Breeding grey seals are still rare in the southern Baltic Sea, Danish Straits and KattegatSubmitted by editor on 12 October 2020.
By Anders Galatius
This new study compiles all available data on grey seal occurrence and breeding in the southern Baltic Sea, Danish Straits and Kattegat from Denmark, Sweden, Poland and Germany. It thus provides an overview of the recolonization of these areas. Although the first pups were recorded in 2003, after 100 years of absence, only 8 pups were recorded in 2020 in the southern Baltic. As such, the number of breeding grey seals in the area is still precariously low.
Throughout the Baltic Sea, the abundance of grey seals declined dramatically from 1900 to the 1970s because of culling campaigns, extensive hunting and pollution. The latter caused up to 80% of the female grey seals to become sterile and unable to have pups. Protection from hunting and lower concentrations of pollutants have allowed the population to recover. Since the early 2000s, grey seals from the larger colonies in the central Baltic Sea have been visiting the southern Baltic Sea in increasing numbers. The numbers of grey seals counted on land in the southern Baltic Sea during the moult in May-June have increased from about 150 in 2003 to around 2500 individuals. This number has remained stable over the last five years. Although this is far below historic abundances, the increase has sparked protests from fisheries and initiatives to mitigate the conflicts by protective hunting and regulation in southern Sweden and Denmark.
During grey seal recolonization events in the eastern and western Atlantic, viable breeding stocks were established during the first decade, but evidently almost all Baltic grey seals that visit the southern Baltic Sea, Danish Straits and Kattegat breed in Sweden, Finland and Estonia. As the grey seal was previously the most common seal species in the southern Baltic and Kattegat, it is surprising that the fraction of breeding seals relative to the visitors has fallen from 2% in 2003 to 0.5% in 2020. This could be caused by disturbances at breeding haul-outs or that adults breeding in the area are killed, legally or illegally.
Archaeological finds reveal that grey seals occurred in Danish waters for at least 9500 years, since the most recent glaciation. The Baltic proper was probably colonized from here 4000-6000 years ago, during a period with cold climate. During the recent decades, the Baltic Sea ice has steadily diminished. Previously, Baltic grey seals were mostly bred on ice, buy this has now shifted towards more breeding on islets and skerries that are not flooded, are inaccessible to predators and not disturbed by humans. To ensure a viable breeding population of grey seals in the southern Baltic, Danish Straits and Kattegat in accordance with the EU’s Habitats Directive and HELCOM’s Baltic Sea Action Plan, it is essential to protect the few suitable breeding locations around the year and to protect the few adult seals breeding in the area.