First records of severely deformed invasive pink salmon in Arctic waters

Native pink salmon have shown low occurrence of vertebral deformities and large inter-population variation in number of vertebrae in the Pacific Ocean. Invasive pink salmon have, however, not been assessed for these endpoints in the North Atlantic and Arctic oceans. The present study assessed number of vertebrae and vertebral deformities (radiology) in the 2020, 2022 and 2024 year-classes of invasive pink salmon captured in Norway. The prevalence’s of vertebral deformities (mean 29%) were higher than earlier reported in native pink salmon. The observed deformities were worse in the 2024 year-class where two adult fish with severe deformities were observed in Arctic waters. More specifically, one fish with whole body kypho-lordo-kyphosis captured in Finnmark, and one fish with complete ural region vertebral fusion and folding of the tail fin captured in Troms. Finally, a significantly lower number of total vertebrae were observed in the 2024 year-class (mean 70.2) compared to the 2020 and 2022 (both mean 70.8) year-classes. The change in number of vertebrae between year-classes is discussed in relation to early life thermal regime and genotype. The currently observed 3.5% prevalence of severely deformed adult pink salmon in the Arctic may be a warning sign of reduced ecosystem health. The present findings argue for including vertebral deformities and meristics in the surveillance of invasive pink salmon in Norway.