Reconstructing the ecological history of the extinct harp seal population of the Baltic Sea

Abstract

The harp seal (Pagophilus groenlandicus), today a subarctic species with breeding populations in the White Sea, around the Jan Mayen Islands and Newfoundland, was a common pinniped species in the Baltic Sea during the mid- and late Holocene. It is puzzling how an ice dependent species could breed in the Baltic Sea during the Holocene Thermal Maximum (HTM), and it remains unclear for how long harp seals bred in the Baltic Sea and when the population became extirpated. We combined radiocarbon dating of harp seal bones with zooarchaeological, palaeoenvironmental and stable isotope data to reconstruct the harp seal occurrence in the Baltic Sea. Our study revealed two phases of harp seal presence and verifies that the first colonization and establishment of a local breeding population occurred within the HTM. We suggest that periods with very warm summers but cold winters allowed harp seals to breed on the ice. Human pressure, salinity fluctuations with consequent changes in prey availability and competition for food resources, mainly cod, resulted in physiological stress that ultimately led to a population decline and local extirpation during the first phase. The population re-appeared after a long hiatus. Final extinction of the Baltic Sea harp seal coincided with the Medieval Warm Period. Our data provide insights for the first time on the combined effects of past climatic and environmental change and human pressure on seal populations and can contribute with new knowledge on ongoing discussions concerning the impacts of such effects on current arctic seal populations.