Gyrodactylus salaris is an extremely dangerous parasite which currently is not present in the British Isles. It is a parasitic freshwater fluke which is indigenous to Baltic Sea rivers in Russia, Norway and Sweden, where salmon are resistance to it. However, G. salaris has spread to Atlantic seaboard rivers in Norway, Sweden, Finland, Denmark, Germany, France, Spain and Portugal where Atlantic salmon have no resistance, resulting in mass mortality of juvenile fish. In Norway infected rivers lost 98% of their salmon within 5 years. It is a minute, external parasite, less than half a millimetre long and can live in the gravel, away from its fish host, for a considerable time. Methods being used in Norway to eradicate the fluke involve poisoning all the fish in the river and erecting barriers to stop salmon entering the river to spawn and thus re-infesting the system. G. salaris can survive for 5 to 7 days without a host in damp conditions such as might be provided by clothing, waders, wet reels, lines or landing nets that have been used in the at risk locations listed above.
The River Annan Board is determined to keep G. salaris away and requests that all anglers who have visited the areas at risk within that timescale ensure that their equipment is thoroughly dried and then disinfected.
Where and When Might it Occur?
Only Atlantic salmon are severely affected by this parasite, although G. salaris has been reported from Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), Arctic char (Salvelinus alpinus), North American brook trout (S. fontinalis), grayling (Thymallus thymallus), North American lake trout (Salvelinus namaycush) and brown trout (Salmo trutta). Salmon from Scottish rivers have also been shown to be susceptible to this parasite.
What is it?
Gyrodactylus are small, leech-like parasites. Over 400 species have been described, from fish and frogs, in fresh and salt water. In Norway, catastrophic losses of Atlantic salmon were seen following the introduction of G. salaris to the country in the 1970s. As of 2001, 41 Norwegian rivers have been infected and their salmon populations effectively exterminated. These parasites are remarkable in that they give birth to live young. The daughter parasite is the same size as the mother, and inside this daughter there is already a developing granddaughter, in a 'Russian doll' arrangement.
The parasite can be seen under low power magnification, for example, with a good hand lens. Without magnification, heavily infected parr appear greyish, with excess mucus, and possibly concurrent fungal infections.
They attach to the host by the attachment organ or opisthaptor at one end of the body and feed using glands at the other end. Attachment can cause large wounds and feeding can damage the epidermis, allowing secondary infection. G. salaris can build up to very high infection intensity of several thousand parasites on a single salmon parr.
Rotenone treatment has been used to eradicate G. salaris from some rivers in Norway. This kills all fish in the river, and restocking is carried out from eggs and juveniles collected prior to treatment. Not all rotenone treatments have been successful, and such drastic treatment is only possible in short rivers with favourable biological and geographical conditions. The parasite cannot survive full strength sea water, so natural migration of fish is unlikely to spread infection.