PET goldfish released into the wild are turning into 2ft killing machines which gorge on insects and tadpoles, Experten behaupten.
They are wreaking havoc on native species, outcompeting rivals and threatening biodiversity.
And they also contribute to poor water quality by mucking up the bottom sediments and uprooting plants.
Researchers fear that getting rid of pet goldfish into fresh waterways will end up having catastrophic consequences for the planet.
Dr James Dickey, of Queen’s University Belfast, sagte: “Our research suggests goldfish pose a triple threat. Not only are they readily available but they combine insatiable appetites with bold behaviour.
“While northern European climates are often a barrier to non-native species surviving in the wild, goldfish are known to be tolerant to such conditions.
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“They could pose a real threat to native biodiversity in rivers and lakes, eating up the resources that other species depend on.”
Owners releasing unwanted animals into the wild is a major problem.
Das goldfish is popular across the world — often awarded to children as a prize at funfairs.
The Belfast team’s findings, published in the journal NeoBiota, come after aquarium owners in Minnesota asked people to stop releasing fish into US waterways after several huge goldfish were pulled from a lake.
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The largest goldfish caught in the UK is said to be a 7lb 5oz specimen in Norfolk in 2010.