An evolutionary island
The Caspian seal (Pusa caspica) is the only marine mammal found in the Caspian Sea, the world's largest inland body of water. Genetic analysis suggests their ancestors became isolated from their Arctic relatives, the ringed seals (Pusa hispida), around two to three million years ago. This occurred when the ancient Paratethys Sea, which once connected the modern Caspian to other oceans, began to shrink and fragment. Trapped in this new, landlocked environment, the seals adapted over millennia, resulting in a distinct species.
Caspian seals are among the smallest members of the true seal family (Phocidae). Adult males reach a maximum length of about 1.5 meters, with females slightly smaller at 1.4 meters. Their weight fluctuates seasonally but typically averages around 55 kg. Their fur is grayish-yellow to dark gray, with males having more prominent dark spots across their bodies compared to the lighter-spotted females. Morphologically, their skulls are very similar to those of Arctic ringed seals, but their evolutionary paths have diverged significantly over their long separation.
A life cycle tied to ice
The survival of the Caspian seal depends on seasonal ice sheets of the northern Caspian Sea. Each winter, from late December onwards, the shallow, less saline waters of the north freeze over, creating the platform for the seals' entire reproductive cycle. More than 99% of the population gives birth on this winter ice field, primarily in the territory of Kazakhstan and Russia.
Pregnant females haul out onto stable ice, often at least 20 cm thick, to give birth from late January through February. They typically deliver a single pup, which weighs about 5 kg and measures 64 to 79 cm long. The pups are born with a dense, white or silver lanugo coat, which provides camouflage against the ice and insulation until they develop sufficient blubber. Unlike their Arctic relatives, Caspian seals do not build snow lairs for their young, a possible adaptation to the less predictable snow cover on Caspian ice. The pups nurse for about three to five weeks before their lanugo is shed and they can enter the water.
A precarious existence
The Caspian seal is classified as Endangered by the IUCN, with its population having declined by more than 90% since the early 20th century. The initial drop was driven by intensive commercial hunting for blubber and pup fur. Today, the primary threats are multifaceted. Climate change is reducing the extent and stability of the winter ice sheets important for breeding.
Industrial pollution has led to the accumulation of heavy metals and organochlorine compounds in the seals' bodies, which can impair their immune and reproductive systems. Outbreaks of canine distemper virus (CDV) have also caused mass mortality events. Bycatch in illegal fishing nets, particularly those set for sturgeon, is now one of the most serious factors in the population's decline, killing thousands of seals annually. The current population is estimated to be fewer than 70,000 individuals, with a continuing decline of 3-4% per year. In response, Kazakhstan has listed the seal in its national Red Book and is working to create a state nature reserve to protect remaining habitats.