Suspected koi in the Torrens put native wildlife recovery on guard

For people working to bring native wildlife back to Adelaide’s River Torrens, an unwanted aquarium fish is not simply someone else’s discarded pet. It can become another competitor for the animals they are trying to protect. That is the concern behind an effort to catch brightly coloured fish suspected to be koi, after sightings in two stretches of the river. ¹

More than a dozen were spotted between St Peters Weir and Hackney Bridge, near Hackney, in the week before ABC News’s October 27 report. The group included bright orange, black and multicoloured fish. A smaller number were reported farther upstream at Dernancourt, where nets are being used to try to catch them. ¹

Those are sightings, not a census. The identification remains provisional: no capture-based confirmation or examination result has been established. Nor has a catch total been announced. Nets in the water mean an interception attempt is under way, not that either stretch has been cleared. ¹

Stuart Collard, from Green Adelaide, which manages the Torrens environment, told ABC the concentration was unusual.

Green Adelaide suspects members of the public released the fish. That explanation has not been confirmed, and nobody responsible has been identified. The unusual group is a reason to investigate and act early; it is not proof of who put the fish there, or even how they entered the river. ¹

The strongest local warning comes from goldfish. Green Adelaide regularly catches them during river surveys, and Collard says there is understood to be a self-sustaining population in the Torrens. “People do release them and we understand that there's a self-sustaining population within the river,” he told ABC. “It's not the right place for these species to be.” ¹

That history explains the urgency, but it does not settle the status of the newly sighted fish. An established goldfish population shows that aquarium fish can persist in this waterway. It does not show that the suspected koi have reproduced, or that they have already formed a lasting population.

Collard described two ways koi can put pressure on the river. Like European carp, they compete with native fish for food. As bottom feeders, they also stir sediment and disturb plants on the riverbed, affecting plant growth. These are the ecological risks behind the response—not measured damage attributed to this particular group of fish. ¹

James Johnston of the Invasive Species Council expressed disappointment at the sightings, given the money and effort being spent to help native fish, rakali and other wildlife thrive. The concern is about protecting that recovery work from another avoidable pressure, rather than declaring an ecological loss that has already been counted. ¹

The river’s ambitions extend to platypus. The South Australian government announced an additional A$4 million in August to help return them to the Torrens, where they were last recorded in the 1880s. Collard said the river was getting close to being able to sustain them again, with reintroduced animals likely to come from Victoria. That remains a restoration goal, not a completed return. ¹

Keeping koi in South Australia requires a permit from the Minister for Primary Industries because they are treated as a noxious species. ABC also reports that releasing koi, goldfish or other non-native fish into the state’s rivers is illegal. Possession under a permit and disposal into a waterway are distinct matters: permission to keep a fish is not permission to release it. ¹

The widely reported fine needs more care. ABC warns that dumping these fish could attract a penalty of up to A$120,000. But the primary statutory text established here is section 78(1) of the Fisheries Management Act as originally enacted in 2007. It set maximum penalties of A$120,000 for a natural person and A$250,000 for a corporation for unauthorised dealings in, or possession or control of, noxious aquatic species. That historical provision does not establish the current October 2026 maximum for releasing every kind of non-native fish. ^{1 2}

The practical instruction is less complicated than the penalty question. South Australia’s primary industries department, PIRSA, tells aquarium owners: “Never release or dispose of unwanted aquarium fish into waterways…” An unwanted pet must not become the river’s responsibility. ³

Collard’s priority is to catch invasive fish early, before they expand and occupy more of the river. That is the sensible measure of this response: not whether the colourful fish briefly disappear from view, but whether they can be intercepted before another persistent population takes hold. ¹