Pinnipeds: Freshwater Seals

Caspian seal

Seals live, generally speaking, in the world's seas and oceans. But, as I noted last time, there is one species of ocean-going seal with a population found in freshwater. This is the ringed seal of the Arctic and Baltic, one population of which became cut off during the last Ice Age and now has descendants living in Lakes Ladoga and Saimaa in Russia and Finland. Lake Saimaa drains into Lake Ladoga, which in turn drains, via the Neva River, into the Baltic, so while seals do not regularly swim in that river, the geographic isolation is, at least in theory, not absolute.

However, two other populations of ringed seals (or their immediate ancestor), became separated from their kin at a much earlier date. Unlike the Ladoga and Saimaa populations, they had the time and isolation to develop into entirely new species, notably different from their relatives out in the ocean.

The Caspian Sea is the world's largest inland sea (or salt water lake, depending on your choice of definition). At around 85% of the size of the Black Sea, it is nearly three times the size of Lake Superior, the world's largest freshwater lake. It borders no less than five countries, with Russia and Azerbaijan on its western shores, Kazakhstan and Turkmenistan to the east, and the north coast of Iran as its southern boundary. Fed by, among others, the mighty Volga River, it has no outflows, and has a surface 28 metres (92 feet) below the world sea level.

It was once connected to the world's oceans via the Black Sea, but became separated around the dawn of the Pliocene epoch, a little over 5 million years ago. It consequently missed out on the Zanclean Flood, and was at first very salty. Enough rivers flow into it, however, that that didn't last. Today, its salinity noticeably varies across its area, being lowest at the mouth of the Volga, and highest at the large, but extremely narrow-mouthed, Garabogazköl Bay in Turkmenistan. Typically, though, while it's brackish, rather than truly freshwater, it has only around one third the salinity of regular seawater.

Throughout this body of water lives the Caspian seal (Pusa caspica). Physically, it looks very like the ringed seal, except that it doesn't have the pattern of rings on its back, and is instead a smoother brown colour, with some darker spots on the back, and, in the case of the males, also on the underbelly. While a small number are apparently able to breed on the scattered islands in the southern reaches of the Sea, most, like their ringed relatives, still rely on ice, and head to the north in winter. At this time of year, floating ice begins to form, and reaches out into the middle of the Sea, giving the seals a safe place to breed and to raise their white-furred pups.

Around 40% of Caspian seals remain in northern waters year round, but the remainder head south in the spring, presumably to avoid too much competition for food. Like ringed seals, they don't dive particularly deeply when foraging for food, usually no more than 50 metres (165 feet), although they can reach four times that depth if they have to. The Caspian Sea is over a kilometre deep in places, so this is clearly just an issue of how far down they expect to find food, not of some geophysical restriction.

During the winter, that food seems to consist primarily of crustaceans, but during the summer, fish predominate, with the local sprats being preferred, in the absence of the cod and herring that ringed seals consume. They also eat smelt, carp, and roach, with some Volga pikeperch at the estuaries of the Volga and Ural Rivers. This basically sums up what we know about them, which is surprisingly little when you consider how long we've been hunting the animals.

Because we have been hunting them a lot, with the earliest evidence stretching back to Stone Age peoples in what is now Iran around 20,000 years ago. It then continued, on a small scale, for thousands of years, until it really began in earnest around 1740. By the end of that century, we were killing around 100,000 of them every year, and this number only increased as the years went by, peaking at around 227,000 in 1935. While it's hard to know the exact figure, there were probably no more than one-and-a-half million of them in the Caspian Sea at the start of the nineteenth century, so, with that level of exploitation, it's perhaps remarkable that it took until the 1940s for us to start running out of seals to kill.

But run out we did, and efforts to limit the cull in the 1960s were wholly inadequate, still permitting unsustainably high levels of hunting. The population finally crashed in the late 20th century, making it small enough to become vulnerable to other hazards that it would otherwise have easily weathered. Three times, in 1997, 2000, and 2001, Caspian seals were hit by a virus related to canine distemper, killing off large numbers of the animals. A warming world, reducing the already limited amount of sea ice in the north of the Sea that they use for breeding, isn't helping matters. At the time of writing, the latest estimate I'm aware of is that around 31,000 breeding females remain, which implies that the population has dropped by about 90% since 1955.

The Caspian seal, unsurprisingly, is formally listed as an endangered species.

Not to dismiss the importance of that, but one interesting question about the seal is how on Earth it managed to get into such an isolated sea in the first place. The obvious answer is that they must have been trapped there since the Caspian Sea became cut off from the world's oceans at the dawn of the Pliocene. But, while it may seem obvious, and for a long time was indeed the prevailing theory, as it turns out, it's also wrong.

That's because a study in 2006 showed that Caspian seals can't possibly have been separated from ringed seals for any more than about three million years, which would mean that they must have reached the Sea at least two million years after it was cut off. The same study, based on genetic evidence, also seemed to show that Caspian seas are most closely related, not to ringed seals, but to harbour seals, while one in 2007 said, no, it's not that either, it's the grey seal.

That seemed a bit odd, since they do look quite a lot like ringed seals. Fortunately, in 2009, improved genetic evidence confirmed that the anatomically-based "ringed seal" theory had been correct all along. But, if anything, we've only pushed the origin date for the species into even more recent times since; the current best estimate is that they first appeared a little over one million years ago. This presumably means that, during the Ice Ages, or one of the warmer breaks between them, the Caspian Sea was somehow connected to the Arctic Ocean, since that was the only place that ringed seals could have come from by that time.

Quite what form that connection took remains a mystery.

Baikal seal

Still, if Caspian seals live a long way from the world's oceans, it's as nothing compared to their other close relative, the Baikal seal (Pusa sibirica). Lake Baikal lies deep inside eastern Siberia, not far north of the Mongolian border, and, at around 20% larger than Lake Erie, is the largest freshwater lake in Asia. Moreover, at a remarkable 1.6 km (5,200 feet) in depth, it is not only the deepest lake anywhere in the world, but also the largest by volume.

Unlike the Caspian, however, Lake Baikal does have an outflow, draining via the Angara and Yenisei Rivers into the Arctic Ocean. Which, despite the large distances involved, at least means that there's a clear route by which seals could have reached it - although why they'd have bothered is another question, and it's certainly possible that the geography was different back then. At any rate, with the lake having been around for a whopping 25 million years, and never having been part of any sea, they can't have been there when it formed.

In fact, they seem to be the result of a more recent migration than Caspian seals, having reached their lake around 0.5 million years ago - so recently that their kidneys are still adapted to life in salt water. (As an aside, I note from the linked study that the researchers tried making the seals drink a litre of fresh water to see what would happen. Answer: they peed a lot. Ah, science...)

Baikal seals are slightly smaller than ringed and Caspian seals, but only slightly, so the lake is evidently large enough for them to avoid any significant "island dwarfism". While some have a few faint spots, most have a bland and uniform dark grey coat that becomes paler on the underparts. Anatomically, they closely resemble ringed seals, including the long, heavy claws on the front flippers.

They need these because Lake Baikal freezes over entirely in winter, and while they do starve themselves for a month or so during the spring moult, they certainly couldn't survive through the winter without diving beneath the waters and digging breathing holes through the ice. Like ringed and Caspian seals, they also dig shelters into the snow in which to birth their pups.

In terms of biology and behaviour, they seem very similar to ringed seals, as one might expect, given their relatively recent separation from that species. One notable difference is that they give birth to twins more often than any other seal species. Which is still only 4% of the time, but it's only 2-3% in humans, so that's not entirely insignificant.

Because the lake is entirely fresh water, and the seals live nowhere else, cod and herring are no more an option for Baikal seals than they are for the Caspian sort. Instead, golomyankas and sculpins are their typical prey, and they dive up to 200m (650 feet) below the surface to find them. Dives can last up to 40 minutes, although much longer than 15 is apparently unusual, especially at night, when their dives tend to be much shallower.

Golomyankas are deep-diving fish unique to this one lake, and, perhaps fortunately, neither they nor the local sculpins are of the slightest interest to fishermen, so the latter do not have to compete with the seals. They are hunted by the locals, but this has been far less intensive than for Caspian seals, and has declined in recent years. Their population has proved hard to estimate, but is currently thought to be around 100,000, which is probably higher than that of Caspian seals, and about the most that the lake could support anyway.

This means that they're as safe as they reasonably could be, considering where they live. The lake does receive pollution from industrial towns upriver, but neither this, nor a 1987 outbreak of a similar virus to the one that later decimated the Caspian seals seem to be doing them any lasting harm at the moment. Climate change could be more of a threat; for the time being, ice cover actually seems to be increasing, despite the warmer summers, but if that ever changed their inability to go anywhere else would obviously be a real problem.

One issue they don't have to face, however, is competition from other seals. For ringed seals, in the admittedly much greater expanse of the Arctic Ocean, that's not at all the case. I've already described a number of species of ice-breeding Arctic seal, plus the land-breeding harbour and grey seals. But there are still two others I haven't mentioned yet, so it is to them that I will turn next...

[Photos by Andrew Butko and Per Harald Olsen, from Wikimedia Commons.]