Record floods have propelled an aggressive invasive fish species across a south-east Queensland river catchment, compromising efforts to save endangered and ancient fishes and turtles.
The Moonaboola (Mary) river catchment is home to several threatened species, including the Mary River turtle, the white-throated snapping turtle (known for breathing through its bottom), the Mary River cod and the Australian lungfish, which has survived for 150m years and is considered a living fossil.
The Burnett Mary Regional Group (BMRG), a conservation group, led an eight-week survey of the river and its tributaries including a 200km canoe expedition and sampling at 61 sites using environmental DNA and netting. According to the findings, record floods in 2022 enabled an exotic fish called the Mozambique tilapia to proliferate.
Prof Mark Kennard, the deputy director of the Australian Rivers Institute at Griffith University, was concerned by how “massively” tilapia had spread. “It’s quite alarming how abundant they are,” he said. “Sometimes you’ll see thousands of fish swimming around.”
The tilapia invasion was an unwelcome additional pressure for endangered fish and turtles, he said, competing for their food and preying on their eggs and young.
Kennard said the Mary River was a “hotspot of threatened aquatic species” and one of the last remaining large rivers on the east coast without a large dam on it.
The riverbank was already degraded due to human occupation and agriculture, he said. Then extreme flooding scoured the riverbed, removing crucial spawning habitat for lungfish and stripping out the large hollow, submerged logs where cod lay their eggs.
Unfortunately, the survey revealed the numbers of endangered fish and turtle species had not improved despite conservation efforts over the past three decades, Kennard said.
Dr Anthony Chariton, who researches aquatic ecology at Macquarie University but was not involved in the Mary River study, said while flooding was a natural part of Australian aquatic environments climate change was affecting the dynamics. “So sometimes floods are more frequent and larger, and then you get prolonged periods of drought.”
He said extreme floods could introduce a lot of material – pollution, rubbish, sediment and nutrients from the soil – into waterways, with potentially long-term effects for river ecosystems.
Tom Espinoza, the CEO of the BMRG, said there were about eight invasive fish in the river that should not be there, but the floods had enabled the tilapia to reach previously inaccessible areas.
“Tilapia have a distinctive feature in their ecology that they chase flow,” he said. “So they’re really good colonisers. Every time the river flows, they’ll chase it upstream.”
The study, supported by commonwealth and Queensland disaster recovery funding, brought together local traditional owners, environmental and resource management groups and scientists from Griffith University and the National Environmental Science Program’s Resilient Landscapes Hub.
Espinoza said the community was “really invested and environmentally informed” and keen to protect the river and its species.
The study has provided a stocktake of where threatened species were living, which could help target habitat restoration efforts, including planting aquatic plants and adding hollowed out logs and other structures that provide protection to baby turtles.
Kennard said this was the beginning of a longer-term resilience strategy for the river. “We’re in it for the long term”, he said. “I’m concerned that these rare, endemic species are going to continue to decline progressively over the next 50 years, unless we can do something about it now.”
