Research over the past few decades has shown that Baltic Sea populations of a majority of species are genetically distinct. The uniqueness of the Baltic salmon was recognised already in the 1980s and the cod east of Bornholm could, based on recent genetic data, be regarded as a separate species. The flounder and the bladderwrack have both diverged in the Baltic Sea and formed new and endemic species.
The marine species colonized the Baltic Sea less than 8 000 years ago. Since then, the Baltic populations have been shaped by natural selection and rapid evolutionary adaptation to the environment, not least the low salinity. If Baltic Sea populations go extinct they may not be easily replaced, because of the genetic adaptations that must take place.
Marine species have key roles
The Baltic Sea is a rare brackish environment. Only a few marine species survive in the low salinity, but these species typically have key ecological roles. As an example, cod has top-down regulatory effects on the whole ecosystem. Bladder wrack and eelgrass forms important habitats for juvenile fish, invertebrates and other species of algae. In addition, freshwater species, including coastal fishes such as pike and perch, survive in the brackish water and add important contributions to the ecosystem.