Pusa sibirica
A seal that has never seen the ocean, living under the ice of the oldest lake on Earth.
This claim gets repeated carelessly, so it is worth stating precisely. Several seals live in fresh water. The Saimaa ringed seal in Finland does, the Ladoga seal in Russia does, and a small group of harbor seals in the Lacs des Loups Marins region of Quebec does too. But every one of those is a freshwater population of a species that also lives in the sea.
The Baikal seal has no marine population at all. Every individual of Pusa sibirica alive today lives in one Siberian lake, which makes it one of the very few pinniped species that is exclusively freshwater. The Caspian seal is the closest comparison, and even the Caspian is brackish rather than fresh.
Seals have been in Baikal for something like two million years. How they arrived is unresolved. They may have swum up rivers from the Arctic, or Baikal may once have connected to a vast ice-age body of water such as the West Siberian Glacial Lake. Nobody has settled the question, and the honest answer is that the route is still unknown.
Lake Baikal freezes for months, often under more than a metre of ice. An air-breathing mammal living beneath that has one problem it cannot negotiate around, and Baikal seals solve it with their claws. They scrape breathing holes through the ice, work them open all winter, and keep several going across their patch of the lake.
Females take the idea further. A pregnant female opens a hole, then hollows out a lair in the snow above and around it, so her pup is born inside an enclosed chamber with an escape route straight down into the water. The pup is hidden from view, sheltered from wind, and much warmer than it would be on open ice. The white lanugo coat, useless as camouflage in open water, makes sense inside a snow cave.
The pup nurses in that lair for as long as the ice holds. When the snow collapses in spring the lair is gone, and by then the pup has moulted into adult grey and can swim and dive.
Baikal seals eat one thing above all others. The golomyanka, or Baikal oilfish, is a small, scaleless, nearly see-through fish found in this lake and nowhere else. It has no swim bladder, it is up to a third body fat, and a dead one left in the sun largely melts. It makes up more than 90 percent of a Baikal seal's winter and spring diet, with other sculpins and various invertebrates filling the gaps.
Golomyanka migrate up and down through the water column, and that shapes how the seals hunt. A 2004 study of summer foraging found the seals switching strategies with the light: by day they chase fish visually in shallower water, and at night they take amphipods, which rise toward the surface in darkness, using touch instead of sight. Dives go deepest at dawn and dusk, tracking prey as it moves.
Baikal seals are often blamed for the decline of the omul, the lake's commercially important fish. The blame is misplaced. Omul is estimated to make up about 0.1 percent of a Baikal seal's diet.
Baikal seals dive to roughly 300 to 400 metres. Most dives are short, two to ten minutes, but an alarmed seal has been recorded staying under for around 70 minutes. That kind of endurance requires unusual oxygen storage in blood and muscle, and Baikal seals carry a notably high blood volume for their body size.
The oversized eyes serve the same environment. Baikal water is exceptionally clear, but very little light reaches the depths where the seals feed, and large eyes gather whatever there is. On a round head with a short muzzle, they make the animal look permanently startled.
They also appear to be the longest-lived seals known, with reported ages up to about 56 years. Long life, small size, an isolated population and one dominant prey species add up to an animal finely tuned to a single lake, and correspondingly fragile if that lake changes.
Baikal seals have already been through one catastrophe in living memory. In 1987 and 1988 a canine distemper outbreak killed an estimated 5,000 to 6,500 animals. Researchers link the severity to contaminant load. DDT, hexachlorocyclohexane and industrial waste, much of it from the Baikalsk pulp and paper plant, concentrate up the food chain into seal blubber and are thought to weaken immune function. Distemper is still present in the population but has not caused mass mortality since.
The chemical picture is mixed rather than simply improving. DDT and non-ortho PCB levels in the lake have fallen since the 1990s, mono-ortho PCB levels are unchanged, and perfluorochemicals have increased. Baikal seals currently carry lower contaminant burdens than seals in Europe or North America, but higher than Arctic seals.
The threat most researchers now name as the most serious is warming. Baikal is a closed, cold-water ecosystem that took millions of years to assemble, and a seal whose whole reproductive cycle depends on thick ice and deep snow has no backup arrangement. Shorter ice seasons mean fewer snow lairs, and fewer snow lairs mean fewer pups. The species is listed as Least Concern, but that status describes today, not the trajectory.
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