Winter swimmers in the Aland Islands between Finland and Sweden have been putting up with more than the cold recently.

The Baltic Sea has frozen over, and an ice-hole kept open for swimmers in the Lilla Holmen Park in the islands' capital, Mariehamn, has been taken over by a large and fearless muskrat.

"With 40cm of ice on the bay, it's hard for animals and birds to find open water," said Maria Bjorke, who swims twice a week – even when air temperatures fall to minus 20C. "We have a heated changing room, and the town council keeps the ice-hole open." About 150 people use the facility.

The muskrat has built a nest under the steps leading into the ice-hole. Though he dives when swimmers approach, he is liable to pop up at any moment – giving bathers a nasty shock. When he charged at one elderly female swimmer, the council decided enough was enough. A trap was set, but the wily creature has proved elusive.

Muskrats, which are herbivorous, can grow up to 2ft long. "We've had to share our ice-hole with ducks as well, but no one minds about them," Ms Bjorke said.