Goldfish the size of footballs and plates have been found invading freshwater rivers in Western Australia, posing a risk to native species.
It has led to calls from scientists at Murdoch University warning people not to flush unwanted goldfish down the toilet because they are becoming a pest, PerthNow reports.
"This is the first time we have found goldfish in an estuary during summer when the water is a lot more salty. This shows they are tolerating the salt for long periods," Fisheries scientist Dr James Tweedley explains.
"The fear is that the goldfish could start using the estuaries as saltbridges to travel to new river systems where they colonise and impact native species."
Local authorities fear the goldfish could introduce disease and cause problems because of their eating habits, eating the eggs of native species in the same environment.
Some of the goldfish caught have been found to weigh up to 2kg each.