A review of protistan parasites in and around Norwegian marine fish farms

Protistan parasites are an integral part of marine ecosystems and can serve as indicators for environmental health and ecosystem status. This study provides a review of the protistan parasite taxa reported in and around Norwegian marine fish farms, the functional groups they represent, and which potential implications their occurrences have for environmental stress. By focusing on the parasite life cycles, pathogenicity, and host specificity, we aimed to determine whether they are internal or external, monoxenous or heteroxenous, and generalist or specialist parasites for inference of the functional groups. Our findings reveal that the majority of reported parasites are external or internal, monoxenous, and generalist taxa, such as Paramoeba perurans, Spironucleus salmonicida, Ichthyobodo, and Ichthyophonus hoferi, which can indicate that the environment along the Norwegian marine shoreline is experiencing moderate to high levels of environmental stress and some degree of degradation. By contrast, the presence of only a few parasitic taxa such as Trichodina cooperi and Trypanosoma pleuronectidium is indicative of low levels of stress and mildly degraded conditions. Notably, parasites typically associated with healthy environments, such as external, heteroxenous specialists, were largely absent. These patterns suggest that Norwegian coastal ecosystems in and around fish farms are subject to continuous environmental stress, potentially due to the farming conditions and other anthropogenic pressures. While not all detected protistan parasites are currently associated with disease outbreaks in Norway, many are known pathogens elsewhere or may act as opportunists under changing environmental conditions. Their presence highlights the importance of continued research efforts on protistan parasites of fish, especially in the context of climate change and evolving fish farming practices.