Late on Monday night, a reporter at the Doncaster Free Press received an unusual phone call. A piranha, one of the world’s fiercest predators normally found stalking the waters of the Amazon basin, had apparently been discovered in a lake in Doncaster.

There were dramatic rumours of ducks being massacred and other wildlife being torn to shreds by the razor-toothed fish in the freezing waters of Martinwells Lake.

Too late to be an April Fools’ Day prank, the story seemed so far-fetched that many locals simply confined it to the territory of urban legends and demanded further proof.

Incredibly, photographs have begun to emerge of a couple of fish that look just like the South American predator. It is thought they may have been pets, which an owner then released into the water when they became too big.

Doncaster council has passed on the dead carcass of what is believed to be the freshwater fish to the Environment Agency for further investigation.

Davey White and Lisa Holmes, both 37, were the first to provide evidence. Enjoying the spring sunshine at the lake with their youngest child, Sonny, eight, they made what they described as a “terrifying” discovery.

Holmes, a carer from Doncaster, said: “My partner is a fisherman and was looking around the edge of the lake when he suddenly spotted this fish floating near one of the pegs.

“He managed to get it out of the water and although he’s a keen angler, he wasn’t sure what type of fish it was straight away. But then we started looking at it more closely and saw the teeth, we realised it was a piranha.”

They used the internet to identify their find at home. “It was quite a shock. We couldn’t believe that we’d found a piranha fish. It’s not the kind of thing you expect to find in Doncaster,” Holmes said.

The lake is in a former clay pit and is usually well-stocked with carp, tench, bream, perch, pike and chub, along with the usual pond life of newts, frogs and toads, plus birdlife including ducks, coots, water hens and swans.

Another Doncaster local, Toni Hooper, 32, found a second suspected dead piranha when out walking with her family on Sunday. “When we realised what it was, it sent shivers down my spine. This is a popular spot among families, dog walkers and fishermen. It’s always busy here,” she said.

“There’s a play park nearby, so you get kids here paddling in the water, teenagers will go swimming here.

“You wouldn’t catch me going in the water. We came here to feed the ducks and on Sunday we noticed there was only one duck and two ducklings. I’m concerned about where the wildlife is. I’ve spoken to others who have said they’ve noticed there aren’t as many ducks.”

Gill Gillies, the assistant director of environment at Doncaster council, said that given the natural habitat of piranhas, it was “highly unlikely” either of the fish were alive at any time in the lake.

“We assume that these were pets that were placed in the lake, something that we would strongly discourage people from doing. The presence of a piranha is somewhat of a novelty, so they have since been taken away by the Environment Agency for testing,” she said.

“In any event, we would always strongly advise against anyone paddling or swimming in this or any lake due to the dangers of deep water.”

Known for their sharp teeth and powerful jaws, piranhas are known to have killed humans, though attacks on people are rare. Human attacks normally take place when the fish are stressed, such as when water levels are lower during the dry season and food is scarce.

The Environment Agency was unable to confirm whether it had received the fish carcass.