has involved the poisoning of whole catchments.

Report all suspected cases of this species. 

Invasive Species Ireland

Invasive Species Ireland