Hundreds of captive-bred, critically endangered frogs are managing to survive after being released at a fast-flowing mountain stream near Mount Beauty in Victoria, with some moving a “considerable distance” from the release point.
More than 600 spotted tree frogs have joined dwindling wild populations in the Kiewa River, as part of a Zoos Victoria conservation breeding program to boost numbers and genetic diversity after 50% of the frog’s Victorian habitat was severely burnt in the 2019-20 black summer bushfires.
This year a team of biologists from Zoos Victoria and Wild Research released 265 one-year-old frogs, following a 2024 cohort of 300 tadpoles and 70 juvenile frogs.
So far more than half of the animals released have been found again, according to the Wild Research director, Dr Matt West, who has been part of a team tracking their survival after release.
“Quite a large proportion of those animals are actually surviving, which is exciting and brings us a huge amount of hope that we might be able to recover this population of spotted tree frogs.”
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It was promising news for a species facing numerous threats, including a disease called the chytrid fungus, predation by introduced fish species like trout, flash flooding and bushfire.
Even though the frogs were individually marked (with a skin clip on their toe), finding them again could be challenging, West said. Spotted tree frogs grew to about 6cm long and were well camouflaged with their vivid green spotty skin.
“They are very difficult animals to find because they look very similar to the types of things they like to sit on – granite boulders, ferns and sedges,” he said.
Team members walk up and down the stream with headlamps, looking for frog eyes reflecting back and listening out for calls. Photograph: Zoos Victoria
The task of finding them was made even more difficult after many of the captive-bred animals had moved “a considerable distance” – about a kilometre in a year – from the section of stream where they were released, West said.
“It’s challenging walking up and down the stream, just for a couple of kilometres. But when you’ve got to extend that out to 10km, it makes it even more difficult to find a frog.”
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Spotted tree frogs live in mountain streams of the Great Dividing Range between Victoria’s central highlands and Mount Kosciuszko in New South Wales.
Dr Kirsten Parris, an amphibian ecologist at the University of Melbourne, said spotted tree frogs were “beautiful” and “super cute”, and one of approximately 80 tree frog species (in the Hylidae family) in Australia. “They’re all classified as tree frogs, but they don’t necessarily all climb trees.”
Deon Gilbert, a threatened species biologist with the zoo, said the Kiewa River reintroduction site was chosen for its existing, albeit small, wild population and lower trout abundance, meaning fewer predators.
The limited canopy in that section of stream also offered more sunlight – providing opportunities for the frogs to sunbathe and warm up on the rocks – an activity thought to be protective against chytrid fungus.
The survey required a huge team effort. Traditional owners, recreational fishers and community members joined biologists from Zoos Victoria and Wild Research, walking up and down the stream with headlamps, looking for frog eyes reflecting back and listening out for calls.
Gilbert said the team would continue to monitor the health and survival of the released frogs, but it was promising to see them persisting and moving within the landscape. “Fingers crossed things start looking up for them.”
