Ulcerative dermal necrosis
Encyclopedia
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
and sea trout
Brown trout
The brown trout and the sea trout are fish of the same species....
stocks in the late 1960s, the 1970s and 1980.
Affected fish developed severe skin lesions over large parts of their body which penetrated into skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
. The onset of symptoms only occurred after migration into freshwater. Lesions became quickly infected with overgrowths of Saprolegnia
Saprolegnia
Saprolegnia is a genus of freshwater mould often called a "cotton mould" because of the characteristic white or grey fibrous patches it forms. Current taxonomy puts Saprolegnia as a genus of the heterokonts in the order Saprolegniales.-Habits:...
fungus giving the affected fish an appearance of being covered in slimy white pustules. The most severely affected fish frequently die before spawn
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
ing.
Although the worst effects of the disease were seen in the 1970s and 1980, even now large numbers of salmon will succumb to the disease after spawning. This is thought be due in part to their weak post-spawning condition, and lack of food for several months whilst in the river.
Those fish that do make it back to the sea are thought to make a good recovery.