Salmon parasite Gyrodactylus salaris

Gyrodactylus salaris is about 0.5 mm long parasite living on the fins, skin and gills of salmon (Salmo salar). It has sharp hooks that it uses to attach itself to the surface of salmon fry. The parasite feeds on cells and mucus around the attachment site. Both the attachment and feeding disturb the normal function of the skin and gills. A mild infection does not harm the fish very much, but in case of more serious infection the mechanical irritation will lead to excessive excretion of mucus. The damaged skin is vulnerable to infection and fungal diseases (Saprolegnia). Mortality depends on the fry resistance and environmental conditions.

The parasite is dangerous for salmon stocks migrating to the Atlantic or Arctic Ocean, but salmon in the Baltic Sea catchment area are not affected.

Gyrodactylus produces live offspring. The parasite carries the next generations inside itself, so it can reproduce very fast under favorable conditions.

The parasite is not transmitted to humans or domestic animals, and does not prevent the use of fish for human consumption.


Gyrodactylus salaris hardly ever causes any symptoms in Finnish fish farms or in wild salmon in the Baltic Sea catchment area. Once attached on the skin of wild salmon migrating to Atlantic, the parasite reproduces exponentially and thousands of parasites on the skin cause fluid imbalance and make fish vulnerable to secondary pathogens. Infected salmon have greyish skin and tattered fins.

Diagnosis and sampling

Gyrodactylus salaris cannot be diagnosed visually. Newly killed or anesthetized fish can be examined while still in water using a dissecting microscope or a good magnifying glass. Also, ethanol preserved whole fish or fins can be used for examination. G.salaris parasites are most often located on pectoral and pelvic fins.

Several species of Gyrodactylus family are known. If Gyrodactylus parasites are found, the species must always be determined. The determination of the species is nowadays done using PCR methodology.


Gyrodactylus salaris mainly spreads from infected fish but also via fishing gear, water and other goods. It can also reproduce on the skin of rainbow trout, arctic char, grayling and brown trout and live for short periods on the skin of many other fish species. Gyrodactylus is a freshwater parasite but it can reproduce in up to 5 ‰ brackish water and survive short periods of time at a higher salinity. The parasite survives well in Finnish sea areas.

Control and prevention in Northern Lapland

Gyrodactylus salaris is a notifiable fish disease in Finland. River catchments of rivers Tenojoki and Näätämöjoki are officially free of this parasite. River catchments of rivers Paatsoki, Uutuajoki and Tuulomajoki form a buffer zone. The cathment areas of these five rivers that drain to the Arctic Ocean form a zone where Gyrodactylus salaris finding would lead to control measures.

It is forbidden to transfer live fish or non-disinfected fish eggs  from the other parts of Finland to the Upper Lapland or from any other area that is not officially proven free of Gyrodactylus salaris. It is also forbidden to transfer live fish or non-disinfected fish eggs from the water catchments of River Paatsojki, Tuulomajoki or Uutuajoki to the river catchments of Tenojoki or Näätämöjoki.

The use of live or dead bait fish is banned in angling, ice-fishing and lure-fishing In Upper Lapland. Bait fish may not be brought from other river systems to the Upper Lapland watercourses, nor may they be moved between these areas.

All boats and canoes as well as fishing gear, diving gear and any other equipment intended to be used in water sports etc. brought from other river systems to the Upper Lapland region must be completely dry or disinfected before they are used. G. salaris can survive several days without a host fish and can thus spread in water or through wet fishing gear and equipment.

Gutting fish caught from other river systems or discharging fish waste in natural waters is prohibited in the Upper Lapland region.

The bilge water of boats, household water from caravans or camper vans or pontoon water from aircraft must be absorbed into soil at a sufficient distance from the shore.

Find out more from Evira’s brochure Protect the salmon – Prevent the Spread of Salmon Parasite Gyrodactylus salaris.


Roadside signs remind tourists of G salaris protection zone in Northern Lapland.


Fin samples from wild fish from the free zone and buffer zone and from a fish farm in river Paatsjoki area are examined yearly. 

Occurrence in Finland and elsewhere

The parasite is quite commonly found in Finnish rainbow trout farms in Baltic Sea catchment area. It is also common in the best wild salmon spawning river of the Baltic Sea, River Tornionjoki. Rivers draining to the Arctic Ocean are free of this parasite.

G salaris is also found in the neighbouring countries of Finland. In Norway, G salaris has been found in over 40 salmon rivers, the noththest infected river being river Skibotten. In Russia, Gyrodactylus salaris has been found both in Baltic Sea catchment area and in rivers Kieretti and Vienan Kemi that drain to the White Sea.