The Baikal seal (Pusa sibirica), also known as Lake Baikal seal or nerpa (), is a species of earless seal endemic to Lake Baikal in Siberia, Russia. The Baikal seal is one of the smallest earless/true seals, and one of the few exclusively freshwater pinniped species. The related Caspian seal inhabits another lake, albeit being saltwater, the Caspian Sea. Both species are closely related to the ringed seal, which inhabits the Arctic Ocean.
A subpopulation of inland harbour seals living in the Hudson Bay region of Quebec, Canada (the Lacs des Loups Marins harbour seals), as well as the Saimaa ringed seal and the Ladoga seal (both ringed seal subspecies), are also found in fresh water, but these seals are part of species that also have marine populations, instead of being distinct species. The maximum reported size is in length and in weight. There are significant annual variations in the weight, with lowest weight in the spring and highest weight, about 38–42% more, in the fall. The animals show very little sexual dimorphism; males are only slightly larger than females. In prehistory, they may have swum up rivers and streams, or there is the possibility that Lake Baikal was linked to the sea at some point through a large body of water, such as the Paratethys Sea, West Siberian Glacial Lake or West Siberian Plain, formed in a previous ice age. The seals are estimated to have inhabited Lake Baikal for some two million years.
Distribution
thumb|left|Group of Baikal seals hauling out on a rock around [[Ushkan Islands, Barguzinsky district]]
The Baikal seal lives only in the waters of Lake Baikal; they are endemic there.
The areas of the lake in which the Baikal seals reside change depending on the season, as well as other environmental factors. They are solitary animals for the majority of the year, sometimes living kilometres away from other Baikal seals. In general, a higher concentration of Baikal seals is found in the northern parts of the lake, because the longer winter keeps the ice frozen longer, which is preferable for pupping. The remaining food sources for this seal are various other fish species, especially Cottocomephorus (about 7% of the diet during the winter and spring) and Kessler's sculpin (about 0.3% of the diet in the winter and spring), but it may also take some invertebrates such as Epischura baikalensis, gammarids and molluscs. they have specialized teeth that allow the seals to expel water while feeding, allowing them to gather large amounts of pelagic amphipods while swimming.
According to a 2004 paper on the foraging tactics of Baikal seals, during the summer nights these seals are known to have different foraging strategies during night time and during day time. During the day, these seals use visual clues to search for their prey, which is mainly fish, while during the night they use tactile clues to hunt amphipods. Since it is brighter during the day, the seals are able to see much better in order to hunt for the fish. Since there is no light at night, they have to hunt with tactile cues. The amphipods they hunt at night have a diel migration, so they come up into shallower waters during the night, and swim to deeper waters during the day to escape predators. These seals were observed to dive deeper during dawn and dusk in order to get to these amphipods as they were swimming shallower and deeper, respectively.
The Baikal seal has been blamed for drops in omul numbers, but this is not the case. It is estimated that omul only comprises about 0.1% of its diet.
Relation to humans and conservation
thumb|A Baikal seal mascot at the Nerpa Festival in [[Irkutsk]]
Since 2008, the Baikal seal has been listed as a Least Concern species on the IUCN Red List.