Diseases of wild and farmed Finfish

Some fish and shellfish diseases of particular significance in Scotland.

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What is it?

Gyrodactylus salaris (Gs) is a small parasite (0.5mm in length)  infecting the skin, gills and fins  of fish and is capable of rapid multiplication causing serious physical damage to Atlantic salmon parr. It has been responsible for significant mortalities (up to 98%) of wild Atlantic salmon populations in Norway, and as a result some salmon stocks have been lost completely.  Gs is a listed disease that must be reported under Schedule 1 of the Aquatic Animal Health (Scotland) Regulations 2009. The U.K. is currently recognised as being free from Gs although evidence exists to suggest that our Atlantic salmon populations are highly susceptible to both infection and mortality.

Once introduced it may be impossible to eradicate Gs. Scotland is renowned as being one of the best fishing destinations in the world. We want to protect the health of our rivers and our fish stocks.

Where is it?

Gs is restricted in its distribution to Europe. It occurs naturally in some Baltic areas of Finland, Sweden and Russia where some Atlantic salmon populations tolerate the parasite. Widespread distribution in Norway has resulted following the accidental introduction of the parasite in the 1970s. The parasite is also common within farmed rainbow trout populations, where it has a much lower impact and is capable of going undetected. Gs has been identified in Denmark, France, Germany, Portugal, Spain, Italy, Poland and Macedonia. The UK and Ireland are the only areas known to be free of the parasite. Across Europe many countries have an undetermined status and it is likely that the parasite is present in more countries than is presently known.    


Due to its small size, low power magnification is required to see the parasite, e.g. through the use of a good hand lens. Heavily infected parr appear greyish, with excess mucus, and possibly concurrent fungal infections. Morphological examination can lead to the identification of Gs by a trained morphologist, but the main method for identification and confirmation is the use of molecular techniques involving PCR, sequencing and phylogenetic analysis.

What can I do?

One of the most significant risks to the introduction of Gs is through the movement of live fish. Fish farmers have a legal responsibility to ensure that any stock introduced is accompanied by an appropriate health certificate granting specific assurances with respect to Gs where appropriate. Strict biosecurity measures should be implemented and followed at all times.

There are also risks of introduction through the movement and use of equipment associated with angling and water sports.  If travelling from a region where Gs is present or its status is undetermined all equipment associated with angling and water sports should be thoroughly cleaned and treated to kill the parasite by either:

  • Drying at a minimum temperature of at least 20 °C for at least 2 days

  • Heating for at least 1 hour at temperatures of above 60 °C

  • Deep freezing for at least 1 day

  • Immersion in a solution* suitable for killing the parasite for a minimum of 10 minutes

    *Virkon ® Aquatic (1%),Wescodyne (1%), sodium chloride (3%), and sodium hydroxide (0.2%).

Contingency Planning

Scotland has a contingency plan outlining the steps which will be considered should Gs be identified in Scotland and the UK. The Scottish Government’s response was most recently tested in 2015 through a joint exercise involving UK government and a wide participation of internal and external stakeholders.

Please note: The contingency plan is currently under review and is being updated.

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