Biologists concerned over skinny Baltic seals

The number of seals in the Baltic colony continues to grow, with the current population estimated at 25,000 animals compared to 4,000 in the 1970s. But today's seals are skinnier than earlier generations and many of them suffer from bowel ulcers, according to the Swedish Museum of Natural History.

“The seals’ subcutaneous layer of fat has become considerably less thick since the 1980s,” said Olle Karlsson, who conducts research on seals for the museum.

According to Karlsson, the layer of fat for grey seals should measure 30-35 millimetres. But almost half of the animals examined in the area south of the island of Åland had a layer of fat measuring less than 26 millimetres.

“The subcutaneous layer of fat functions as insulation against the cold and is important as an energy reserve for female seals when they are rearing their young. We have not yet seen any negative effects, but we are concerned,” said Karlsson.

There is no good explanation as to why the seals have shed so much fat. No discernible change has been noted in the quality of the herring they eat, and the change in water temperature has been too marginal to cause any changes of this kind.

The frequency of bowel ulcers is also a cause for concern.

“The ulcers are quite a common cause of death, but we have no satisfactory explanation as to what causes them,” said Karlsson.

While the number of seals in the Baltic has increased dramatically over the last thirty years, levels are still far below those of a century ago, when the seal population was estimated at 100,000.

“There are hardly any grey seals at all in the southern Baltic. There is plenty of room there for strong growth in the colony,” said Karlsson.