Picture by Rick Stevens

Story by James Woodford 

This winter 1,000 of the nation’s rarest tadpoles will be taking a freezing bath.

In the past decade scientists working to save the Corroboree Frog have been unable to bring the species back from the brink of extinction.

Only 65 males are known to exist in the wild and every year their numbers seem to be falling.

The prognosis for the diminutive amphibians is so poor that it is no longer thought to be safe for them to be left undisturbed in the high alpine grasslands.

Once corroboree frogs were so common their survival was taken for granted and their angry calls were almost deafening.

“Prior to the mid-80s,” says NSW Department of Environment and Climate Change, threatened species officer Dave Hunter, “you could come up here and see the Corroboree Frogs walking around everywhere.”

In recent years a mere handful of humans have seen the species in the wild.

For the past few years scientists have been harvesting the last of the species’ eggs from the wild in order to bring them into captivity.

It is a last desperate attempt to protect tadpoles and young metamorphs from a deadly fungus, called chytrid. The fungus has ravaged populations of frogs both in Australia and overseas.

The equivalent of Noah’s Ark for Corroboree frogs are several refrigerated shipping containers, located at Taronga Zoo in Sydney and the Amphibian Research Centre near Melbourne. It is inside these that the captive breeding program for the species has been headquartered.

But attempts at large-scale captive breeding have not yet proven successful. Instead efforts to date have involved harvesting as many eggs as possible, keeping these in captivity until they are four year old frogs, and then releasing them back into the wild. Unfortunately these valuable young seem to be also highly susceptible to the disease.

So this year the recovery team is taking a radically different tack.

Rather than store them for years in a shipping container in Melbourne or Sydney the eggs have only been kept until they were ready to hatch. In the last few weeks 20, 200 litre tubs have been placed in the wilderness and the eggs released into those. There are four sites, each with five tubs and into each tub has gone 50 tadpoles.

The idea is that they will be within their own environment but away from contact with other frog species carrying chytrid fungus. It is thought that the most dangerous time for infection is when the frogs are tadpoles. In fact, a recent experiment demonstrated that 60% of the tadpoles in natural pools become infected, whereas no tadpoles in the artificial tubs became infected.

When the spring comes and they metamorphose they will climb a ramp out of the tubs and theoretically begin their lives as wild Corroboree Frogs.

It is an extraordinary thing to have to do. But the chytrid fungus has infested these high swamplands and another common species that co-exists with the Corroborees, called the Common Eastern Froglet, appears to be one of the main problems. It is a carrier of the fungus and shares the same pools with the endangered corroboree tadpoles, readily infecting them.

Part of the difficulty of captive breeding this species is that because they are a cold climate animal they must be kept in chilled facilities, additionally, because they have evolved to cope with winter snows and short summers, their life cycle is unusual compared to other Australian frog species.

It takes nearly three weeks for Corroboree Frog eggs to be ready to hatch. Once ready to hatch they go into a state called diapause, which basically means they do nothing until a downpour of rain triggers them to hatch into tadpoles. They will then stay in their pools, under ice and snow throughout the winter and then metamorphose in the spring.