What does a ringed seal need to live? Plenty of fish and some Mollusca to eat, as well as cold winters and lot of hummock ice in which the females can dig their pupping caves.
Even when there’s enough food, continuously warmer winters may make the ringed seal (Pusa hispida botnica) extinct – such is the gloomy conclusion made by Mart Jüssi, a seal biologist, in his freshly defended doctoral thesis where he analysed the impact of changes in climate for three distinct landlocked species of seals: the grey seal (Halichoerus grypus) found in the Baltic Sea, the ringed seal, and the Caspian seal (Pusa caspica). He was interested in how the warming climate influences seals’ reproduction and chances for survival.
There are thousands of miles between the Baltic Sea and the Caspian Sea, both located in a different climatic zone. Still, the two bodies of water have a lot in common: They are of the same size and have similar depth patterns, as well as similarities in the change of salinity from north to south. The human impact is similar as well, and it has constantly increased during the last hundred years.
At the end of the 19th century, oil tankers started to cross the Caspian Sea. At the same time, seals’ habitat in the Baltic Sea began to become fragmented. In 1894, the Small Strait Dam was built between Saaremaa and Muhu, thus blocking marine mammals’ access to the Gulf of Riga.
Today, the Baltic Sea has problems with eutrophication and very busy ship traffic. The Caspian Sea’s health is endangered by both the oil industry and invasive species. Seals of both seas have similar living conditions. The Caspian Sea seals live in complete isolation. The pinnipeds of the Baltic Sea are also restricted in their ways, as the sea is connected to the Atlantic Ocean solely through narrow Danish straits.
Living conditions are quite extreme in both of the seas. In the summer, the bodies of water are very warm and in the winter very cold. The fluctuating temperature and oxygen content of the water alters the small animal population, thus impacting the seals’ diet.
All the seal species researched by Jüssi have a connection to ice because during winters their habitats freeze completely or partially. Sea ice is a temporary but extremely important place for living.
The grey seal is the most flexible of the three, as its different populations can pup on both ice and dry land. The ringed seal of the Baltic Sea only gives birth to its pups on ice. Some female Caspian seals have been known to breed on land, but most of them prefer ice.
In his doctoral thesis, Jüssi finds that climate changes can render seals’ ice-dependent breeding strategy harmful. Unstable ice conditions bring about weak baby seals. Few portions of the sea covered with ice become ecological traps – a lot of ringed seal mothers come to breed there, but young seals attract predators and birds of prey.
Warming by just a couple degrees already endangers populations of ringed seals in the southern Baltic Sea and Caspian seals. The southern ringed seal populations could probably become extinct in a short period of time.
The grey seal is more flexible and may also pup on small islets during ice-free winters. Still, a lack of ice decreases the distribution of this species as well. As the climate becomes warmer and the sea level rises, small islands suitable for breeding may be left underwater. Human activity amplifies changes in the environment, as pollution, disturbances and excessive fishing all affect seals.