Hydrological Connectivity Versus Anthropogenic Fragmentation: Population Genetic Structure of Eurasian Perch Across Latvian and Lithuanian River Basins

Freshwater ecosystems are increasingly affected by habitat alteration and reduced connectivity, which can influence the genetic structure of aquatic organisms. This study investigated the population genetic structure of Eurasian perch (Perca fluviatilis L.) in major river systems of Latvia (Daugava River) and Lithuania (Nemunas and Neris rivers) using nine microsatellite loci. The quantitative parameters of genetic diversity were assessed using standard population genetic analyses. The evaluation of population genetic structure was also based on Bayesian clustering and phylogenetic reconstruction. All of the studied populations exhibited relatively high genetic diversity, with the number of alleles per locus ranging from 4 to 24 and observed heterozygosity ranging from 0.536 to 0.716. Genetic differentiation among populations was low to moderate but significant, with FST values ranging from 0.021 to 0.079 within the Daugava River basin and reaching 0.034 between the Nemunas and Neris rivers. Bayesian clustering and Neighbor-Joining analyses revealed genetic structuring at both regional and local scales, including pronounced substructuring within the Daugava River, which is divided into a few fragments due to three hydroelectric plants. The observed patterns indicate that population connectivity is not uniform across the study area and likely reflects the combined influence of historical and contemporary processes. These findings improve our understanding of perch population structure in Baltic river systems and provide relevant information relevant for the conservation and management of freshwater fish genetic resources.