Fearless Jeremy Wade, 53, spent weeks hunting for the fish in remote Papua New Guinea after locals reported a mysterious beast which was castrating young fishermen.
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He finally unmasked the perpetrator as the Pacu fish - known locally as ‘The Ball Cutter’ - and managed to catch one in his small wooden fishing boat.
Mr Wade wrestled the 40lb monster on to the floor of his boat and opened its snapping jaws with his naked hands - to discover a jaw-dropping array of human-style teeth.
The Ball Cutter boasts an impressive set of man-like molars, which tear off the testicles of unwitting hunters, leaving them to bleed to death.
Mr Wade, from Bath, Somerset, told how he reeled in the Ball Cutter as part of his new series of River Monsters, aired on ITV next week.
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He said: '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.
'It is a hot and dirty area so the people would often go to the water with their children to wash but obviously they were very worried about this thing in the water.
'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.'
At least two fishermen have bled to death after being bitten by the beast although Jeremy believes they were 'pretty unlucky' as it is quite shy.
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Pacu fish are usually found in the Amazon, where they need their teeth to crack into the tough cases of nuts and seeds.
The previously vegetarian fish were introduced to Papua New Guinea 15 years ago to increase stocks.
They quickly used their special technique to chomp meat due to a lack of suitable vegetation in the waters - making short work of human testicles.
Former biology teacher Jeremy luckily just sustained a small nick to his knuckles during his encounter with the Ball Cutter, despite wearing just shorts and a T-shirt.
The angler of 40 years said: 'It is about going in to these situations with the right information to know what you are dealing with.
'It is all about prevention. But there is this fear running through you. As long as I know what I am about to face and we have all of the precautions then I am happy.
'The fish was remarkably muscular, it was kicking so hard to get away from me but I wasn’t really injured.'