Archival and modern DNA SNP data of 13 Baltic salmon populations

Intra-species genetic homogenization arising from anthropogenic impacts is
a major threat for biodiversity. However, few taxa have sufficient
historical material to systematically quantify long-term genetic changes.
Using archival DNA collected over ~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
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. 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 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.