Detection of Non-Native Pink Salmon (Oncorhynchus gorbuscha) in Swedish Rivers Using eDNA

Biological invasions are increasing on a worldwide scale and can have severe ecological and economic consequences. Pink salmon (Oncorhynchus gorbuscha) have a native range in the Pacific Ocean but have been introduced and are now spreading in the Arctic and North Atlantic Oceans, including Sweden. A comprehensive eDNA sampling scheme was conducted in 27 river systems in south-western Sweden in 2023, with the aim to detect adult spawning pink salmon. We applied qPCR and dPCR analysis on aquatic eDNA samples. Results indicated the presence of pink salmon at 24 sites across 11 river systems. Pink salmon were not detected in any of the sampled rivers that drain into the Baltic Sea. However, pink salmon were present in a river only 35 km from the entrance to the Baltic Sea, which means there is a high risk pink salmon will spread into the Baltic Sea in the coming years. Catch reporting is generally low and camera fish counters are few across these systems; therefore, the strength of incorporating eDNA methods is beneficial for the development of pink salmon monitoring programs and aquatic invasive species management.