The Baltic Sea basin is a dynamic and heterogeneous environment varying over both time and space, due to its fairly recent establishment on the evolutionary scale and increasing anthropogenic forcing, such as sea warming. In this project, I will use both published and newly generated whole genome sequencing data of multiple keystone fish species (the Thorny Skate Amblyraja radiata, the Greater and Lesser Sandeel Hyperoplus lanceolatus, Ammodytes tobianus) that were sampled all over the Baltic Sea (with the extension to the North Sea and the greater Atlantic) covering the well-described salinity and temperature gradient, or sub-basins with distinct water depth profiles and other environmental characters. Leveraging these data, I will investigate their population structure, past demography, genomic background between historical colonization/regional distribution and local adaptation across heterogeneous environments. Additionally, I can conduct genome comparative studies in relation to adaptation to the same environmental changes between e.g., ray-finned bony fish and cartilaginous fish species or between sister species occupying similar ecological niches.