A fisherman caught the pacu, almost 8 inches long, in the Oresund, The Local.se reported.
The Natural History Museum of Denmark in Copenhagen issued a warning statement: "Keep your swimwear on if you're bathing in the Sound these days -- maybe there are more out there!"
The pacu has a reputation for going after men's testicles, Henrik Carl, an ichthyologist at the museum, told the Local. But he said swimmers are far more likely to drown in the Oresund than to be attacked by a fish, especially one not native to those waters.
"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," Carl said.
The museum is testing the apparent pacu to see if it has the right DNA.