Three seal species occur regularly in the Baltic Sea: grey seal Halichoerus grypus, harbour seal Phoca vitulina, and ringed seal Phoca hispida. Grey seals occur throughout the Baltic Sea and the population grew rapidly from 2000 to 2014, before levelling off at above 30 000 individuals. Harbour seals mainly occur in the southern Baltic Sea and the population in this area had an estimated growth rate of 8.4% between 2002 and 2014. The neighbouring Kalmarsund population had a lower growth rate. The population of ringed seal in the Gulf of Finland is low, at around 100 animals, and is listed as vulnerable by IUCN. This is probably due to recent lack of ice for breeding during the winter. The Bothnian Bay population of ringed seal exceeds 10 000 animals.
The only cetacean species to occur regularly in the Baltic Sea is the harbour porpoise Phocoena phocoena. East of the Transition Area (Figure 2), a large population decline has occurred in the past 50–100 years. With an estimation of 447 individuals (95% CI: 90–997), this population is listed as critically endangered by IUCN. The Belt Sea population has a much higher abundance, estimated at 40,475 (95% CI: 25,614-65,041).