To finish out our series highlighting different pinnipeds, Research Assistant Kat tells us all about one of the world’s strangest seals: one that lives in a lake…
Pinnipeds are very diverse. They range from small, like the harbor seal, to large like the elephant seal; they range from strictly coastal to offshore deep divers; and they are found in a wide variety of locations all around the world. But arguably none are quite so odd as this one…because it lives in a lake!
The Baikal seal (Pusa siberica), named after its home Lake Baikal, is the only true freshwater seal in the world. Other seal species can tolerate fresh water, and several seal species use freshwater systems to hunt or breed (e.g. grey seals, ringed seals), however the Baikal seal is the only one to live exclusively in freshwater.
Located in the Russian part of Siberia, Lake Baikal itself is a pretty remarkable spot. It’s the world’s deepest and largest lake by volume, reaching a depth of 5,387 feet and holding 23,615k3 of water – that’s more water than all the North American Great Lakes combined! Formed by an ancient rift valley, Lake Baikal is also one of the clearest lakes in the world and contains high levels of oxygen, making it very productive. The sunlight can penetrate deeper into the water making it a hotspot of primary productivity, creating the foundation for a very diverse ecosystem.
So ok, let’s address the elephant in the room: how on earth did a seal, which are oceanic creatures, end up living in a lake completely cut off from the sea?! Well, it is thought that seals reached Lake Baikal by travelling up river and drainage systems that ran from the lake to the Arctic Ocean during the Pleistocene era, around 400,000 years ago. The ringed seal, which is an Arctic species, is thought to be the closest ancestor of the Baikal seal, and some subspecies of ringed seal are known to spend time in freshwater systems as well. Changes in waterways and the physical geography of this region since the Pleistocene era mean that Baikal seals are now confined to Lake Baikal and its tributaries.
Interestingly, in such a restricted environment the seals also seem to engage in niche-partitioning within the lake itself, i.e. they will use different parts of the lake at different times/for different reasons. Mostly, the seals can be found in the north and central parts of the lake; however during the winter when thick ice covers much of the lake (0.5-1.5 meters thick), some populations will move farther south to avoid the ice coverage. The ice and shorelines are used as haul out spots by the seals, enabling them to rest and warm up just like seals in the ocean will haul out on rocks or the beach. Baikal seals will also separate themselves geographically based on age, with adults more commonly found in the north and the juveniles and young seals typically inhabiting the south!
During the winter months, pregnant females come out on the ice to give birth. They will dig lairs in the ice to protect themselves and their young from predators such as eagles, hawks and wolves which are common in this region. (Fun fact: Baikal seals have one of the highest rates of twins of any pinniped species, with around 4% of births resulting in twins!) Females typically start reproducing around 5-6 years old, however some do not begin having pups until they are 9-10 years old. Males become reproductively active earlier, typically between age 3-7 years.
So what do these unusual seals eat in their lake environment? Given the rich diversity of their ecosystem, Baikal seals have their choice of prey items but most often seem to feed on pelagic fish. Their large eyes are thought to be an adaptation to help them to forage for these fish deep in the water and they can dive up to 100m to hunt. The seals can still use eyesight to hunt even at depth, since sunlight can penetrate much deeper through the clear waters of Lake Baikal. Baikal seals spend most of their time in the water, foraging and socializing.
As one might expect, seals living in a lake are vulnerable to a number of threats, many of them human-based. Like many marine mammal species, climate change is one of the number one threats facing Baikal seals. Surface temperatures of Lake Baikal have increased by almost 1.5°C/2.7°F in the last 50 years alone, which means a shorter ice period during the winter and potential changes in prey species that can survive in these warmer waters. If you’ve ever gone swimming outside, you will know that lakes heat up much quicker than the ocean or wider expanses of water, and even though Lake Baikal is deep it is likely to experience more drastic temperature changes more quickly than oceanic environments.
Humans are also a threat to these little seals, with many of them still hunted for food and materials. Archaeological records suggest that seals have been harvested in this region for thousands of years. At one time there was a commercial harvest for Baikal seals, which allowed up to 9,000 seals to be commercially hunted, however this was terminated in 1980. Today, seals are still hunted by individuals for their meat, blubber, pelts, etc. and are often caught unintentionally during fishing efforts, resulting in an estimated mortality of 3,000-4,000 seals per year.
As one might expect, living in such close proximity to humans has other knock-on effects on the seals. For example, they have high levels of organochlorines from waste run-off from industrial plants. As the top predator in their lake system, the seals are eating contaminated fish that have eaten other contaminated fish, resulting in so-called “biomagnification” of these organochlorines in the seals.
Contamination is known to reduce immune function, making one of the other main threats to these seals becomes more dangerous: morbillivirus. The first morbillivirus infection in Baikal seals was seen in 1987 and was linked to an outbreak of canine distemper virus in domestic dogs. The following year, a large-scale outbreak of phocine distemper virus (PDV), the “seal version” of canine distemper virus, was reported in European and Baltic waters mostly affecting harbor seals. These types of outbreak may put Baikal seals at greater risk due to their high contaminant loads and suppressed immune systems, yet unfortunately due to their close proximity to humans and canines, both wild and domestic, it is likely that morbillivirus may continue to threaten these seals.
Well hopefully you have enjoyed learning about this adorable and highly unique little seal and it’s home, they are certainly one of my favorite pinnipeds and I think they are a pretty incredible example of how diverse and adaptable animals can be!