There’s a good chance that, as a child or a parent, you might have read the book “A Fish out of Water”, by Helen Palmer, the wife of Dr Suess.

The boy in the story buys a goldfish from Mr Carp, who warns him not to feed it too much. But the boy does exactly that, and the result is a giant goldfish which eventually has to be housed in the local swimming pool.

It’s not strictly a fairytale. Look:

Picture: Stephen Beatty, Freshwater Fish Group and Fish Health Unit, Murdoch University

That’s a koi carp captured in a waterway in southwest Western Australia.

When pet fish are flushed down the loo or lovingly set free to beautify a suburban manmade lake, they can grow to up to 10 times the size of local species, meaning they can easily outcompete natives for habitat and food when they inevitably escape into waterways.

Murdoch University’s Freshwater Fish Group and Fish Health Unit, together with the Department of Fisheries, has an incredible collection of escaped koi pictures, but they’re not Photoshop jobs. Escaped koi can reach up to 1m in length, easily big enough to bully the 5-10cm native species out of existence.

Picture: Stephen Beatty, Freshwater Fish Group and Fish Health Unit, Murdoch University

Research Fellow at the Fish Health Unit, Stephen Beatty, told Business Insider that goldfish caught in the Vasse River in WA’s southwest have the fastest known growth rates for that species anywhere in the world. Here’s a recent example:

Picture: Stephen Beatty, Freshwater Fish Group and Fish Health Unit, Murdoch University

And while this pearl chiclid doesn’t look impressively large, Beatty says it’s actually a world record as far as his team knows. And it’s supposed be at home swimming up a river in South America.

Picture: Stephen Beatty, Freshwater Fish Group and Fish Health Unit, Murdoch University

Beatty said the team was exactly sure how long it takes a koi to grow to more than a metre in size once it’s in the wild, but estimates around 10 years.

ABC’s Radio National Afternoons last week spoke with Dr Jeff Cosgrove, the Swan River Trust’s river science environment officer. He says it’s a massive environmental issue for the region.

“They are eating up the food resources and using up the habitat that our native fish would otherwise be using,” Dr Cosgrove told the ABC, adding that the ferals can also spread exotic diseases.

“This is an added pressure, on top of all the other pressures our aquatic wildlife is experiencing,” he said. “Once the fish is in the system, they are extremely difficult to eradicate.”

Business Insider Emails & Alerts Site highlights each day to your inbox. Email Address Join

Follow Business Insider Australia on Facebook, Twitter, LinkedIn, and Instagram.