Seals in the Baltic Sea

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Seals in the Baltic Sea

Discover Sörmland Blog
Published by Stefanie Schlosser in Nature · 17 March 2022
Tags: sealsBalticSea
In the 1970s, the populations of the different seal species in the Baltic Sea had reached a historical minimum. In fact, the seals were about to disappear from the Swedish coasts. The main reason for seals to almost go extinct were the environmental toxins PCB (Polychlorinated biphenyl) and DDT (Dichlorodiphenyltrichloroethane). As top predators, seals stand at the upper end of the food chain and are therefore likely to accumulate those toxins in their bodies.

Top predators play an important role in the ecosystem, since they regulate the balance between different species. Seals catch the fish which is most common and preferably they hunt those individuals which already are weakened because of some reason.
All seals belong to the biological clade of the pinnipeds (Pinnipedia), which includes three taxonomic families – the eared seals (Otariidae), the earless seals (Phocidae) and the walrus (Odobenidae). All three species occurring in Sweden belong to the earless seals. The common seal (Phoca vitulina) only lives along the Swedish west coast and the Kalmar Straight. The ringed seal (Phoca hispida) appears in the very North of the Baltic Sea, along the Swedish and Finnish coasts. In Sörmland, you can find the grey seal (Halichoerus grypus), which is Sweden’s biggest seal species.
The Newsletter of this month will be about grey seals, their communication and reproduction behavior. If you haven’t signed up yet, make sure you don’t miss it!

All seals are not only amazing swimmers but great divers as well. Their bodies are physically adapted to diving in various ways. Before descending into the depths of the waters, seals exhale all air. What might seem strange to us humans, protects the lungs of the seals, as they therefore are able to entirely close their nostrils and throat during diving. Land mammals wouldn’t even be able to inflate the lungs after their complete collapse. Seals, however, are able to do so. Since their blood contains more red blood cells, and therefore more hemoglobin, it is able to store more oxygen than for example human blood. Furthermore, the seal’s muscle tissue contains more myoglobin, which is an oxygen-binding protein.

As a result of those adaptations, seals can dive for about 20-25 minutes even though they exhale in the beginning. Everyone, who already went diving – with or without bottle – will know about the pressure increase already after a slight descent under water. How do seals deal with this? Obviously, they cannot hold their nose and breath out to equalize the pressure. Instead, their middle ear contains sinuses that fill with blood during dives. Blood, unlike air, won’t get compressed and therefore protects the seal from ear injuries.
How do seals sleep? Since they can spend several months out in the sea, they have to be able to sleep in the water as well. Doing so, they only sleep for some minutes at a time and alike whales and dolphins, half of their brain still stays awake. In case seals sleep on land, their entire brain goes to sleep.

Even though all seal populations around Sweden have increased during the last decades and the worst environmental toxins (PCB and DDT) are banned today, there are other threats appearing. Climate change highly affects the extent and period of marine ice cover. The ringed seal is entirely dependent of sea ice in order to give birth and nurse its young. Even the breed of grey seals, at the coast of Sörmland, show a higher survival rate when birthed on ice. Compared to nursing on land, the seal mother encounters a calmer environment and more space out on the ice. This prevents as well contagious infections to spread through entire populations, which often are concentrated on small areas on land.
Today, tourism has been created around the seal populations and seal safaris are offered for example in the Stockholm’s archipelago.
Remember: The newsletter of March will give you more information about grey seals. Sign up here.

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