Noxious koi carp have been spotted in the River Torrens.
It's illegal to keep koi in South Australia, and those dumping them — or even their pet goldfish — in rivers could be fined up to $120,000.
Nets are being used to try to catch the brightly coloured koi.
Adelaide residents are being urged not to release noxious koi carp or their unwanted goldfish into the River Torrens, following sightings of more than a dozen suspected koi at Hackney, and a smaller number upstream at Dernancourt.
The group of bright orange, black and muti-coloured fish at Hackney appeared last week between the St Peters Weir and Hackney Bridge.
Stuart Collard from Green Adelaide, the organisation responsible for managing the environment of the Torrens, said it was illegal to keep koi carp in South Australia and it was unusual to see such a large number in the river.
Some of the large group of koi spotted by an ABC reporter in the River Torrens at Hackney last week. ( ABC South East SA: Caroline Horn )
"We haven't seen those numbers, or in fact many records at all, of koi within the river," he said.
Mr Collard said Green Adelaide suspected the fish had been released by members of the public.
Koi are considered a noxious species under the SA Fisheries Management Act and cannot be owned without a permit from the Minister for Primary Industries.
It is illegal to dump koi, goldfish or any non-native fish, into South Australian rivers and people can be fined up to $120,000 for doing so.
Stuart Collard is urging people not to dump koi, goldfish or other non-native fish into the River Torrens. ( ABC News: Daniel Taylor )
Mr Collard said Green Adelaide regularly caught goldfish while carrying out surveys in the river.
"People do release them and we understand that there's a self-sustaining population within the river," he said.
"It's not the right place for these species to be."
Nets are being deployed at Dernancourt to try to catch the rogue koi carp. ( ABC News: Daniel Taylor )
Nets are currently being used at Dernancourt to try to catch the population of koi recently spotted in that part of the river.
Mr Collard said, like European carp, koi carp competed with native fish for food.
He said as bottom feeders, they stirred up the sediment that disturbed plants at the bottom of the river, impacting their growth.
"It's really important for these types of invasive fish species that we get on top of those populations early so that they don't expand and take up more space within the river," he said.
Matt Jacobs from the Glenelg Nature Trust undertaking work to catch the koi at Dernancourt. ( ABC News: Daniel Taylor )
James Johnston from the Invasive Species Council said it was disappointing to hear of the koi carp being spotted in the river, particularly with the funds and effort being used to try to help native fish, rakali and other species thrive.
The South Australian government announced in August that it would provide $4 million in additional funding to try to return platypus to the Torrens.
Work is being done to prepare the River Torrens for the reintroduction of platypus. ( ABC News: Malcolm Sutton )
The unique animals were last recorded in the waterway in the 1880s, but Mr Collard said the river was getting close to being able to sustain a population again, with those reintroduced likely to be relocated from Victoria.
