This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Seals are, as a group, thoroughly marine animals, built for salt water and, with only a handful of exceptions, found nowhere far from an ocean coastline. Lake Baikal holds the exception that has puzzled naturalists since it was first formally described in the nineteenth century: the Baikal seal, known locally as the nerpa, the world's only seal species that lives its entire life in fresh water, in a lake sitting more than a thousand kilometers from the nearest sea by any waterway a seal could plausibly have swum.
The leading explanation researchers have converged on is that an ancestral population of ringed seals, a species still found today across Arctic seas, made its way up the river systems connecting the Arctic Ocean to the Baikal basin at some point during the repeated sea-level and river-connection changes of the ice ages, when the drainage pattern of the region looked considerably different than it does now, and became isolated in the lake as those connections later closed off. Cut off from the ocean population for a very long stretch of time by the reckoning of a mammal's evolutionary clock, though a comparatively short one against the lake's own twenty-five-million-year age, the isolated population diverged into what taxonomists now recognize as its own distinct species, adapted entirely to fresh water and to prey, chiefly a small, oil-rich fish called the golomyanka found only in Baikal itself, unlike anything its Arctic relatives eat.
The golomyanka connection is itself part of the reason the nerpa could establish itself in the lake at all: nowhere else does a freshwater lake support a food web rich and stable enough, across a large enough water volume, to sustain a top predator built on a seal's energetic needs, one more downstream consequence of Lake Baikal behaving, ecologically, more like a small ocean than like an ordinary lake. The population today, estimated at somewhere in the range of eighty to one hundred thousand individuals by various surveys, though as with several other Baikal population figures precise agreement across sources is limited, remains a live subject of population monitoring and a widely used indicator species for the health of the wider lake ecosystem.