Fishermen in Queensland, Australia suspect a human hand is helping invasive fish to spread throughout the state’s rivers and dams. African tilapia fish have spread considerably further than predicted by biologists and fishery authorities and local fishermen now worry that the approaching wet season will end efforts to control the tilapia invasion in already infected waters.

According to Peter Jackson, a freshwater fisheries scientist with the Queensland Government, the findings of tilapia populations at the top of river systems suggest that someone is deliberately introducing this fish to Queensland’s waterways.

“Our pest fish strategy has always been one of trying to contain fish to where they are and not let them spread, Jackson says. Unfortunately recently it’s obvious that someone’s been spreading them around.”

There exists many different species of tilapia but now of them are native to Australia. Tilapias are popular farm fishes and have been kept in ponds for thousands of years; the Ancient Egyptians did for instance raise tilapia in ponds along the River Nile. Tilapia is also an appreciated aquarium fish among aquarists with large enough tanks.

Tilapias are adaptable fish and the fact that they thrive in tropical Queensland comes as no surprise. Once tilapia has established a breeding population in a lake or waterway, it is almost impossible to eradicate them – at least if you aren’t willing to sacrifice a lot of native species in the process. In the warm waters of Queensland, tilapia can breed several times per year and since they are a mouth-brooding species each batch have a high survival rate.

Tilapia is now found in one of Queensland’s biggest river systems, the Burdekin, where they compete with native species for food and space. According to David Bateman, executive officer of the amateur fishing lobby group Sunfish, tilapias also cause trouble for native species by disturbing the water when they build their circular nests to breed.

Jackson now calls for more research to determine the risks of having tilapia in Queensland waters. “Research needs to be done to really determine exactly what their impact is, but we’re taking the precautionary approach and saying well, let’s not find out, let’s keep them out of as many river systems as we can – because once they do get into a river system, unless we find out really quickly, and they’re in an isolated area, our chances of getting them out are almost zero”, Jackson explains.

Jackson fears it might already be too late to save the Burdekin from the tilapia invasion, since the wet season is rapidly approaching. Resources will instead have to be focused on protecting remaining river systems.