Genomic analysis of NE Atlantic sardine () reveals reduced variation in a recently established North Sea population and directs reconsideration of management units.

The European sardine () is under intense fishing pressure and exhibits distributional/abundance shifts linked to environmental change. The current understanding of population demographics needed for sustainable management is uncertain due to concerns that previous genetic studies lacked resolution and limited sampling of sardine north of the Bay of Biscay. To address these issues, we performed mtDNA sequencing and genome wide SNP analysis of samples collected across the Bay of Biscay, Celtic Sea, English Channel and North Sea. The complete SNP data reported a lack of structure throughout the sampled area compatible with high gene flow. A consensus suite of positive outlier SNPs was identified which reported a significant correlation with geographical distance with the largest differentiation between the southern Bay of Biscay and North Sea samples which also reported a significant mtDNA Φ. While the roles of dispersal limitation and environmental heterogeneity underpinning this require further study, this adds to growing evidence that selection is influencing sardine population structure against a background of high gene flow. The results indicate that while there may be a level of demographic independence between North Sea and South Biscay sardine, the current delimitation of central (Biscay) and northern (Channel and Celtic Sea) operational stocks may misrepresent connectivity between the Biscay and Channel. The North Sea sample exhibited markedly lower mtDNA and nuclear variation than other samples. As sardine have only recently invaded the North Sea such reduced genetic variation is compatible with predictions for peripheral leading-edge populations but contrasts with patterns for other small pelagic species and emphasises the need to consider species-specific genetic structure in ecosystem-based management. Nascent management of the North Sea sardine fishery must ensure that current low levels of genetic diversity are not eroded further as this may decrease the species adaptive potential and inhibit its expansion.