--- title: Pet Fish Freed in Australian Waterways Are Turning Into Monsters description: Goldfish and koi set free in Australian lakes are mutating into enormous sizes, decimating local species. canonical: https://time.com/3814921/abandoned-pet-fish-australia/ author: David Stout article:opinion: false article:content_tier: free article:published_time: 2015-04-09T08:53:42.000Z article:modified_time: 2026-05-11T05:04:37.802Z article:section: Science og:title: This Pet Fish Grew Into a Feral Aquatic Killer og:description: It's what happens when you ditch little Otto into a local lake og:url: https://time.com/3814921/abandoned-pet-fish-australia/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt021055f9aa6bf19a/6987ea3e0b72e353d76e1626/feral-fish.jpg?branch=production&width=1200&quality=75&auto=webp&crop=16:9 og:image:width: 1500 og:image:height: 844 og:image:alt: Freshwater Fish Group & Fish Health Unit og:type: article twitter:card: summary_large_image twitter:title: This Pet Fish Grew Into a Feral Aquatic Killer twitter:description: It's what happens when you ditch little Otto into a local lake twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt021055f9aa6bf19a/6987ea3e0b72e353d76e1626/feral-fish.jpg?branch=production&width=1200&quality=75&auto=webp&crop=16:9 ---  # Check Out This Photo of a Pet Fish That Grew Into a Feral Aquatic Killer > Sponsored content. 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Future Proof is free at time.com/newsletters. ### Sources * TIME Newsletters: [time.com/newsletters](/c/time-nl-futureproof-2026q3/newsletters?i=1762a73d-3c8d-41a7-8794-49733fc2d62b&cr=time-nl-futureproof-2026q3-v1) by [David Stout](https://time.com/author/david-stout/) ## David Stout Apr 9, 2015 8:53 AM UTC  Photo provided by Murdoch University's Freshwater Fish Group & Fish Health Unit by [David Stout](https://time.com/author/david-stout/) ## David Stout Apr 9, 2015 8:53 AM UTC Setting a pet fish free in a local lake or waterway is not an act of liberation — it’s ecological sabotage. According to researchers at Australia’s Murdoch University, once pet fish are introduced into lakes and rivers they have the potential to decimate an ecosystem. While a goldfish or koi might remain miniature when housed in a bowl or aquarium, once freed into a larger environment the fish are able to grow at an exponential rate. They also eat up food that native species depend on and introduce exotic diseases. A [study published](http://time.com/wp-content/uploads/2015/04/vasse%5Fwonnerup%5Ffishes%5Fscientific%5Freport%5Fffgfhu%5Fcffr%5Ffinal%5F2014.pdf) last year by Murdoch University’s Freshwater Fish Group and Fish Health Unit claims “introduced freshwater fishes are one of the major global threats to aquatic biodiversity.” “They are eating up the food resources and using up the habitat that our native fish would otherwise be using,” Jeff Cosgrove, a Postdoctoral Research Fellow at Murdoch University, [told](http://www.abc.net.au/radionational/programs/rnafternoons/aquarium-fish-hurting-waterways/6364144) the Australian Broadcasting Corporation. In addition, once they’ve been introduced into a new ecosystem they can be “extremely difficult to eradicate,” says Cosgrove. _Read next:_ [_The Brontosaurus Comes Back from the Dead_](http://time.com/3773770/brontosaurus-dinosaur/?iid=time%5Freadnext) ### Photos: Inside a Hospital for California's Stranded Seals and Sea Lions  A California sea lion sits on the edge of a pool at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  California sea lions swim and sit out in the sun while visitors look on at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Harbor seal pups wear plastic identification tags that help caregivers tell them apart at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Herring breakfasts are organized and labeled for each seal and sea lion at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  A harbor seal pup wears a plastic "hat tag," used to identify individual animals at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  A malnourished elephant seal pup looks outside a pen at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Volunteers tube-feed a malnourished harbor seal pup at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  A volunteer restrains an elephant seal pup while the animal is fed through a tube at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  A shaved patch on the back of a harbor seal shows where blood was drawn during an admit exam at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Seals and sea lions released back into their natural habitat are added to annual graduation posters at the Marine Mammal Center in Marin County, Calif., May 9, 2014. Corey Arnold for TIME  Researchers perform a necropsy on a California sea lion that died of malnutrition at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Dr. Shawn Johnson, Director of Veterinary Science, listens to the lungs of a young sea lion under anesthesia at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  A California sea lion pup undergoes surgery for a lymph node abscess at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Students on a field trip learn what to do if they find a stranded seal on the beach at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  Siobhan Rickert, 18, has been a volunteer at the Marine Mammal Center for four years She is one of a dedicated network of 1,100 volunteers who help run the facility in Marin County, Calif., May 9, 2014. Corey Arnold for TIME  An elephant seal pup lies in an enclosure at the Marine Mammal Center in Marin County, Calif. on May 9, 2014. Corey Arnold for TIME  The Marine Mammal Center sits where the U.S. military once had a missile site outside San Francisco, in the Marin Headlands, Calif., May 9, 2014. Corey Arnold for TIME [_Listen to the most important stories of the day._](http://time.com/newsletter/the-brief/?sl%5Fautoplay=true&xid=spl-fullbrief-article) ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/3814921/abandoned-pet-fish-australia/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/3814921/abandoned-pet-fish-australia/"},"headline":"Check Out This Photo of a Pet Fish That Grew Into a Feral Aquatic Killer","datePublished":"2015-04-09T08:53:42.000Z","dateModified":"2026-05-11T05:04:37.802Z","description":"It's what happens when you ditch little Otto into a local lake","url":"https://time.com/3814921/abandoned-pet-fish-australia/","keywords":["Environment","Australia","fish"],"thumbnailUrl":"https://static.time.com/v3/assets/bltea6093859af6183b/blt021055f9aa6bf19a/6987ea3e0b72e353d76e1626/feral-fish.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","author":[{"@type":"Person","name":"David Stout","jobTitle":null,"url":"https://time.com/author/david-stout/"}],"articleSection":"Science","image":[{"@type":"ImageObject","url":"https://static.time.com/v3/assets/bltea6093859af6183b/blt021055f9aa6bf19a/6987ea3e0b72e353d76e1626/feral-fish.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","width":1200,"height":675,"headline":"Freshwater Fish Group & Fish Health Unit","caption":"","creditText":"Photo provided by Murdoch University's Freshwater Fish Group & Fish Health Unit","representativeOfPage":true}],"publisher":{"@type":"Organization","name":"Time","url":"https://time.com/","logo":{"@type":"ImageObject","url":"https://time.com/images/logo.png","width":528,"height":156},"foundingDate":"March 3, 1923","sameAs":["https://www.facebook.com/time","https://www.instagram.com/time/?hl=en","https://twitter.com/time","https://www.pinterest.com/timemagazine"]}},{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/section/science/","name":"Science"}},{"@type":"ListItem","position":2,"item":{"@id":"/tag/environment/","name":"Environment"}},{"@type":"ListItem","position":3,"item":{"@id":"https://time.com/3814921/abandoned-pet-fish-australia/","name":"Check Out This Photo of a Pet Fish That Grew Into a Feral Aquatic Killer"}}]}] ```