Skip to main content
Dryad

The chalk streams of southern England and northern France harbour substantial unique components of the overall genetic diversity of Atlantic salmon (Salmo salar L.)

King, Andrew1; Evanno, Guillaume2; Stevens, Jamie1

Published Apr 08, 2026 on Dryad. https://doi.org/10.5061/dryad.2bvq83c3n

Data files

Apr 08, 2026 version files 1.04 MB

Click names to download individual files

Abstract

Aim: Atlantic salmon continue to suffer marked declines in abundance due to stressors acting in both their freshwater and marine habitats. It is therefore imperative to identify populations in need of conservation intervention, with the aim of preserving as much genetic diversity as possible. Previous analyses have shown the chalk rivers of southern England and northern France to hold genetically distinct populations of salmon. However, these populations have never been investigated in the same study. We sought to understand the relationship between English and French chalk salmon and assess their conservation value.

Location: British Isles and France.

Methods: A suite of 93 single nucleotide polymorphism loci were genotyped in 1640 samples from 42 British Isles and French rivers. Genetic structuring was assessed; SNPs potentially under selection and related to climate adaptation were identified, and the contribution of each river and region to genetic diversity was evaluated.

Results: We demonstrate the French and English chalk salmon to be closely related and confirm their distinction from salmon inhabiting non-chalk rivers. The identification of a small number of significant FST outliers and loci associated with environmental variables suggests that this distinction is driven by local adaptation. We propose that the chalk and non-chalk salmon be designated as two distinct Evolutionarily Significant Units that each contain multiple Management Units. The chalk river salmon, especially those from southern England, are identified as making a significant contribution to the overall diversity of the species within the English Channel region.

Main conclusions: We propose that chalk stream salmon populations may meet the criteria for recognition as a distinct subspecies of salmon, Salmo salar calcariensis. Taken together, the results highlight the urgent need for enhanced conservation and protection for the Atlantic salmon populations inhabiting the chalk rivers of southern England and northern France.