Unwanted goldfish dumped into rivers are threatening freshwater fish

Pet goldfish being discarded in wetlands are expanding to several times their typical size and spreading diseases to the native fish.

The unwanted fish are being dumped in wetlands south of Perth and are making their way to rivers housing freshwater fish, choking the native habitat as they feed and grow to become up to two kilograms, Murdoch University professors told .

Dr Stephen Beatty with the university's school of Veterinary and Life Sciences said he and his coworkers often find goldfish weighing over one kilogram in the Vasse River near Busselton Environmental Policy And Associated Schemes Introduced For Wetlands are working to remove the pests.

Discarded pet goldfish (pictured) dumped in wetlands south of Perth are travelling to freshwater rivers where they are expanding to several times their size

Discarded pet goldfish (pictured) dumped in wetlands south of Perth are travelling to freshwater rivers where they are expanding to several times their size

'Perhaps they were kids' pets where the family have been moving house and their parents, not wanting to take the aquarium, have dumped them in the local wetlands,' he said.

'Unfortunately a lot of people don't understand that wetlands connect up to river systems and introduced fish, once they get in there, can do a lot of damage to native freshwater fish and the aquatic habitat.'

The goldfish can disrupt the aquatic plants by stirring up algae, disturb their feeding strategy and possible eating other fish's eggs, Dr Beatty said.

The goldfish (pictured) can disrupt the aquatic plants by stirring up algae, disturb their feeding strategy and possible eating other fish's eggs

The goldfish (pictured) can disrupt the aquatic plants by stirring up algae, disturb their feeding strategy and possible eating other fish's eggs

It is believed that they have also introduced at least one disease to native fish populations, he said.

'It causes lesions on the skin, it's pretty horrible to look at.'

Dr Beatty is asking people to have unwanted goldfish to simply return them to the pet shop or euthanize them by placing them in a plastic bag in the freezer.