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Are sea trout stocks prone to diseases and parasites?

Yes, sea trout and salmon are prone to a wide range of these pathogens because they live in both freshwater and marine environments.  Diseases and parasites are widespread and are a natural means of controlling the relative abundance of natural populations. Normally in the wild, however, sick and dying animals are soon removed by predators, or scavengers and by fungi and bacteria. Therefore, the impacts of pathogens can be hard to detect.  In contrast, the severely damaging outbreak of Ulcerative Dermal Necrosis (UDN), which occurred in the UK and Ireland in the mid-1960s, at a time of high abundance of both sea trout and salmon stocks, was very obvious. This was because of the presence in many rivers of so many dead and dying fish which were secondarily infected by fungus, although the causative agent was never fully determined.  Most disease or parasites are not normally so virulent and the rare, more obvious, outbreaks may indicate periods of stressful environmental conditions.