Gyrodactylus salaris , commonly known as salmon fluke ,  is a tiny monogenean ectoparasite which lives on the body surface of freshwater fish.  This parasite-like parasite has been implicated in the Atlantic salmon populations in the Norwegian fjords .  It also parasitises other species, including rainbow trout .  G. salaris requires fresh water ,  but can survive inbrackish water for up to 18 hours. 
The parasite is 0.5 mm (0.02 in) long,  and can not be seen with the naked eye, but it can be seen with a magnifying glass .  On its posterior end is a haptor , a specialized organ for attaching to the host fish, which has sixteen hooks around its edge.  The parasite is viviparous , that is, it produces live offspring.  The parasites give birth to live as they grow up, and they are even growing up in the neonates. 
Interactions with host fish
When feeding, the parasite attaches its anterior end to the fish with cephalic acorns. It is a pharynx through the mouth and a digestive solution with proteolytic enzymes which dissolves the salmon skin. Mucus and dissolved skin are then sucked into the gut. Attachment of many parasites can cause wide wounds, which causes secondary infections. 
G. salaris was first described in 1952,  after being removed from a Baltic strain  of Atlantic salmon kept at the Hölle Laboratory in Sweden, near to the river Indalsälv.  At the time, it was not possible to cause illness in the host fish.  The presence of G. salaris on fish became a World Organization for Animal Health notifiable disease in 1983. 
Catastrophic losses of Atlantic salmon occurred in Norway in the 1970s following the introduction of G. salaris . By 2001, the salmon populations of 41 Norwegian rivers had been virtually wiped out in this way. 
Historically, Gyrodactylus -infected rivers have been treated with the indiscriminate pesticide / rotenone piscicide . A newer method of treatment employs small volumes of aqueous aluminumand sulfuric acid in the river. A huge advantage of this method is its ability to kill parasites without harming the hosts. This new method has been shown in Batnfjordelva and Lærdalselva, two rivers in Norway . [ quote needed ]
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