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 (F_ST 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.