Giant Goldfish Demonstrate Why Not To Release Pets Into The Wild


Katy Evans

Katy is Managing Editor at IFLScience where she oversees editorial content from News articles to Features, and even occasionally writes some.

Managing Editor

One of the goldfish found in the Vasse River. Murdoch University.

Who knew something as little as a goldfish could turn out to be so destructively big? Well, researchers in Northern Ireland, actually, who have been studying the impact of goldfish dumped by pet-owners into the waterways of the UK for the past year.

Authors of the new study in NeoBiota discovered that goldfish consume more than comparable species in UK waters, calling them a “triple threat”: they are readily available, have insatiable appetites, and are bolder than most fish.  


Thought to have been released into the waters by pet owners, goldfish are one of the world’s worst invasive aquatic species. In the wild, they can grow to the size of a football, destroying the habitats of native fish by stirring up sediment from the riverbed, uprooting plants, introducing diseases, and competing with native populations, putting them and their ecosystem under pressure.

They also eat pretty much anything, including other fish eggs. Essentially, the freshwater fish can’t win when goldfish are around.

While the study couldn’t confirm whether more goldfish had been released into the wild by people who bought fish as companions during the last few years’ lockdowns (perhaps they didn’t turn out to be such good company) but anecdotal evidence looks that way.

Although, giant goldfish menaces have been reported in countries from Australia to the US and Canada, little has been known about the movement patterns of goldfish in the wild, making it hard to form an effective control program.


A 2016 study in Western Australia discovered goldfish growing up to 30 centimeters (12 inches) and weighing up to a kilogram, with the largest tipping the scales at 1.9 kilograms (4.2 pounds). Over in Canada, goldfish just as big have been causing havoc in their waterways. The US even has a Don’t Let It Loose™ campaign, advising on the dangers of invasive species and what you should do should you no longer want your pet.  

Needless to say, all of these researchers are hoping their findings will act as a deterrent for people who think that dropping their fish off in the nearest waterway is a quick and harmless way of getting rid of an unwanted pet. It’s really not.


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  • australia,

  • invasive species,

  • goldfish,

  • giant goldfish,

  • introduced species