Ulcerative dermal necrosis

Ulcerative dermal necrosis (UDN) is a chronic dermatological disease of cold water salmonid fish that had a severe impact on north Atlantic Salmon and sea trout stocks in the late 1960s and 1970s–1980.

[1] Affected fish develop severe skin lesions which begin on the head and back, and near the tail.

[2] Lesions become infected with overgrowths of oomycetes, such as Saprolegnia,[1] giving the affected areas a slimy blue-grey appearance.

[2] The most severely affected fish frequently die before 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.