| Summary: | Environmental DNA (eDNA) monitoring methods are increasingly used as a supplement or substitute to conventional monitoring. This rapidly advancing research field promises improvements for aquatic species conservation and the detection of invasive species and pathogens. The eDNA dynamics of some groups of organisms like fish have been extensively studied, in particular fish of commercial interest or where there is a high invasive potential. However, there are still many knowledge gaps on eDNA dynamics and monitoring potential for rare and elusive species, and for host-pathogen complexes. The overarching goal of this thesis was to explore, develop and evaluate the potential of targeted eDNA detection and quantification as surveillance and biosecurity tool both in nature and aquaculture. For this purpose, we chose two dissimilar host-pathogen complexes, which are of economic importance and relevance regarding species conservation: The Atlantic salmon and the salmonid parasite Gyrodactylus salaris and freshwater crayfish with their obligate parasite Aphanomyces astaci. The salmon fluke G. salaris has caused significant damage to indigenous Atlantic salmon populations in Norway, and the Norwegian Government is working towards the eradication of this parasite. The oomycete A. astaci, carried and transmitted by American freshwater crayfish species, causes crayfish plague and is the largest threat to endangered European crayfish species, and is registered as a list 3 disease (national disease) in Norway. The same applies for G. salaris. In these host-pathogen complexes, fish shed much larger amounts of eDNA than crayfish as they are covered with a mucus layer. Conversely, the sporulating oomycete A. astaci is readily detectable using the eDNA methodology while the flatworm G. salaris assumingly only shed minute amounts of eDNA. Three main research questions were asked: 1) Can the eDNA methodology work equally well or better than conventional methods for biomonitoring of the host-pathogen models, particularly at low ...
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