Balancing the restoration of a native fish and the risks of hitch-hiking invasive species

Reintroducing a species to an area where it is locally extinct may contribute to reestablishing the ecosystem.However, we show that such reintroductions can pose a risk to biodiversity by introducing hitchhiking invasive organisms together with the donor population.This risk was caused by more severe impacts of invasive organisms in the habitat of the donor population than the receiving environment.The freshwater resident Atlantic salmon (Salmo salar) in Sweden's River Klarlven perform feeding migrations to Lake Vnern.The upper part of the watershed, in Norway, lost its salmon population due to hydropower development that obstructed upstream migration.We conducted a risk assessment of the potential impacts on native ecosystems from invasive organisms associated with reintroducing Atlantic salmon into the Norwegian part of the watershed by importing adult salmon spawners.This assessment is crucial due to differences in the development of invasive organisms in the Swedish and Norwegian parts of the watershed.The risk of impacts was evaluated for invasive species, parasites, bacterial pathogens, and viruses that are present or likely present in the lower part of the watershed, or at risk of being introduced.We found a high risk of negative impacts associated with the parasite Gyrodactylus salaris, viral hemorrhagic septicemia virus (VHSV), Aphanomyces astaci causing crayfish plague, Renibacterium salmoninarum causing bacterial kidney disease, and Tetracapsuloides bryosalmonae causing proliferative kidney disease.In addition, 20 invasive species and pathogens were associated with a medium risk and three with a low risk.The case study contributes to a deeper understanding of how reestablishing locally extinct or diminished species can influence biodiversity conservation efforts and the health of aquatic ecosystems, underlining the importance of comprehensive planning in restoration projects.We highlight the importance of risk assessment of invasive species when considering the reintroduction of native species or dam removal.