14-05-2025
Mary River cod turns the tables on one of its biggest threats by eating them
A critically-endangered fish could be turning the tables on one of its biggest threats by eating them.
In the Mary River, stretching 291 kilometres from the Sunshine Coast to the Great Sandy Strait, the invasive pest species tilapia have been an ongoing problem.
They compete for food and habitat, as well as eating the eggs and juveniles of the river's endangered species such as the "bum-breathing" Mary River turtle, Australian lungfish, the white-throated snapping turtle, and the Mary River cod.
During a routine survey last month by Burnett Mary Regional Group (BMRG) conservationists tilapia remains were found in the mouths of two Mary River cod.
Its chief executive Tom Espinoza said they were not surprised by the finding, even documenting a cod eating a tilapia, but were excited to finally have evidence of the predation.
"There was always the hope and suspicion that they [cod] were eating everything," Mr Espinoza said.
"One of the cod that had the tilapia in its mouth would have been about 80cm long, and the tilapia was close to 40cm long, so they can eat a fish almost half their size — which is great.
"It's provided a lot of hope for people that are invested in healthy cod populations across the board."
While the Mary River cod is considered an apex predator in the river system, it now occurs in less than 30 per cent of their former known range.
Griffith University's Professor Mark Kennard, who works with the National Environmental Science Program, said the cod would eat a lot of different types of large animals.
"We've collected cod that have regurgitated possums and lizards so they are a large, top-level predator in freshwater systems," Professor Kennard said.
He said researchers believed tilapia would be a food source for the cod.
"But there hasn't been much documented evidence of our large-body native species actually eating non-native fish like tilapia," Professor Kennard said.
Environmental groups, researchers and traditional owners have been co-ordinating conservation efforts along the Mary River for decades.
Professor Kennard said tilapia had increased in abundance in the Mary River, especially since the last big flood in 2022.
Tilapia are so successful as an invasive fish that eradication is not considered possible with the species found in 26 of Queensland's 67 catchments.
"Part of the solution is things like enhancing benefits for native species," Professor Kennard said.
Professor Kennard was part of an extensive mapping of the unique river system, which runs south to north.
It has led to innovative conservation ideas such as cod logs and tilapia fertiliser.
"There's still a lot of work to do. The threatened species have not come back as much as we would like," he said.
"They're still rare and restricted in certain parts of the catchment despite 30 years of on-ground work doing restocking of fish, that kind of thing.
"So I think we do need to be making a concerted effort now and in the coming years."
Mr Espinoza said the finding would "increase our resolve" to put in place short and long-term solutions for managing invasive species in the delicate system.
"Hopefully in 10, 15, 20 years' time we've got a population of large cod that can do the rest of the job themselves," he said.
"It's a real natural, sustainable solution."
