"Down by the water there was something large, floating. At first we thought that is was a large piece of plastic, but then we saw an eye. I went down to check and saw that it was a very strange looking fish," Eriksson said to the Svenska Dagbladet daily.
The unexpected find proved to be a very rare Giant Oarfish also known as the "King of Herrings" and reported to have inspired the myth of sea snakes.
The fish, which can grow up to around 12 metres long and weigh 272 kilogrammes, is typically found in the world's oceans, at depths of 300-600 metres.
The fish generally only comes to the surface when it is dead, as in the case of the 3.5 metre beast landed by Eriksson and his friends.
The fish is now being care for at The House of the Sea (Havets Hus) in nearby Lysekil while the authorities decide what to do with the exceptional find.
"It is totally incredible. I still have trouble believing that it has happened, it is unbelievable," said Roger Hansson at Havets Hus to the newspaper.
The only other time a Giant Oarfish has been sighted in Swedish waters was off Koster in 1879 and the new find has raised the interest of the Natural History Museum in Stockholm who are considering whether to preserve the monster fish.