2.1 Sampling and laboratory procedures
The study was based on 74 tissue samples collected in eight different regions across the North Atlantic and adjacent waters (Figure 1): eastern Canada (CA), Iceland (ICE), Barents Sea (BAS), North Sea (NOS), Belt Sea (BES), Proper Baltic Sea (PBS), Iberia (IBE), and Black Sea (BLS). The NOS-BES border was located at the latitude 56.95ºN, as a straight line from Denmark to Sweden (Sveegaard et al., 2015), while the BES-PBS border was placed as a diagonal line from the Swedish Hanö island (56ºN 14.7ºE) to the village of Jarosławiec in Poland (54.5ºN 16.5ºE) (Carlén et al 2018; Amundin et al., 2022). All sampling was performed on by-caught or stranded carcasses, and no live harbour porpoise has been targeted in the scope of this study.
We extracted total genomic DNA from skin or muscle tissue using one of the three following methods: NucleoSpin Tissue Kit, DNeasy Blood & Tissue Kit, or Phenol-Chloroform extraction. DNA concentration and quality were measured using a Qubit Fluorometer and Fragment Analyzer to ensure that chosen samples were not fragmented and at least 300ng of DNA per sample were available. Library and whole-genome resequencing was performed at GENEWIZ from Azenta Life Sciences in Leipzig, Germany, on a NovaSeq 6000 platform with a S4 flow cell and using 150-bp paired-end reads.