This article was amended on Friday 4 September 2009

In the article below we said that a piranha had been found in the river Torridge in Devon. The Environment Agency, which put out the story and photo, subsequently said it had misidentified what was, in fact, a different South American fish, the pacu.

A giant piranha – the most ferocious freshwater fish in the world – has been found in a Devon river, the Environment Agency said today.

Bathers in the UK's rivers and streams should be reassured that the fish was dead and is thought to have been an abandoned pet. Its discovery will add, however, to experts' anxieties about non-native wildlife being introduced to the UK.

The fish, of a species more accustomed to the Amazon river basin, was discovered in the river Torridge by Environment Agency workers conducting a wildlife survey. Eddie Stevens, one of the three-man monitoring team, said they were left open-mouthed by their discovery.

"What we actually came across was something which we would not expect to find in our wildest dreams. We could hardly believe our eyes," he said.

Used to more temperate climes, the fish probably could not tolerate the cold waters, the agency said. A closer examination revealed that its stomach was full of sweetcorn – further evidence it may have been kept as a domestic pet.

At a length of 36cm (14in), the red-bellied piranha was a giant of its kind – more than eight inches longer than an average specimen. The team first spotted its tail sticking out of an undercut bank on the far side of the river.

"Our first thought was that a sea trout had become lodged in amongst the rocks and debris collected under the bank," Stevens said. "But when it was removed from the river we were speechless to find it was a piranha."

The introduction of non-native species poses a serious threat to native wildlife, according to the agency. Mark Diamond, an ecology manager, said: "Whilst piranhas can't survive the colder climates of the UK, this latest find highlights a real issue – that releasing unwanted exotic pets or plants into rivers can have serious consequences for native wildlife.

"Rather than dumping things in the wild, we would urge people to seek advice about what to do with exotic species."