Record floods have accelerated the spread of an invasive fish species in a southeast Queensland river, threatening efforts to protect endangered fish and turtles that are unique to the region.

The Moonaboola (Mary) River catchment is home to several rare and ancient species, including the Mary River turtle and the white-throated snapping turtle, both of which possess distinctive adaptations. The endangered Mary River cod and the Australian lungfish, a species that has survived for over 150 million years and is often regarded as a “living fossil,” also inhabit the river.

However, conservation efforts face a significant challenge with the recent spread of the Mozambique tilapia, an invasive fish species.

The Burnett Mary Regional Group (BMRG), a conservation organisation, led an eight-week survey of the river and its tributaries. This study involved a 200 km canoe expedition and sampling at 61 sites using environmental DNA testing and netting techniques.

Their findings revealed that record-breaking floods in 2022 allowed Mozambique tilapia to spread widely across the river system, worsening the situation for native species.

Prof. Mark Kennard, deputy director of the Australian Rivers Institute at Griffith University, expressed deep concern about the extensive spread of the tilapia, describing it as a major new threat to native aquatic life.

“It’s quite alarming how abundant they are,” he said. “Sometimes you’ll see thousands of fish swimming around.”

According to Kennard, tilapia compete for food with endangered species, preying on the eggs and young of these native fish and turtles. He noted that the Mary River is considered a critical habitat for threatened aquatic species and remains one of the few undammed major rivers on Australia’s east coast.

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.

Human activities and agriculture have already degraded sections of the riverbank, and recent floods further damaged the ecosystem. Floodwaters scoured the riverbed, removing critical spawning areas for the Australian lungfish and clearing away large, submerged logs where Mary River cod typically lay their eggs.

Unfortunately, the survey found that populations of endangered fish and turtle species have not shown significant improvement despite over 30 years of conservation work.

Dr. Anthony Chariton, an aquatic ecologist from Macquarie University, who was not involved in the study, explained that while flooding is natural, climate change is altering its impact on river ecosystems.

“So sometimes floods are more frequent and larger, and then you get prolonged periods of drought.”

Extreme floods, he said, can carry pollutants, sediments, and nutrients into waterways, causing long-term ecological changes.

BMRG CEO Tom Espinoza noted that while around eight invasive fish species are present in the river, the floods allowed 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 disaster recovery funding from the Commonwealth and Queensland governments, involved collaboration with local traditional owners, environmental groups, and scientists from Griffith University and the National Environmental Science Program’s Resilient Landscapes Hub.

The research provides an assessment of where endangered species reside, which could help guide future habitat restoration efforts.

Planned strategies include planting aquatic vegetation and placing hollow logs and other structures to shelter young turtles.

Kennard emphasised that this marks the beginning of a long-term resilience plan for the river ecosystem.

 

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