Populations of koi carp, goldfish, pearl cichlids, swordtails, guppies, silver perch and spangled perch have been detected in the Swan-Canning catchment.
Swan River Trust senior environmental manager Jeff Cosgrove said the feral fish likely entered the Riverpark by people dumping unwanted fish.
‘The Trust urges people not to release aquarium fish into ponds, rivers, creeks, dams, artificial lakes or stormwater drains because they can connect to our waterways,’ he said.
‘Introduced freshwater fish can affect our catchment in much the same ways that feral foxes, cats and rabbits cause damage to the landscape, because exotic fish compete with native species for habitat and food.
‘Small ornamental fish, common in the freshwater aquarium trade, cause big problems in our waterways because they can flourish and compete with native species. This puts further pressure on native fish already stressed by other environmental pressures such as climate change. ‘
Efforts have been made to remove feral fish from the Riverpark and the catchment.
The Trust, Murdoch University and the Department of Fisheries have most recently been targeting pearl cichlids.
Mr Cosgrove said it was important people chose aquarium fish carefully to ensure fish were suitable and would not cause problems.
‘Any unwanted fish should be returned to the aquarium dealer or from where they were purchased or should be disposed of humanely,’ he said.