You may think dumping goldfish into a river is a harmless act, but in reality the fish can become destructively big.

Researchers from the Centre of Fish and Fisheries at Murdoch University have been trying to control goldfish for 12 years in the Vasse River, located in the southwest of Western Australia.

An invasive species, the goldfish is causing havoc for native fish and its surrounding ecosystem, which is why Stephen Beatty and his fellow researchers spent a year studying the little-known movement patterns of the goldfish in the wild. The results of the study, now concluded, has been published in a paper in the journal, Ecology of Freshwater Fish.

Image: murdoch university

Beatty told Mashable Australia that the study was conducted because of the sharp rise in aquarium species, such as the goldfish, being detected in the Vasse River in the last 15 years. That's all thanks to people letting them go in local waterways.

"We think it's a major factor, people letting their aquarium species go. Unwanted pets, basically. They can do quite a lot of harm," Beatty said.

Goldfish are omnivores in the wild, and they can have destructive feeding habits. They deteriorate the quality of water by stirring up sediment on the bottom of river beds, dig up vegetation and also consume anything edible that comes before them — including native fish eggs.

On top of that, the goldfish compete for space and resources with native fish, and have been responsible for introducing disease.

Most startling is the fact that goldfish in the wild can grow to massive sizes, and they have an unprecedented ability to travel long distances. One goldfish found by researchers weighed 1.9 kilograms (4.1 pounds), while another was tracked travelling a marathon 230 kilometres (142 miles) in a year.

"We didn't think goldfish were that mobile," Beatty said. "What this study shows is that they are quite mobile, but I think it's mostly to do with with feeding and foraging."

A Vasse River goldfish. Image: murdoch university

Beatty said they were able to discover evidence of goldfish undergoing migration into nearby wetlands, where they are reproducing — a.k.a. a spawning migration. It's a discovery which will help researchers figure out how to stop the invasive species.

"It gives us better clarity on how to control them. That wetland has only a small opening, so we think we can manufacture some sort of trap to catch them when they go in to breed," he said.

Other removal methods include nets and electrofishing, but Beatty admits it can be difficult to remove an alien species without damaging the local fish population. As always, prevention is the cure.

"The key thing is if you've got unwanted pets, you can see if the pet shops will take them back. But if you're going to euthanise them, putting them in the freezer is the most humane way," Beatty explained.

"But just letting go of a pet, no matter how innocuous you think it is in your aquarium, or how pretty it is, can potentially cause a lot of damage. Not all fish you let go will form a self-maintaining population, but we're finding more and more that do."