Morphological organ alterations and infections diseases in brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss exposed to polluted river water

Poor water quality is discussed as a major factor causing a decline of brow n trout populations in Swiss rivers. For our study we have chosen a river i n the Swiss midlands, where the brown trout population has decreased dramat ically during the last 10 yr and where feral fish have shown distinctive pa thological alterations. The objective of our study was to investigate wheth er river water may be responsible for impaired fish health leading to an in creased mortality in the river. In an active monitoring program, groups of brown and rainbow trout were exposed to polluted river water for 24 mo. Fis h held in tap water served as a reference. Mortality, macroscopic and histo pathologic changes, and infectious agents were investigated. Compared with the reference group, high mortality rates and severe pathological alteratio ns of the inner organs were observed in fish held in river water. Especiall y gills, liver and kidney of these fish showed significantly higher changes than fish from tap water. These changes were dominated by degenerative and inflammatory reactions. Additionally, several infectious agents were diagn osed in fish exposed to river water. The most important findings were furun culosis and proliferative kidney disease. Brown trout seemed to be more sen sitive than rainbow trout to environmental stress and infectious agents.