Low genetic variation and strong genetic structure across a range of geographical scales in European smelt (<scp><i>Osmerus eperlanus</i></scp> L.)

Populations of anadromous European smelt (Osmerus eperlanus L.) are declining across its range with mitigation efforts for this ecologically important species hindered by a lack of demographic information. Here, mitochondrial DNA (mtDNA) and microsatellite analyses were used to describe historical and recurrent demographics for the species across a large part of its range. mtDNA revealed a shallow phylogeographic structure indicating a cohesive ancestral population, low overall haplotype and nucleotide diversities. However, microsatellites revealed unexpectedly high genetic structuring (F_ST = 0.15; p < 0.0001), including (i) isolation by distance effects over various scales, (ii) separation between Baltic and Atlantic samples and (iii) the highest interpopulation divergence and the lowest intrapopulation variation among UK sites. The results indicate that despite considerable dispersal potential, there is strong structuring among rivers, which should be recognised as separate management units. Furthermore, individual clustering analyses revealed further population separation within waterways and the need to resolve isolating mechanisms. Overall levels of genetic variation were found to be lower than those reported for other osmerids, with evidence suggesting that a considerable portion of ancestral variation has been eroded. As such, low genetic variation may limit resilience to environmental change. Proactive management strategies are discussed, with the prioritisation of UK populations recommended.