The 40lb Pacu fish, known as the “Ball Cutter”, was caught by Brit Jeremy Wade in Papua New Guinea.
The ex-biology teacher told The Sun: “I had heard of a couple of fishermen in Papua New Guinea who had been castrated by something in the water.
“The bleeding was so severe that they died. The locals told me that this thing was like a human in the water, biting at the testicles of fishermen. They didn’t know what it was.
“Amazingly, these things are quite elusive so we had to be patient catching one. We put a line into the water and waited for it to bite.
“When I reeled it in, it had this mouth which was surprisingly human-like, it is almost like they have teeth specially made for crushing.
“They are like human molars and the fish have powerful jaw muscles. They are very deep bodied and solid like a carp, with strong muscles,” said the 53-year-old.
The fish is usually found in the Amazon where they need their teeth to crack open tough cases of nuts and seeds.
The fish was introduced to Papua New Guinea 15 years ago, and began eating meat for lack of options.
Thie catch was filmed as part of new TV series, River Monsters, to be screened on ITV in the United Kingdom.