Plastic caves will save seal pups in he Baltic Sea

27 February, 2024

Ringed seal pups become homeless as the Baltic Sea’s ice shrinks.

In an interesting experiment, researchers build caves of plastic and plywood to help the cubs survive in an increasingly hot climate.

The ringed seal pup’s fur cannot stay in the water for long periods of time and is dependent on snow caves as protection from predators and harsh weather. Archive image. Photo: Danny Green

The ringed seals need a special kind of nursery: Snow caves on the ice formed when snow is collected by the wind in broken-up drift ice.

The caves provide protection against both hungry predators and freezing cold before the seal pups have a protective layer of blubber and can stay in the water for longer periods.

But the specific requirement for these nurseries rhymes badly with the fact that the Baltic Sea is getting warmer. The extent of the ice has already decreased as a result of climate change – and is predicted to decrease even more in the future.

Mortality among ringed seal pups is high when they are forced to give birth on land .

– Predators such as sea eagles and foxes kill them and they lose too much heat without the caves that protect against harsh weather, says Markus Ahola, who works with the monitoring of the Baltic Sea’s seal populations at the Swedish Museum of Natural History.

A nest in a compost bin

Trials are now underway to build artificial caves for the ringed seals on cliffs in the Archipelago Sea. The project is run by the University of Applied Sciences in Turku and this winter 13 caves have been built out of plywood and plastic, among other things, to see what the females like best.

How things have gone will be revealed later this spring when the caves are inventoried after the pups have left the nest.

– But last year, an artificial cave became a home for a ringed seal, says Markus Ahola, who participates in the project as a volunteer.

– That particular cave consisted of a plastic compost bin, he says.

A black compost bin became home to a seal last winter. Foto: Yrkeshögskolan i Åbo

The southern sub-populations of ringed seals in the Baltic Sea are thought to have already been affected by a warmer climate. The extent of the sea ice naturally varies from year to year, but since the 1990s the Swedish Meteorological Service measurements clearly show that the extent of the ice in the Baltic Sea has decreased.

– We attribute that to climate change, says Per Pemberton, an oceanographer at the Swedish Meteorological Service.

Affects more species

The authority’s scenarios for the next 50 years show that smaller and smaller parts of the Baltic Sea will be covered by ice during the winter, as well as that the ice will become thinner and have a shorter season.

– How big the changes will be depends on how the emissions of greenhouse gases develop, says Per Pemberton.

In addition to affecting animals that depend on ice, it can affect the ocean’s ecosystem in several ways, but how big the effects will be is still uncertain.

– Finding ways to help the ringed seals survive in the Baltic Sea is an important piece of the puzzle for the entire ecosystem, says Markus Ahola. For example, anglers like to eat turbot, which has increased enormously in the Baltic Sea and become a concern.

Seals are top predators that balance ecosystems. If they disappear, it can also affect many other species.


Ice extent in the Baltic Sea, historical and future scenario. How much the extent of the ice decreases in the future depends on how large the emissions of greenhouse gases will be. The graphic shows a future scenario based on one of the emissions scenarios used by the UN Climate Panel (called RCP4.5), where the world takes relatively strong climate action. Graphics: Anna-Lena Lindqvist/TT
Markus Ahola, first curator at the National Museum of Natural History. Archive image. Foto: Johanna Hanno

Ringed seals are the smallest of the Baltic Sea’s three seal species, the others being gray seals and harbor seals.

During the ice-free part of the year, the ringed seal lives largely in open water. During the winter, it creates underwater territories and digs breathing holes in the ice.

A ringed seal can be 40-50 years old.

The population in the Baltic Sea crashed in the 20th century due to hunting and environmental toxins but has grown since then. Now there are estimated to be between 15,000 and 20,000 individuals. But the situation is still judged to be bad for the southern part of the population, probably due to changed ice extent.

Sources: Havs- och vattenmyndigheten, Markus Ahola.

Text: Hanna Odelfors/TT
Photo: Danny Gren, Yrkeshögskolan i Åbo, Johanna Hanno
Graphics: Anna-Lena Lindqvist/TT

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