The endangered "punk" "bum-breathing" Mary River turtle is facing a frightening new threat, with fears thousands of juvenile turtles have been swallowed whole by mystery predators.
A viral photograph of a turtle growing an algae mohawk on its head helped raise international recognition of work to save the freshwater turtle.
Dubbed the "bum-breathing" turtle, it has the unusual ability to absorb oxygen from water through gill-like structures in its anus.
The remarkable species only lives in the Mary River in south-east Queensland and is fighting for survival in the lower reaches, where population estimates have dropped to just 5 per cent of 1970s levels.
Conservationists have dramatically increased the survival rates of hatchlings on land by protecting turtle nests for the past 20 years.
But in the light of new research, Tiaro Landcare's Mary River turtle project leader Marilyn Connell now feared that thousands of those juveniles have perished under water.
"We're still analysing the data, but what we think is that post six-months, probably next to none of these little fellows have survived," Ms Connell said.
Tracking the juvenile turtles
The disturbing discovery was made after a conservation project undertaken with Charles Darwin University, where juvenile turtles were raised to a larger size before being released aged 12–15 months old.
Last year, acoustic tags were attached to the shells of almost 60 juvenile turtles.
Hydrophone listening stations were anchored to the bed of a 4-kilometre-long stretch of the river to record the date and individual details of tagged turtles that passed them.
The hydrophones survived near-record flooding earlier this year, but when they were pulled up several weeks ago, the preliminary results from the sonar recordings were far from good news for the turtle conservation program.
"The problem is that most of these tags are travelling very fast between hydrophones — much faster than a turtle can swim — which suggests that they're in the belly of something," Ms Connell said.
"The tags are still attached, so the turtle is not motoring along on its own accord.
"What we do know is that they're still in the river so they're not being taken by raptors, like sea eagles."
What is the turtle-eating predator?
The mystery predators have been swallowing turtles that are just a little smaller than the size of a bread-and-butter plate.
"So, it is something with a big mouth and we've got our suspicions that it may well be the fork-tailed catfish," Ms Connell said.
"When we did our turtle population survey, we trapped everything that was in the river and one night we caught 100kg of fork-tailed catfish.
"We really need to do another tracking study. We need to catch fork-tailed catfish, put tags on them and see if their movement patterns match the movement patterns we've already got of these little [turtle] tags swimming much faster between each hydrophone than little turtles can."
Native fork-tailed catfish have thrived since a tidal barrage was installed downstream of Tiaro in 1982.
Mary River Catchment Coordinating Committee project officer Brad Wedlock said it prevented predatory species like bull sharks and large barramundi from travelling upstream to eat the catfish.
"You've taken that apex predator out of the system and only smaller barramundi can move through," Mr Wedlock said.
"We've modified the system, so it is tweaked in favour of the catfish.
"We catch 2.5-foot-long [76cm] catfish in the lower reaches, so there's a significant population of them but we need a lot more information, a lot more research on this."
Ms Connell said the preliminary results reinforced the need to monitor all stages of the conservation program.
"We can't just assume that enough of the hatchlings have survived to become adults for the population to become sustainable," she said.
