Dumping goldfish into a river may seem like a harmless act, but in reality, doing this can cause the fish to grow so large they become harmful to the ecosystem.
Researchers from the Centre of Fish and Fisheries in Western Australia have been trying to control the goldfish in the Vasse River for more than 12 years.
Goldfish are an invasive species, which can cause serious problems for native fish and the surrounding ecosystem.
Stephen Beatty and his team of researchers have spent the last year studying the movement patterns of goldfish in the wild. The results have been published in a paper in the journal, Ecology of Freshwater Fish.
Beatty says he believes the rise of goldfish in the wild is due to people letting them go in local rivers.
"We think it's a major factor, people letting their aquarium species go," he told Mashable. "Unwanted pets, basically. They can do quite a lot of harm."
Goldfish are omnivores and will eat anything from the eggs of other fish to vegetation. Their eating habits are destructive. They have been observed lowering the quality of water by stirring up sediment on the bottom of river beds while trying to dig up vegetation.
In the wild, goldfish can grow to startling sizes and can travel extremely long distances. Researchers found one goldfish that had grown to more than 4 pounds, while another had traveled more than 142 miles in one 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."
Beatty said researchers are working on ways to control and remove goldfish from the wild. In the meantime, he encourages those with unwanted goldfish to either return them to the pet store or, "if you're going to euthanize them, putting them in the freezer is the most humane way," he says.
"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."