Weak fine-scale genetic structure and considerable life-history diversity in a near-pristine subarctic Atlantic salmon (Salmo salar) river

The Näätämö River, in the far north of Europe, hosts a large Atlantic salmon (Salmo salar) stock with high socio-economic value. The catchment has near-pristine environmental conditions, although there are recent signs of stock declines. It nevertheless offers a good opportunity to monitor the status of a near pristine wild salmon system. This study aimed to characterize the fine-scale genetic structure of Atlantic salmon in the Näätämö River and evaluated the possibility to assign individuals to their population of origin. We genotyped juveniles sampled from eight locations using a 60 K single nucleotide polymorphism (SNP) panel to characterize within-river genetic diversity and structure. We also tested the performance of a previously designed 180 SNP panel to assign individuals to their population of origin. The genetic structure of the Näätämö River salmon population appears weak (FST ranging from 0.001–0.035), possibly due to natural straying. The performance of the predesigned panel in assigning individuals to their population of origin was moderate to high (82–85%), but customization could further enhance its accuracy. Additionally, we described the life-history diversity of salmon within the river using a long-term scale archive of c. 22,000 individuals collected over 48 years, identifying 59 unique life-history strategies. We suggest that to protect the self-sustaining salmon population in the Näätämö River, it would be important to manage the salmon fishery and conserve the population by incorporating up-to-date knowledge of stock status, considering their genetic structure, substantial gene flow and diverse life history.