Swimmers in Sweden have been warned about a ball crunching fish with a certain fetish for testicles that have been hanging around the area.

The alert was raised after a fisherman in Oresund Sound caught 21cm pacu fish swimming around. The pacu is a relative of the piranha and commonly found in the Amazon region.

This is the first time a pacu has been caught in Europe and they suspect that the freshwater fish was dumped into the water by a collector.

"Keep your swimwear on if you're bathing in the Sound these days - maybe there are more out there!" warned the National History Museum in Denmark.

The pacu can grow up to 90 centimetres and weigh up to 25kg. It’s been nicknamed the "ball cutter."

"The pacu is not normally dangerous to people but it has quite a serious bite, there have been incidents in other countries, such as Papua New Guinea where some men have had their testicles bitten off," said Henrik Carl, a fish expert at the Danish museum.

"They bite because they're hungry, and testicles sit nicely in their mouth," he told The Local.

"And its mouth is not so big, so of course it normally eats nuts, fruit, and small fish, but human testicles are just a natural target. It's not normal to get your testicles bitten off, of course, but it can happen, especially now in Sweden."