The bad news: The Environment Agency today said it’s spotted a piranha in the River Torridge in Devon. The good news: The deadly, razor-toothed Amazonian fish was harmless … because it was dead.

The discovery, however, highlights the dangers of releasing non-native species into the wild in the UK.

Agency staff specialists discovered the dead piranha in the river’s East Okement tributary while conducting a survey of fish species using electric fishing equipment. According to the agency, piranhas would not survive in UK rivers (yet anyway — could climate change one day help the toothy tropical species better tolerate a local swim?).

“What we actually came across was something which we would not expect to find in our wildest dreams — we could hardly believe our eyes,” said Eddie Stevens, one of the three-man monitoring team. “After completing 20 metres of the survey, a large tail emerged from the 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. But when it was removed from the river we were speechless to find it was a piranha.”

The piranha, native to the Amazon River basin, can grow up to 35 centimetres in length and is regarded as the world’s most ferocious freshwater fish. It’s infamous for both its razor sharp teeth and its habit of hunting prey in packs.

An autopsy carried out by the Environment Agency found the River Torridge piranha had a stomach full of sweetcorn, which suggests it might have been kept as a domestic pet. Agency experts speculate the piranha was placed in the river once the fish became too large for its tank, and was found dead as the fish could not tolerate the low temperature of the water.

Other invasive species are surviving in the UK, and wreaking havoc with natural ecosystems. For example, the floating pennywort, originally from North America, was brought to Britain in the 1980s as a plant for tropical aquaria and garden ponds. It’s since made its way into the wild, where it’s starving rivers across the south of England and south Wales of light, nutrients and oxygen. As a result, many other river species are killed.

By blocking waterways, pennywort can also increase the risk of flooding.

Other invasive species that are currently a concern for the Environment Agency include:

• Signal crayfish and Chinese mitten crabs that are weakening river banks (signal crayfish also carry a deadly fungal disease that kills native crayfish);
• Dense growths of Japanese knotweed along river banks that are increasing the risk of flooding;
• Topmouth gudgeon, which are displacing native fish in lakes; and
• Mink, which are affecting the number of  water voles.

Fish species which the team would commonly expect to find in the river include salmon, brown trout and less commonly the bullhead, stone loach and minnow.

“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,” said Mark Diamond, Ecology Manager at the Environment Agency. “Rather than dumping things in the wild, we would urge people to seek advice about what to do with exotic species.”

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