Holocene environmental changes and the seal (Phocidae) fauna of the Baltic Sea: coming,going and staying

• 1 The occurrence of different species of seal (Phocidae) during the Holocene in north‐eastern Europe was influenced by a changing geological situation over the last 12 000 years, characterized by alternating freshwater, brackish and marine stages and by spatially and temporally limited opportunities for migration of marine species into the Baltic basin. The patterns of immigration by ringed seal Phoca hispida, harp seal Phoca groenlandica, grey seal Halichoerus grypus and harbour seal Phoca vitulina are reviewed here in order to understand present distributions and population sizes. Furthermore, the future of their populations is analysed.
• 2 Phoca hispida immigrated into the Baltic basin during its brackish Yoldia Stage at the Late Glacial/Early Holocene boundary and has persisted in some disjunct populations until now. A second immigration followed at the beginning of the Littorina Transgression – which resulted in the present stage of the Baltic Sea, obviously caused by the strong cooling event that occurred 6200 calibrated ¹⁴C years BC. Its present status is not threatened, but global warming will restrict suitable habitat.
• 3 Phoca groenlandica had a mid‐Holocene intermezzo in the Baltic Sea. Its later disappearance was probably caused by hunting of subadult animals by humans.
• 4 Halichoerus grypus and Phoca vitulina also immigrated at the beginning of the present stage of the Baltic Sea. Whereas H. grypus has been common from this beginning point until modern times, the population of P. vitulina persisted at a low level for millennia. Only since the 18th century has its population increased and spread over the whole Baltic Sea.