A shocked angler caught an unexpected haul while fishing in the River Don in Sheffield.
Oliver Mappin was fishing for perch at the Round Dam in Rivelin when he hooked a foot-long terrapin.
The 68-year-old said: “I thought it was an octopus or something at first.
“It caused quite a stir with the people around me. I was in total amazement at catching it.”
Oliver had been at fishing at the dam for three hours when he decided to call it a day.
He said: “I started to reel in my line when I felt something on the end.”
The terrapin – a species of turtle which lives in fresh water – bit through his line and his landing net.
Oliver released it back into the pond after taking a photograph of his catch.
Now he is warning others to be careful if they do come across the creature.
He said: “They tend to nip.
“The damage it could do in a pond is horrendous.
“It will kill newts and frogs and ducklings too and drag them under the water.
“If there’s more of them they will breed and they are immune to freezing temperatures as well.
“They could decimate any normal wildlife not used to that kind of predator.”
Terrapins became popular pets after the original Teenage Mutant Ninja Turtles film 25 years ago.
But animal welfare charity the RSPCA said animal rescue centres were inundated with abandoned terrapins in the 1990s and they were spotted in public ponds around the country.
The charity said ‘ill-informed’ people bought them as babies after watching the film and TV series, without realising how big they grow.
A new Teenage Mutant Ninja Turtles film last year sparked fears of more abandoned terrapins.
An RSPCA spokesman said: “Sadly, many owners who buy exotic pets on impulse after seeing a film or TV show don’t find out how to care for the animals first.
“When they then realise how much space and care the animal requires they can lose interest, or feel unable to care for them anymore.
“As a result exotic pets, such as terrapins, are often abandoned, given to animal rescue centres or released into the wild.
“Terrapins are complicated animals to care for.
“We discourage anyone from buying any pet on a whim and urge people to think carefully first before buying an exotic pet. Tortoises, turtles and terrapins may seem like relatively easy pets to keep, but the reality is many terrapins are abandoned in lakes and ponds when they grow too big. They start off at a size similar to a 50p piece but can grow to the size of a dinner plate.
“Releasing unwanted exotic pets into the wild is cruel and illegal. Most exotic pets are unlikely to be able to survive in the wild in Britain – and non-native species could pose a serious threat to our native wildlife.”
“It is illegal under the Wildlife and Countryside Act 1981 to release, or to allow to escape, any species that are not normally native to the UK.”