The Baikal seal is the only species of seal that lives exclusively in freshwater. This small species is found only in Lake Baikal in Siberia. It is the smallest species of true seal. It thrives in the lake which continually holds about 100,000 specimens in spite of its polluted state. Hunting of the Baikal seal is legal and common, but the numbers never drop. This may be due to the hardiness of the Baikal oilfish which is the seal's primary food source.
The freshwater sub-populations of the ringed seal include the Ladoga ringed seal and the Saimaa ringed seal. It is hypothesized that the subspecies developed when populations of ringed seals got trapped in freshwater lakes when pursuing prey. The Lagoda seal consists of a population of around 3000 seals in Lake Lagoda, Russia. The Saimaa seal, which is endangered, includes only around 300 specimens in Lake Saimaa, Finland.
Because I am always interested in organizing and categorizing information, I also looked into the classification of the seal group in general. Seals are divided into two groups: eared seals which include sea lions and fur seals, and true seals which include ringed seals, spotted seals, harbor seals, harp seals, leopard seals and many other species. Together with walruses, they make up the clade known as pinnepeds. (A clade is a group of animals that is believed to have all descended from a common ancestor.)
Seals are closely related to both bears and musteloids. The musteloids include red pandas, weasels, otters, martens, badgers, skunks, raccoons and several raccoon-like animals that I don't know much about like coatis and olingos.