A live goldfish just caught by a fisherman lies on the bank of Teller Lake #5 outside of Boulder, Colorado before being thrown back April 10, 2015. Reuters/Rick Wilking

Researchers in Western Australia have found that giant goldfish weighing up to 1.9 kilograms are threatening freshwater species in WA waterways. People are dumping unwanted pet goldfish in the waterways and they are reaching massive sizes.

Dr. Stephen Beatty from the school of Veterinary and Life Sciences at Perth's Murdoch University and his colleagues are regularly finding goldfish weighing over one kilogram. They have been working for the past 12 months on a control program for Busselton's Vasse River.

In most cases, the goldfish were released into the waterways innocently but they grew to such massive sizes that the fish are choking the habitat for native fish. While the fish were dumped innocently, people should be aware that wetlands connect up to river systems and fish introduced in the waterways can do a lot of damage to aquatic habitat and native freshwater fish. Once established in freshwater, goldfish cause a host of problems.

“They cruise along the bottom stirring up the substrate with their feeding strategy. This can re-suspend nutrients into the water column which exacerbates things like algal blooms. They can also disrupt aquatic plants and eat other fish's eggs ... We know that one disease has been introduced and we think it has probably come in on goldfish. It causes lesions on the skin, it's pretty horrible to look at,” Beatty told the ABC.

Preserving freshwater fish is important as they do a lot of good to our ecosystem. As they are carnivores, they eat mosquito larvae in the water. Beatty attached acoustic transmitters to 15 goldfish in the Vasse River in WA's South West.

“What we found is that they actually migrate off the main channel into the Vasse River into a wetland system to spawn. Given that there is only a narrow channel to get into the wetland, we think that could be targeted to control the species in that system a lot more efficiently than we have been doing,” he added.