The impact of climate change on the adaptation of marine fish in the Baltic Sea

The biota in the Baltic Sea has formed under wide variations in environmental conditions induced by climate change since the last glaciation. Areas of the Baltic Sea display marked differences in salinity, oxygen content and temperature of water layers, timing of the peak production cycle, and other basic environmental conditions. Some of the most important adaptations of marine fish to the conditions in the Baltic Sea facilitate reproduction at low and changeable salinity and oxygen deficiency in deeper layers, and enhance the survival of their offspring in early stages of development. In sea herring, flounder, cod, and other late immigrants (which probably colonized the Baltic after the Ancylus stage), abundant year classes occur in periods of higher salinity. During periods of low salinity, the abundance and spatial distribution of marine species is reduced. Probable earlier immigrants (e.g. gulf herring) are less dependent on saltwater inflows, but they depend more on local processes. The success of marine species in different areas of the Baltic Sea has been variable, and species adaptation, mediated by climate oscillations, has resulted in the formation of local populations.