Gyrodactylosis is due to Gyrodactylus salaris a freshwater external parasite of salmon which infects the skin, gills and fins of salmon, trout and some other types of freshwater fish. The parasite is less than a half millimetre in size and so is barely visible to the naked eye and causes serious mortalities to Atlantic salmon (Salmo salar). It has largely exterminated salmon in 41 Norwegian rivers since its introduction. Tests have shown that populations of Scottish salmon are also susceptible to the parasite and so a similar catastrophic impact is expected if it is introduced and becomes established in Scotland.

Signs of Infection

Heavily infected fish appear greyish, with excess mucus and possibly concurrent fungal infections. The parasite can be seen under low power magnification such as a hand lens.


The parasite is very hardy and may be introduced by fishermen and other water users returning from infected countries as it is capable of surviving for several days in damp conditions such as plastic bags, watersports equipment (such as waders, fishing equipment, bags, canoes and windsurf gear, clothing) and on the surface of dead fish.

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