Intra-species genetic homogenization arising from anthropogenic impacts is a major threat to biodiversity. However, few taxa have sufficient historical material to systematically quantify long-term genetic changes. Using archival DNA collected over approximately 100 years, we assessed spatio-temporal genetic change in Atlantic salmon populations across the Baltic Sea, an area heavily impacted by hydropower exploitation and associated with large-scale mitigation stocking. Analysis was carried out by screening 82 SNPs in 1680 individuals from 13 Swedish rivers. We found an overall decrease in genetic divergence and diminished isolation by distance among populations, strongly indicating genetic homogenization over the past century. We further observed an increase in genetic diversity within populations consistent with increased gene flow. The temporal genetic change was lower in larger wild populations than in smaller wild and hatchery-reared ones, indicating that larger populations have been able to support a high number of native spawners in relation to immigrants. Our results demonstrate that stocking practices of salmon in the Baltic Sea have led to the homogenization of populations over the last century, potentially compromising their ability to adapt to environmental change. Stocking of reared fish is common worldwide, and our study is a cautionary example of the potentially long-term negative effects of such activities.


  • Human-driven loss of biodiversity at several levels is threatening the functioning of ecosystems at a global scale [1,2] as we live in the ‘sixth extinction’ geological epoch [3]

  • The temporal genetic analysis spanning a century enabled us to obtain a comprehensive view of the impact of humandriven genetic changes arising from one of the oldest and most extensive stocking experiments in the world

  • Based on comparisons between archival DNA and contemporary samples from rivers on a geographical distance of over 1400 km, we provide strong evidence of genetic homogenization among Atlantic salmon in the Baltic Sea over the last 100 years

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Human-driven loss of biodiversity at several levels is threatening the functioning of ecosystems at a global scale [1,2] as we live in the ‘sixth extinction’ geological epoch [3]. The loss of biodiversity may occur even without drastic or apparent changes in population abundance. Inter- or intraspecific hybridization can lead to the breakdown of reproductive barriers and previously distinctive populations or species boundaries, a process known as genetic homogenization (GH). Breakdown of historical genetic population structures, leading to potentially locally adapted subgroups of a species becoming genetically admixed, may potentially erase the outcomes of long-term evolutionary processes having shaped specific adaptations [5]. It further may reduce meta-population resilience arising through mechanisms such as the ‘portfolio effect’ [6,7]. Foll M, Beaumont MA, Gaggiotti O. 2008 An diversity losses in European grey wolves


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