These baby turtles are tracked at faster speeds than they can swim, and researchers think they know why

Through the gaping mouth of a fork-tailed catfish, you can spot the tail of an undigested bream, swallowed whole.

Are supersized specimens of the native catfish derailing decades of community and scientific efforts to save precious “bum-breathing” endangered Mary River turtle hatchlings?

“I guess the exciting thing now is that we’ve actually got funding to investigate and find out,” Tiaro and District Landcare turtle project leader Marilyn Connell said.

“It’s no use producing a whole lot more turtles if they are just going to get eaten in the river — that’s pointless.”

Researchers funded by the Queensland government are working to solve the mystery of mass deaths of the endangered freshwater turtles’ babies, in a stretch of river radically changed by a man-made wall.

The “bum-breathing” Mary River turtle – one of the world’s most vulnerable reptile species – can absorb oxygen from water through a gill-like structure in its anus.

Conservationists estimate numbers nosedived by as much as 95 per cent after European settlement due to land clearing, gold mining, sand and gravel extraction, feral animal predation, rapid population growth and nests being trampled by livestock.

It is believed that more than 200,000 eggs were collected for the pet trade between the 1960s and 1980s.

‘In the belly of the beast’

In 2001, passionate Tiaro and District Landcare volunteers launched a community-based conservation campaign to try to stop the Mary River turtle from joining an alarming list of Australian species becoming extinct.

Collaborating with researchers, the volunteers worked to protect nests and about 10,000 Mary River turtle hatchlings were reared for 12-15 months before being released to increase their chances of survival.

But Ms Connell believes next to none of the juveniles freed upstream of Tiaro’s tidal barrage lived more than six months.

Sonar tags attached to the shells of almost 60 young turtles were recorded travelling much faster than the reptiles could swim.

“It sounds like they’re in the belly of a beast,” Ms Connell said.

“The scientific literature says fork-tailed catfish normally grow up to 450 millimetres long and we frequently see them 500, 600, 700 or more millimetres, and the width of their mouth is like 120 millimetres, so they can swallow things that are quite large.”

Disappointed but undaunted

Charles Darwin University lecturer Mariana Campbell said those who worked to save the turtles should not feel that they had failed.

“I think the community group felt like “Wow, have we done all that in vain?” which I don’t think they have at all,” she said.

“We realised that once the hatchlings entered the river, something was happening to them that wasn’t allowing their population to recover, something that probably wasn’t happening before.”

Built in the 1980s, Tiaro’s tidal barrage stops saltwater moving upstream in the Mary River.

The fork-tailed catfish has since thrived, safe from bull sharks and large barramundi that are too big to navigate the fish ladders over the wall.

With the help of Landcare volunteers, researchers caught and operated on 17 fork-tailed catfish and three very large eels to insert acoustic tags and install hydrophone listening stations along a 60-kilometre stretch of the river.

They hope to determine whether the catfish speeds match those recorded from tagged turtles that had been swallowed.

Outdoor operating theatre

Dr Campbell said a great deal of care was taken to insert the tags without harming the fish.

“We make a tiny incision, the fish is anaesthetised, trained people place the tag inside so there’s no effect on the fish,” she said.

“The incision is then closed, antibiotics applied, the fish goes into a recovery tank until it’s well recovered from the anaesthetic, and then it goes back in the river.

“It’s very careful — we want to ensure the fish is well and doesn’t have any pain, and compared to their body size the tracker is absolutely tiny.”

Researchers from the University of the Sunshine Coast assisted in the process and in April a second lot of potential predators will be tagged.

The ear bones of catfish, which can be aged like the rings of a tree, will also be sent away for analysis, including testing the minerals in their bones.

Ms Connell said the prospect of finding answers to a host of questions was “really exciting”.

“Are the catfish staying just above the barrage, are they going down into the saltwater, are they able to come back up, how old are they, what are they eating?” she said.

Dr Campbell praised the efforts of the Landcare volunteers.

“It is just a great example of highlighting how important community involvement is when it comes to not just protecting endangered species, but also looking at the wellbeing and the health of any ecosystem that could be very close to your house, in your neighbourhood, or down the road,” she said.

Tiaro and District Landcare continues to work with the Mary River Catchment Coordinating Committee to help hatchlings upstream of Gympie, where fork-tailed catfish numbers are lower.