Feeding cane toad corpses to freshwater crocodiles could protect species from pest

As cane toads continue their march across the Kimberley, an innovative study using their chemical-laced corpses as bait could help protect the freshwater crocodiles in their path.

Scientists, traditional owners and volunteers have been working to mitigate WA’s cane toad invasion for over 10 years, but despite their efforts and the Kimberley’s rugged terrain, the pests have continued to spread.

The latest monitoring efforts by the Department of Biodiversity, Conservation and Attractions (DBCA) estimated the front line would reach Derby this year and Broome by 2025.

If their expectations are realized almost the entirety of the Kimberley will soon be covered; leaving populations of the region’s important predators, like freshwater crocodiles, vulnerable.

Last year researchers found 60 of the dead reptiles at Danggu Gorge National Park (formerly known as Geikie), but expected the current toll to be much higher.

Freshwater crocodile at Bandilgnan
Freshwater crocodiles can be seen in their hundreds at Bandilngnan (Windjana Gorge). (ABC Kimberley: Taylor Thompson-Fuller)

Trial could save populations of freshies

Two hours’ drive away at Bandilngnan (Windjana Gorge), scientists are using the corpses of the poisonous pest as bait for freshies, as freshwater crocodiles are known, in a bid to stop deaths of the reptiles.

The study, run by Macquarie University (MQ), Bunuba Rangers and the DBCA, has cane toads gutted, washed and injected with nausea inducing chemicals.

A photo of windjana gorge with stakes
Two stakes ready to bait with cane toad corpses for freshwater crocodiles at Bandilngnan (Windjana Gorge). (ABC Kimberley: Taylor Thompson-Fuller)

Lead researcher MQ ecologist Dr Georgia Ward-Fear said when crocodiles took the bait, they got a bout of nausea, associated cane toads with the feeling and didn’t eat one again.

“We’ve essentially created a food poisoning response in the native predators to the taste and the smell of [cane] toads,” she said.

Dead cane toad being washed
The dead cane toads are gutted, washed and injected with nauseating chemicals before being set out as bait. (ABC Kimberley: Taylor Thompson-Fuller)

The approach, called “conditioned taste-aversion” seemed to be working on freshwater crocodiles which quickly developed a distaste for the amphibian warty.

“There’s a high uptake of baits as we’ve started the baiting trials, and that very quickly declines within a couple of days of the baits being out and replenished,” she said.

Cane toad gets it's head chopped off
Dead cane toads have their heads cut off before they are washed and injected with a nauseating chemical.(ABC Kimberley: Taylor Thompson-Fuller)

The hope was the technique, which had also been used to a similar effect with goannas and northern quolls, would buffer the reptiles against the first wave of the cane toad invasion.

“As the toads come into a landscape for the first time, they are a novel prey item and animals haven’t had any interactions with them,” she said.

“And so there’s no evolved resistance to the toxin, predators only have to mouth the cane toad for say 10 seconds and they die.”

Three people hold up a goanna smiling
Dr Georgia Ward-Fear has used the technique to teach goannas not to eat cane toads too. (Supplied: Melissa Bruton)

The results of the study still need to be finalised, but early indications showed fewer freshwater crocodiles were dying where baits had been deployed, Dr Ward-Fear said.

“There were a large number of deaths before we moved into these areas and started training and we have seen a decline in the deaths in areas we have treated,” she said.

Croc approaches cane toad bait
The chemical injected cane toad corpses make freshwater crocodiles sick, so they never eat one again. (Supplied: Department of Biodiversity, Conservations and Attractions)

Cane toads come for Derby

DBCA project officer Tamishia van der Donk, is part of the five-person field team setting up stakes, motion sensing cameras and conducting crocodile headcounts.

Mish setting motion sensing camera
Tamisha Van Der Donk sets up a motion sensing camera to detect when freshwater crocodiles take the bait. (ABC Kimberley: Taylor Thompson-Fuller)

Another critical part of her work is monitoring the cane toad front line as it progresses through the Kimberley, which she said could hit Derby by the end of this year.

“Cane toads move at a rate of approximately 50 kilometers per year and they are 30 kilometers out of Derby, so… they could potentially be in and around Derby this year,” she said.

A map showing spread of cane toads in Kimberley (2022)
A 2022 map showing the gradual spread of cane toads in Australia. (Supplied: Department of Biodiversity, Conservation and Attractions)

Residents of the town and its surrounds might even hear the front line approaching, as the larger toads heading up the invasion come into earshot.

It’s a sound DBCA field officer Miles Bruny said he had heard a few times before.

It still upsets him knowing the type of destruction cane toads would bring.

DBCA Field Officer Miles Bruny holds up cane toad
DBCA field officer Miles Bruny has heard the cane toad front line approach more than once. (ABC Kimberley: Taylor Thompson-Fuller)

“It sounds kind of like 1,000 high pitched helicopters arriving over the horizon and it comes on pretty strong,” he said.

“To know that there’s a wave of one species running towards you that is causing a lot of deaths on the way… it’s a bit hard to get your head around.”

Freshwater croc swims past cane toad
A freshwater crocodile swims past a bait used to deter them from eating cane toads. (Supplied: Department of Biodiversity, Conservation and Attractions)

Mr Bruny said the days of stopping their march were over and it “never really would have worked”, but new research gave him hope more species could be protected in the future.

“It’s more a game of lateral thinking now, we need to come up with really creative solutions, like taste aversion, and be ready by the time they get there,” he said.

Toads could continue their march

If they eventually conquer the Kimberley, Dr Ward-Fear said, cane toads would be “challenged” when traversing the arid landscape of the Pilbara, but could exploit water sources to keep marching south.

Freshwater crocodile found dead
A number of freshwater crocodiles were found dead at Danngu Geikie Gorge National Park last year. (ABC Kimberley: Hannah Barry)

“There are lots of agricultural areas and water points that they may be able to exploit to move through that zone, so never say never, they might end up down at Perth,” she said.

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