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A new study led by Griffith University has tracked two invasive fish species—the Mozambique and spotted tilapia—and found them established in the Mitchell River catchment in Northern Queensland, which flows into the Gulf of Carpentaria.

The study's lead authors say that is concerning, because tilapia can tolerate saltwater as well as freshwater, meaning they could exit the Mitchell River catchment during wet season floods and invade other Gulf rivers and their native ecosystems.

The study, "Tracing the movement of invasive tilapia fishes during a new invasion to inform catchment scale management strategies," has been in Biological Invasions.

"For decades, Biosecurity Queensland has been trying to keep tilapia—a group of ornamental African and Middle Eastern freshwater fish some people keep illegally as pets—out of Australian rivers," said Dr. Kaitlyn O'Mara from Griffith's Australian Rivers Institute.

"But it's been a losing battle—despite harsh penalties for illegal possession and expensive eradication efforts, many eastern Queensland and Western Australian rivers have become invaded by tilapia.

"Attempts to keep tilapia out of Gulf of Carpentaria rivers have been dampened by their invasion of the Mitchell River."

The researchers found tilapia established in three locations within one sub-catchment of the Mitchell River. The movement patterns observed in the study demonstrated habitat-seeking behavior, mostly by juveniles, that could lead to the successful establishment of tilapia at new locations within a catchment following introduction.

Both species of tilapia mostly eat a commonly found food source—aquatic plants—which allowed them to dominate fish communities. They were also found to eat the eggs of native fish.

Tilapia can be aggressive towards native fish and damage riverbeds when they feed and build nests. They can also be better at surviving tough conditions than many natives, making them well suited to the harsh conditions of the Gulf of Carpentaria in the dry season.

Dr. O'Mara said the pristine wetlands on the floodplains of the Gulf rivers produced a lot of the food native fish eat, but they also provide ideal tilapia habitat.

"It's in these wetlands that most of the food for Mitchell River fish is made, and they are culturally significant places that are home to rare native fish species," she said.

"We could see tilapia move here next, using wet season floods to swim there from the upper part of the catchment where they live at the moment.

"Detecting them early will be key to protecting these wetlands and other Gulf rivers.

"Separately, Biosecurity Queensland is now investigating a new public sighting in a separate subcatchment of upper Mitchell (around Mareeba) after cyclone Jasper.

"This sighting and our new study have sparked action for further surveys in the Gulf rivers to understand how they have expanded their range after the cyclone Jasper floods."