Varberg Fortress was built in 1287-1300 by Count Nielsen as protection against a Danish king, who had declared him an outlaw. Varberg is a coastal town south of Gothenburg in
I have been unable to find any more reports but it sounds like a seal and large pike are not unknown to take wildfowl if they get the chance. So the probability is a natural explanation and not an unknown creature. If anyone has any information about recent sightings or more information please post a comment.