Rapid increase in abundance and distribution of invasive pink salmon (Oncorhynchus gorbuscha) within a diverse, large Barents Sea catchment.

Pink salmon originate from the North Pacific area but were introduced into northwest Russia from the late 1950s onwards. Since 2017, the alien species has increased dramatically in abundance and rapidly invaded adjacent areas of the North Atlantic region. In the large Teno River in northernmost Norway and Finland, running to the Barents Sea, various monitoring methods originally designed for assessment of Atlantic salmon populations have been used to observe the development in abundance and distribution of pink salmon in the main stem and in a number of tributaries. In addition, environmental DNA (eDNA) sampling has enabled monitoring of these trends across an even wider set of tributaries. The first observations of pink salmon were made in the 1960s, and variable but mostly low catches were recorded in the following decades. In recent years, the total number of pink salmon entering the Teno system increased rapidly from c. 5000 in 2017 to c. 180,000 in 2023. Initially, the invading pink salmon were occupying the main stem, large tributaries and headwater rivers of the catchment, even up to a distance of 250-350 km from the sea. However, in recent years, a greater number of smaller tributaries have been occupied as demonstrated by eDNA detections and other observations. The largest spawning aggregations of pink salmon have been observed in the main stem of the Teno River. Future development in the abundance and dispersal of pink salmon in the Teno system depends strongly on the extent and success of the mitigation efforts in intercepting and removing pink salmon by a weir and trap close to the estuary.